Stability and three-dimensional aromaticity of closo-NB(n-1)H n azaboranes, n = 5-12

Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for tetrahedral or octahedral molecules, an aromatic closed-shell electronic structure with 6n + 2 electrons. Conversely, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each macrocyclic path, yet their π-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., with π-electron counts different from 6n + 2, and they are related to naphthalene. Finally, we identify tetrahedral and cubic π-conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite the fact that these molecules show substantial π-electron delocalization, they cannot be classified as true 3D-aromatics.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na₄(B₁₂H₁₂)(B₁₀H₁₀) with 3 V-class cathode active materials: NaCrO₂, NaMnO₂, and NaFeO₂. Among these layered sodium transition metal oxide cathodes, NaCrO₂ shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na_0.5CrO₂) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na₄(B₁₂H₁₂)(B₁₀H₁₀) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

Probing a General Rule towards Thermodynamic Stabilities of Mono BN-doped Lower Polyenes

The BN-doped organic analogues are interesting as aliphatic amineboranes for hydrogen storage, precursors for aromatic borazines and adsorbent cage azaboranes. However, BN-doped aliphatic polyenes remained undeveloped. Herein, we perform theoretical calculations on two mono BN-doped aliphatic lower polyenes, 1,3-butadiene and 1,3,5-hexatriene. A general rule is proposed, i.e., isomers with terminal nitrogen and directly BN-connected, N-B(R), in particular, are of significant thermodynamic stability as compared with their inverse analogues (where boron is at the terminal position). The N-B(R) type isomers are found to be the most stable ones in both polyenes. Isomers with terminal B and N are of intermediate stability. Highly destabilized isomers are those with one terminal methylene group and one terminal heteroatom in the butadiene series, and two terminal methylene groups in the hexatriene series. Rules established here may lead researchers to synthesize isomers with particular thermodynamic stability.

Structural Increments for 11-Vertex nido-Phospha- and Aza(carba)boranes and -borates; Dependence of Energy Penalties on the Extent of Electron Localization

Relevant structural features and corresponding energy penalties were determined that allow to easily estimate the relative stabilities of 11-vertex nido-phospha- and aza-substituted boranes, borates, carbaboranes, and carbaborates. For this purpose, density functional theory computations at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)+ZPE level were carried out to determine the relative energies of 95 phospha- and 46 aza(carba)boranes and -borates. Energy penalties assigned to disfavoring structural features show additive behavior and excellent precision with respect to the computed results, as in the case of 6- and 11-vertex nido-carboranes and -borates. An unsubstituted phosphorus atom was found to possess energy penalties quite similar to those of the three-electron-donating H-C group. A bare nitrogen atom has energy penalties much larger than those of a bare phosphorus atom. Four-electron-donating RP and RN moieties, however, have even more adverse energy penalties. The disfavoring effects of heteroatoms in a borane cluster are determined by the amount of electron localization, that is, primarily by the number of skeletal electrons that formally originate from the heterogroup and secondarily by the electronegativity. Heteroatom energy penalties are independent of the type of the other heteroatoms present in the same cluster. Some novel phospha(carba)borane geometries with bare and exo-substituted phosphorus atoms in the same cluster have favorable thermodynamic stabilities competitive with those of known isomers.