The Crystal Chemistry of Inorganic Hydroborates

Protonation of Borylated Carboxonium Derivative [2,6-B10H8O2CCH3]−: Theoretical and Experimental Investigation

The process of protonation of [2,6-B10H8O2CCH3]− was investigated both theoretically and experimentally. The most suitable conditions for protonation of the derivative [2,6-B10H8O2CCH3]− were found. The process of protonation was carried out in the presence of an excess of trifluoromethanesulfonic acid CF3SO3H at room temperature in dichloromethane solution. The structure of the resulting complex [2,6-B10H8O2CCH3*Hfac]0 was established using NMR data and the results of DFT calculations. An additional proton atom Hfac was found to be localized on one of the facets that was opposite the boron atom in a substituted position, and which bonded mainly with one apical boron atom. The main descriptors of the B-Hfac bond were established theoretically using QTAIM and NBO approaches. In addition, the mechanism of [2,6-B10H8O2CCH3]− protonation was investigated.

Structural Phase Transitions in closo-Dicarbadodecaboranes C2B10H12

The crystal structures of three thermal polymorphs (I, II, and III) for each isomer of closo-dicarbadodecaboranes C₂B₁₀H₁₂ (ortho, meta, and para) have been determined by combining synchrotron radiation X-ray powder diffraction and density functional theory calculations. The structures are in agreement with previous calorimetric and spectroscopic studies. The difference between rotatory phases (plastic crystals) I and II lies in isotropic rotations in the former and anisotropic rotations of the icosahedral clusters in the latter. Phase I is the cubic close packing (ccp) of rotating closo-molecules C₂B₁₀H₁₂ in the space group Fm3̅. Phase II is the ccp of rotating closo-molecules C₂B₁₀H₁₂ in the cubic space group Pa3̅. The preferred rotational axis in II varies with the isomer. The ordered phases III are orthorhombic (meta) or monoclinic (ortho and para) deformations of the cubic unit cell of the disordered phases I and II. The ordering in the phase III of the ortho-isomer carrying the biggest electrical dipole moment creates a twofold superstructure w.r.t. the cubic unit cell. The thermal polymorphism for C₂B₁₀H₁₂ and related metal salts can be explained by division of the cohesive intercluster interactions into two categories (i) dispersive cohesive interaction with additional Coulombic components in the metal salts and (ii) anisotropic local interaction resulting from nonuniform charge distribution around icosahedral clusters. The local interactions are averaged out by thermally activated cluster dynamics (rotations and rotational jumps) which effectively increase the symmetry of the cluster. The C₂B₁₀H₁₂ molecules resist at least as well as the CB₁₁H₁₂^- anion to the oxidation, and both clusters form easily a mixed compound. This allows designing solid electrolytes such as Na_x(CB₁₁H₁₂)_x(C₂B₁₀H₁₂)_1-x, where the cation content may be varied and the temperature of transition into the disordered conducting phase is decreased.

Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido-closo-borate Compounds

In this work, we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K₂B₁₂H₁₂ from Fm3̅ to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li₂B₁₀H₁₀ was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group–subgroup relationship, with the notable exception of the room-temperature phase of Li₂B₁₀H₁₀.

Herein, we study the pressure and temperature dependencies of alkali hydrido-closo-borates in extracting the mechanical properties of this class of compounds that have a promising future as solid electrolytes. In our research, we have discovered and determined two new high-pressure and high-temperature crystal structures.

Complex Metal Borohydrides: From Laboratory Oddities to Prime Candidates in Energy Storage Applications

Despite being the lightest element in the periodic table, hydrogen poses many risks regarding its production, storage, and transport, but it is also the one element promising pollution-free energy for the planet, energy reliability, and sustainability. Development of such novel materials conveying a hydrogen source face stringent scrutiny from both a scientific and a safety point of view: they are required to have a high hydrogen wt.% storage capacity, must store hydrogen in a safe manner (i.e., by chemically binding it), and should exhibit controlled, and preferably rapid, absorption-desorption kinetics. Even the most advanced composites today face the difficult task of overcoming the harsh re-hydrogenation conditions (elevated temperature, high hydrogen pressure). Traditionally, the most utilized materials have been RMH (reactive metal hydrides) and complex metal borohydrides M(BH4)x (M: main group or transition metal; x: valence of M), often along with metal amides or various additives serving as catalysts (Pd2+, Ti4+ etc.). Through destabilization (kinetic or thermodynamic), M(BH4)x can effectively lower their dehydrogenation enthalpy, providing for a faster reaction occurring at a lower temperature onset. The present review summarizes the recent scientific results on various metal borohydrides, aiming to present the current state-of-the-art on such hydrogen storage materials, while trying to analyze the pros and cons of each material regarding its thermodynamic and kinetic behavior in hydrogenation studies.

Enhanced Room-Temperature Ionic Conductivity of NaCB11H12 via High-Energy Mechanical Milling

The body-centered cubic (bcc) polymorph of NaCB₁₁H₁₂ has been stabilized at room temperature by high-energy mechanical milling. Temperature-dependent electrochemical impedance spectroscopy shows an optimum at 45-min milling time, leading to an rt conductivity of 4 mS cm^-1. Mechanical milling suppresses an order-disorder phase transition in the investigated temperature range. Nevertheless, two main regimes can be identified, with two clearly distinct activation energies. Powder X-ray diffraction and ²³Na solid-state NMR reveal two different Na⁺ environments, which are partially occupied, in the bcc polymorph. The increased number of available sodium sites w.r.t. ccp polymorph raises the configurational entropy of the bcc phase, contributing to a higher ionic conductivity. Mechanical treatment does not alter the oxidative stability of NaCB₁₁H₁₂. Electrochemical test on a symmetric cell (Na|NaCB₁₁H₁₂|Na) without control of the stack pressure provides a critical current density of 0.12 mA cm^-2, able to fully charge/discharge a 120 mA h g^-1 specific capacity positive electrode at the rate of C/2.

Primary Amine Nucleophilic Addition to Nitrilium Closo-Dodecaborate [B12H11NCCH3]-: A Simple and Effective Route to the New BNCT Drug Design

In the present work, a convenient and straightforward approach to the preparation of borylated amidines based on the closo-dodecaborate anion [B₁₂H₁₁NCCH₃NHR]-, R=H, Alk, Ar was developed. This method has two stages. A nitrile derivative of the general form [B₁₂H₁₁NCCH₃]^- was obtained, using a modified technique, in the first stage. On the second stage the resulting molecular system interacted with primary amines to form the target amidine products. This approach is characterised by a simple chemical apparatus, mild conditions and high yields of the final products. The mechanism of the addition of amine to the nitrile derivative of the closo-dodecaborate anion was studied, using quantum-chemical methods. The interaction between NH₃ and [B₁₂H₁₁NCCH₃]^- ammonia was chosen as an example. It was found that the structure of the transition state determines the stereo-selectivity of the process. A study of the biological properties of borylated amidine sodium salts indicated that the substances had low toxicity and could accumulate in cancer cells in significant amounts.

Boron Hydrogen Compounds: Hydrogen Storage and Battery Applications

About 25 years ago, Bogdanovic and Schwickardi (B. Bogdanovic, M. Schwickardi: J. Alloys Compd. 1-9, 253 (1997) discovered the catalyzed release of hydrogen from NaAlH4. This discovery stimulated a vast research effort on light hydrides as hydrogen storage materials, in particular boron hydrogen compounds. Mg(BH4)2, with a hydrogen content of 14.9 wt %, has been extensively studied, and recent results shed new light on intermediate species formed during dehydrogenation. The chemistry of B3H8-, which is an important intermediate between BH4- and B12H122-, is presented in detail. The discovery of high ionic conductivity in the high-temperature phases of LiBH4 and Na2B12H12 opened a new research direction. The high chemical and electrochemical stability of closo-hydroborates has stimulated new research for their applications in batteries. Very recently, an all-solid-state 4 V Na battery prototype using a Na4(CB11H12)2(B12H12) solid electrolyte has been demonstrated. In this review, we present the current knowledge of possible reaction pathways involved in the successive hydrogen release reactions from BH4- to B12H122-, and a discussion of relevant necessary properties for high-ionic-conduction materials.

Paradigms of frustration in superionic solid electrolytes

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion-cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Paradigms of frustration in superionic solid electrolytes

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion-cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Experimental and computational characterization of phase transitions in CsB3H8

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing B₃H₈^- (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH₄^- (3BH₄^-↔ B₃H₈^- + 2H₂), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsB₃H₈ prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsB₃H₈) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named β-CsB₃H₈), and then into a face-centered cubic phase, here named γ-CsB₃H₈, at 88 °C. These phases are not stable at room temperature thus requiring in situ measurements for their characterization. The phase transitions and decomposition pathway of CsB₃H₈ were studied with in situ synchrotron powder X-ray diffraction (SR-PXD), in situ and ex situ vibrational spectroscopies (Raman and FTIR), and differential-scanning calorimetry combined with thermo-gravimetric analysis (DSC-TGA). The structure determination was validated by vibrational spectroscopy analysis and modeling of the periodic structures by density functional methods. In γ-CsB₃H₈, a significant disorder in B₃H₈^- positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.

Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter

This Special Issue of Chemistry is dedicated to Dr. Howard D. Flack (1943–2017), a renowned crystallographer who transformed the way in which, by using single crystal X-ray diffraction, we are able to determine the absolute structure of a crystalline material, and thereby determine the absolute configuration of molecular species within the material [...]

Polymorphism of Calcium Decahydrido-closo-decaborate and Characterization of Its Hydrates

Metal closo-borates and their derivatives have shown promise in several fields of application from cancer therapy to solid-state electrolytes partly owing to their stability in aqueous solutions and high thermal stability. We report the synthesis and structural analysis of α- and β-CaB₁₀H₁₀, which are structurally and energetically similar, both showing a tetrahedral coordination of Ca²⁺ to four closo-borate cages. The main distinctions between the α- and β-polymorph are found in the crystal system (monoclinic or orthorhombic), topology (wurtzite or cag), and the degree of displacement of Ca²⁺ from the center of the coordination tetrahedron. Neutron vibrational spectroscopy measurements further revealed distinct perturbations in the cation-anion interactions arising from the different crystal structures. We also synthesized and structurally investigated five stoichiometric hydrates, CaB₁₀H₁₀·xH₂O, x = 1, 4, 5, 6, and 7, and discovered an order-disorder polymorphic transition, α- to β-CaB₁₀H₁₀·6H₂O. The hydrates reveal a rich structural diversity with ordered structures, CaB₁₀H₁₀·xH₂O, x = 1, 4, 5, 6, and 7, as well as disordered structures, x = 6 and 8. The latter allow for a continuum of compositions within 7-8 molecules of crystal water. The DFT-optimized experimental crystal structures reveal complex networks of three types of hydrogen interactions: dihydrogen bonds, B-H^δ-···^+δH-O; hydrogen-hydrogen interactions, B-H···H-B; and hydrogen bonds, O-H^δ+···^-δO-H. A rather short B-H···H-B (2.14 Å) interaction is observed for CaB₁₀H₁₀·5H₂O, which is locally stabilized by four hydrogen bonds.

Metal Borohydrides beyond Groups I and II: A Review

This review consists of a compilation of synthesis methods and several properties of borohydrides beyond Groups I and II, i.e., transition metals, main group, lanthanides, and actinides. The reported properties include crystal structure, decomposition temperature, ionic conductivity, photoluminescence, etc., when available. The compiled properties reflect the rich chemistry and possible borohydrides’ application in areas such as hydrogen storage, electronic devices that require an ionic conductor, catalysis, or photoluminescence. At the end of the review, two short but essential sections are included: a compilation of the decomposition temperature of all reported borohydrides versus the Pauling electronegativity of the cations, and a brief discussion of the possible reactions occurring during diborane emission, including some strategies to reduce this inconvenience, particularly for hydrogen storage purposes.

Theoretical Study of closo-Borate Anions [BnHn]2− (n = 5–12): Bonding, Atomic Charges, and Reactivity Analysis

This study has focused on the structure, bonding, and reactivity analysis of closo-borate anions [BnHn]2− (n = 5–12). Several descriptors of B–H interactions have been calculated. It has been found that the values of electron density and total energy at bond critical point are the most useful descriptors for investigation of B–H interactions. Using results from the descriptor analysis, one may conclude that orbital interactions in [BnHn]2− increase with increasing the boron cluster size. Several approaches to estimate atomic charges have been applied. Boron atoms in apical positions have more negative values of atomic charges as compared with atoms from equatorial positions. The mean values of boron and hydrogen atomic charges tend to be more positive with the increasing of boron cluster size. Global and local reactivity descriptors using conceptual density functional theory (DFT) theory have been calculated. Based on this theory, the closo-borate anions [BnHn]2− (n = 5–9) can be considered strong and moderate electrophiles, while the closo-borate anions [BnHn]2− (n = 10–12) can be considered marginal electrophiles. Fukui functions for electrophilic attack have been calculated. Fukui functions correlate well with atomic charges of the closo-borate anions. Boron atoms in apical positions have the most positive values of Fukui functions.

Fused 1,2-Diboraoxazoles Based on closo-Decaborate Anion-Novel Members of Diboroheterocycle Class

The novel members of the 1,2-diboraoxazoles family have been obtained. In the present work, we have carried out the intramolecular ring-closure reaction of borylated iminols of general type [B₁₀H₉N=C(OH)R]⁻ (R = Me, Et, ⁿPr, ⁱPr, ^tBu, Ph, 4-Cl-Ph). This process is conducted in mild conditions with 83–87% yields. The solid-state structures of two salts of 1,2-diboraoxazoles were additionally investigated by X-ray crystallography. In addition, the phenomena of bonding interactions in the 1,2-diboraoxazole cycles have been theoretically studied by the Quantum Theory of Atoms in Molecules analysis. Several local and integral topological properties of the electron density involved in these interactions have been computed.

Die unerwartete Kristallstruktur des Cäsium-Dodekahydro-Monocarba-closo-Dodekaborats Cs[CB11H12]

The cesium dodecahydro-monocarba-closo-dodecaborate Cs[CB11H12] crystallizes with an unexpected trigonal crystal structure having the lattice parameters a = 2094.73(3) and c = 1324.56(2) pm (c/a = 0.632) for Z = 18. The non-centrosymmetric space group R3 allows an ordering of the unsymmetric [CB11H12]– anions in a way that the least electronegative vertices of the pseudo-icosahedral cages avoid close proximity to the Cs+ cations. Hence there are channels at [0 0 z], [1/3 2/3 z] and [2/3 1/3 z], into which the C–H bonds of the [CB11H12]– units are pointing. There are two crystallographically independent Cs+ cations and [CB11H12]– anions present with unsurprising interatomic distances (d(C–B) = 166–181 pm, d(B–B) = 170–183 pm, d(B–H) = d(C–H) ≈ 110 pm) for the latter. Both Cs+ cations have contact to 18 hydrogen atoms (d(Cs–H) = 296–427 pm) stemming from six unevenly face-grafting [CB11H12]– anions, where only B–H bonds are involved. This fact is nicely reflected by IR and Raman spectroscopy. According to a 6/6 motif of the ions with highly distorted mutual octahedral coordination spheres of their centres of gravity, the crystal structure of Cs[CB11H12] follows roughly a rock salt-like arrangement. This becomes even more evident, when order-disorder transitions starting at T = 60 °C lead to more highly symmetrical structures with orientationally disordered [CB11H12]– anions.