Homopolar Dihydrogen Bonding in Alkali-Metal Amidoboranes and Its Implications for Hydrogen Storage

Efficient synthesis of primary and secondary amides via reacting esters with alkali metal amidoboranes

Amides are one of the most important organic compounds that are widely applied in medicine, biochemistry, and materials science. To find an efficient synthetic method of amides is a challenge for organic chemistry. We report here a facile synthesis method of primary and secondary amides through a direct amidation of esters with sodium amidoboranes (NaNHRBH₃, R = H, Me), at room temperature without using catalysts and other reagents. This process is rapid and chemoselective, and features quantitative conversion and wide applicability for esters tolerating different functional groups. The experimental and theoretical studies reveal a reaction mechanism with nucleophilic addition followed by a swift proton transfer-induced elimination reaction.

Understanding the Nature and Properties of Hydrogen-Hydrogen Bonds: The Stability of a Bulky Phosphatetrahedrane as a Case Study

Recently, the first mixed C/P phosphatetrahedranes (^tBuC)₃P and (^tBuCP)₂ were reported. Unlike (^tBuCP)₂, (^tBuC)₃P exhibits remarkable thermal stability, which can be partially attributed to a network of nine hydrogen-hydrogen bonds (HHBs) localized between the tert-butyl substituents. The stabilizing contribution arising from this network of HHBs was obtained from local energy decomposition (LED) analysis calculated at the domain-based local pair natural orbital CCSD(T) (DLPNO-CCSD(T)) level of theory. These calculations suggest that each HHB contributes approximately -0.7 kcal/mol of stabilization; however, the net stabilization energy likely lies between -0.25 and -0.5 kcal/mol because of steric repulsion. Spatial analysis of the London dispersion energy via a dispersion interaction density (DID) plot reveals that the DID surface is localized at key C-H groups involved in HHBs, consistent with London dispersion interactions predominantly arising from HHBs. In addition, we present a computed mechanism that supports a phosphinidenoid species as a key reaction intermediate in the synthesis of (^tBuC)₃P.

Exploring the homopolar dehydrocoupling of ammonia borane by solid-state multinuclear NMR spectroscopy

Solid-state 1H-14NOT HMQC, 11B MQMAS and 1H-11B HETCOR NMR experiments are used to explore the role of homopolar B-B interaction in the thermal dehydrogenation of pure and supported ammonia borane, which is considered as one of the most promising hydrogen storage materials. This work also addresses the subtlety of the homopolar interactions in amine borane compounds, and how they differ from their heteropolar counterparts.

Experimental and theoretical evidence of dihydrogen bonds in lithium amidoborane

In situ high-pressure synchrotron X-ray diffraction, Raman scattering, and complementary first-principles calculations have revealed that structural and spectroscopic properties of lithium amidoborane compound are largely determined by multiple heteropolar dihydrogen bonds. The crystal structure of the compound is stabilized by dimeric complexes, wherein molecular ions bind together by intermolecular dihydrogen bonds of unconventional type. This strong intermolecular coupling determines stable character of the crystal structure in the pressure range up to ~ 30 GPa and is spectroscopically manifested by pronounced changes related to molecular vibrations of the amino group: the splitting of stretching modes, the anomalous behavior of wagging modes as well as Fermi resonance due to vibrational coupling of bending and stretching modes, significantly enhanced above 10 GPa. Unconventional nature of dihydrogen bonds is confirmed by the frequency increase, blueshift, of NH stretching modes with pressure. A role of certain hydrogen mediated interactions in the process of dehydrogenation of ammonia borane and its alkali metal derivatives is speculated. Findings presented here call for reconsideration of hydrogen release mechanism from alkali metal ammonia borane derivatives. The work makes significant contribution towards establishing the general theory of ubiquitous and versatile hydrogen mediated interactions.

Synthesis, X-ray characterization and theoretical study of 3a,6:7,9a-diepoxybenzo[de]isoquinoline derivatives: on the importance of F⋯O interactions

This manuscript reports the synthesis, X-ray characterization and theoretical study of 3a,6:7,9a-diepoxybenzo[de]isoquinoline derivatives focusing on the importance of F···O interactions.

Solid-state hydrogen rich boron–nitrogen compounds for energy storage

Mechanistic studies of hydrogenation and dehydrogenation of boron and nitrogen containing compounds in the solid-state and its applications are reviewed.

Noncovalent Interactions between Molecular Hydrogen and the Alkali Fluorides: H-H···F-M (M = Li, Na, K, Rb, Cs). High Level Theoretical Predictions and SAPT Analysis

Various types of hydrogen bonds have been recognized during the past century. In this research, a new type of noncovalent interaction, the dipole-induced hydrogen bond formed between a hydrogen molecule and an alkali halide, H-H···F-M, is studied. Proposed by Zhang and co-workers ( Phys. Chem. Chem. Phys. 2015, 17, 20361), these systems are extensively investigated initially using the "gold standard" CCSD(T) method in conjunction with augmented correlation-consistent polarized core-valence basis sets up to quadruple-ζ. The full triple excitations CCSDT method has been used to further refine the energies. Several properties including geometries, bond energies, vibrarional frequencies, charge distributions, and dipole moments have been reported. The earlier Zhang research considered only the linear H-H···F-M structures. However, we find these linear stationary points to be separated by very small barriers from the much lower lying bent C _s structures. The CCSDT/aug-cc-pCVQZ(-PP) method predicts the dissociation energies for bent H-H···F-M (M = Li, Na, K, Rb, Cs) are 2.76, 2.96, 3.00, 2.89, and 2.49 kcal mol^-1, respectively, suggesting that the H···F hydrogen bond becomes gradually stronger when alkali metal M goes down the periodic table from Li to K but becomes slightly weaker for Rb and even more for Cs. This Li < Na < K > Rb > Cs order is consistent with that for the dipole moments for the isolated MF (M = Li, Na, K, Rb, Cs) diatomics. Symmetry adapted perturbation theory (SAPT) is used to understand these unusual noncovalent interactions.

Supported ammonia borane decomposition through enhanced homopolar B–B coupling

The thermolytic decomposition of ammonia borane (AB) is known to proceed through the polymeric coupling reaction between –BH₃ and –NH₃ sites of multiple ammonia borane molecules, which results in the release of hydrogen and other by-products, e.g., ammonia, diborane and borazine.

Attractive PHHP interactions revealed by state-of-the-art ab initio calculations

We report in this work a combined structural and state-of-the-art computational study of homopolar P-HH-P intermolecular contacts. Database surveys have shown the abundance of such surprisingly unexplored contacts, which are usually accompanied by other weak interactions in the solid state. By means of a detailed theoretical study utilizing SAPT(DFT), MP2, SCS-MP2, MP2C and CCSD(T) methods and both aug-cc-pVXZ and aug-cc-pCVXZ (X = D, T, Q, 5) basis sets as well as extrapolation to the CBS limit, we have shown that P-HH-P contacts are indeed attractive and considerably strong. SAPT(DFT) calculations have revealed the dispersive nature of the P-HH-P interaction with only minor contribution of the inductive term, whereas the first-order electrostatic term is clearly overbalanced by the first-order exchange energy. In general the computed interaction energies follow the trend: E ≈ E < E < E. Our results have also shown that the aug-cc-pVDZ (or aug-cc-pCVDZ) basis set is not yet well balanced and that the second-order dispersion energy term is the slowest converging among all SAPT(DFT) energy components. Compared to aug-cc-pVXZ basis sets, their core-correlation counterparts have a modest influence on all supermolecular interaction energies and a negligible influence on both the SAPT(DFT) interaction energy and its components.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

Synthesis of Ammonia Borane Nanoparticles and the Diammoniate of Diborane by Direct Combination of Diborane and Ammonia

Pure nanoparticle ammonia borane (NH3 BH3 , AB) was first prepared through a solvent-free, ambient-temperature gas-phase combination of B2 H6 with NH3 . The prepared AB nanoparticle exhibits improved dehydrogenation behavior giving 13.6 wt. % H2 at the temperature range of 80-175 °C without severe foaming. Ammonia diborane (NH3 BH2 (μ-H)BH3 , AaDB) is proposed as the intermediate in the reaction of B2 H6 with NH3 based on theoretical studies. This method can also be used to prepare pure diammoniate of diborane ([H2 B(NH3 )2 ][BH4 ], DADB) by adjusting the ratio and concentration of B2 H6 to NH3 .

Mechanisms of Hydrogen Generation from Tetrameric Clusters of Lithium Amidoborane

The first-principles study of dehydrogenation mechanism of tetrameric clusters of lithium amidoborane LiNH2BH3, (LiAB)4, is presented. The choice of tetramer is based on the suspicion that dimeric cluster models used in previous theoretical studies are too small to capture the essence of the reaction. Dehydrogenation pathways starting from three isomers of (LiAB)4 tetramers were explored by applying the artificial force induced reaction (AFIR) method at the M06 level of theory. All obtained reaction pathways feature initial dimerization of two LiAB molecules in the tetramer. Formation of intermediates containing the Li3H moiety is a very characteristic feature of all pathways. In the succeeding rate-limiting step of the release of H2 molecule, a hydridic H atom of the Li3H moiety activates a protic H atom of the NH2 group with formation of the Li2H2 moiety in transition state. The most kinetically favorable pathway has the activation enthalpy of 26.6 kcal mol(-1), substantially lower than that found for dimeric cluster. The obtained results suggest that only three LiAB molecules directly participate in the elementary reactions.

s-Block amidoboranes: syntheses, structures, reactivity and applications

The highly versatile amidoborane compounds of the group 1 and 2 metals are reviewed, with an emphasis on their synthesis, structures and reactivity.

Tunable dipole induced hydrogen bonds between a hydrogen molecule and alkali halides

Hydrogen bonding (HB) systems are known to be X-H∙∙∙Y type complexes, which are called conventional HB systems if the X and Y are strongly electronegative atoms such as O, N and halides or unconventional systems if the X is replaced by C. In this study, we devise a new dipole-induced HB that is formed between a hydrogen molecule and an alkali halide using ab initio calculations. The HB is depicted as H-H∙∙∙Y-M, in which MY are alkali halides. Analysis of the possible structures and properties of the proposed compounds, including their geometries, frequencies, bond strength, and natural charge distribution, as well as a topological analysis of electronic density, shows that the large dipole moment of the Y-M molecule is responsible for the generation of the proposed HB. We also find that the strength of HB can be tuned by adopting MY with various polarities. We hope that our findings could provide a new insight into HB.

Supramolecular interactions in boron hydrides: how non-classical bonding directs their crystal architecture

The nature and driving forces behind the formation of dihydrogen bonds are explored for several textbook boranes, and their influence on the crystalline architecture is revealed.

Homopolar dihydrogen bonding in main group hydrides: discovery, consequences, and applications

This perspective describes the recent discovery and investigation of homopolar dihydrogen bonding, and focuses on the identification and characterisation of hydride–hydride interactions in compounds of the main group elements.

Metallization and superconductivity of BeH2 under high pressure

Pressure-induced metallization and potential superconductivity of BeH2 has been a topic of interest. In the present study, we extensively explored the crystal structures of BeH2 in a wide pressure range of 0-300 GPa using an unbiased structure searching method coupled with first-principles density functional calculations. A series of pressure-induced structural transformations are predicted for BeH2, as Ibam (α phase) → P-3m1 (phase II) → R-3m (phase III) → Cmcm (phase IV). Calculated pressures of phase transition are 25, 140, and 202 GPa, respectively. The phase II is isostructural to the well-known 1T structure of transition metal dichalcogenides, which is composed of covalent bonded BeH2 slabs stacked along the perpendicular direction by van der Waals forces. The phase III is constructed by the same BeH2 slabs, but differs from the phase II in the stacking sequence. The α phase, phase II, and phase III all have insulating electronic states while their band gaps decrease as pressure increases. We predicted that BeH2 reaches a metallic state by a III → IV phase transition, instead of a direct band gap closure in phase III. The phase IV has a three-dimensional extended Be-H network formed by edge-sharing BeH8 polyhedrons with delocalized electrons. Electron-phonon coupling calculations implemented using linear response theory on the metallic BeH2 predict a large electron-phonon coupling parameter of 0.63, leading to an estimation of superconducting transition temperature (Tc) of ∼38 K at 250 GPa.

Desorption of hydrogen from light metal hydrides: concerted electronic rearrangement and role of H⋯H interactions

The synergic interplay of H⋯H and M–H interactions in the evolution of H₂ from binary metal hydrides is revealed.

The roles of dihydrogen bonds in amine borane chemistry

A dihydrogen bond (DHB) is an electrostatic interaction between a protonic hydrogen and a hydridic hydrogen. Over the past two decades, researchers have made significant progress in the identification and characterization of DHBs and their properties. In comparison with conventional hydrogen bonds (HBs), which have been widely used in catalysis, molecular recognition, crystal engineering, and supramolecular synthesis, chemists have only applied DHBs in very limited ways. Considering that DHBs and conventional HBs have comparable strength, DHBs could be more widely applied in chemistry. Over the past several years, we have explored the impact of DHBs on amine borane chemistry and the syntheses and characterization of amine boranes and ammoniated metal borohydrides for hydrogen storage. Through systematic computational and experimental investigations, we found that DHBs play a dominant role in dictating the reaction pathways (and thus different products) of amine boranes where oppositely charged hydrogens coexist for DHB formation. Through careful experiments, we observed, for the first time, a long-postulated reaction intermediate, ammonia diborane (AaDB), whose behavior is essential to mechanistic understanding of the formation of the diammoniate of diborane (DADB) in the reaction of ammonia (NH3) with tetrahydrofuran borane (THF·BH3). The formation of DADB has puzzled the boron chemistry community for decades. Mechanistic insight enabled us to develop facile syntheses of aminodiborane (ADB), ammonia borane (AB), DADB, and an inorganic butane analog NH3BH2NH2BH3 (DDAB). Our examples, together with those in the literature, reinforce the fact that DHB formation and subsequent molecular hydrogen elimination are a viable approach for creating new covalent bonds and synthesizing new materials. We also review the strong effects of DHBs on the stability of conformers and the hydrogen desorption temperatures of boron-nitrogen compounds. We hope that this Account will encourage further applications of DHBs in molecular recognition, host-guest chemistry, crystal engineering, supramolecular chemistry, molecular self-assembly, chemical kinetics, and the syntheses of new advanced materials.