Amminelithium Amidoborane Li(NH3)NH2BH3: A New Coordination Compound with Favorable Dehydrogenation Characteristics

The monoammoniate of lithium amidoborane, Li(NH₃)NH₂BH₃, was synthesized by treatment of LiNH₂BH₃ with ammonia at room temperature. This compound exists in the amorphous state at room temperature, but at ?20 °C crystallizes in the orthorhombic space group Pbca with lattice parameters of a=9.711(4), b=8.7027(5), c=7.1999(1) Å, and V=608.51 ų. The thermal decomposition behavior of this compound under argon and under ammonia was investigated. Through a series of experiments we have demonstrated that Li(NH₃)NH₂BH₃ is able to absorb/desorb ammonia reversibly at room temperature. In the temperature range of 40–70 °C, this compound showed favorable dehydrogenation characteristics. Specifically, under ammonia this material was able to release 3.0 equiv hydrogen (11.18 wt %) rapidly at 60 °C, which represents a significant advantage over LiNH₂BH₃. It has been found that the formation of the coordination bond between ammonia and Li⁺ in LiNH₂BH₃ plays a crucial role in promoting the combination of hydridic B? H bonds and protic N? H bonds, leading to dehydrogenation at low temperature.     

Comparative Computational Studies of Gaseous Alkali Metal Amidoboranes MNH2BH3 and their Carbon Analogs MC2H5 (M = Li – Cs): Formation and Unimolecular Hydrogen Evolution

Comparative computational studies of reaction mechanisms of formation and unimolecular hydrogen evolution from alkali metal amidoboranes MNH₂BH₃ and their carbon analogs MC₂H₅ (M = Li – Cs) were performed at the B3LYP/def2‐TZVPPD level of theory. Transition states (TS) for the consecutive dehydrogenation reactions were optimized. In contrast to endergonic dehydrogenation of carbon analogs, dehydrogenation reactions of alkali metal amidoboranes are exergonic at room temperature. The nature of the alkali metal does not significantly affect the thermodynamic characteristics and activation energies of unimolecular gas phase dehydrogenation reactions. The influence of the alkali metal is qualitatively similar for amidoboranes and their carbon analogs.     

Structures and Stability of Metal Amidoboranes （MAB）： Density Functional Calculations

Molecule geometry structures, frequencies, and energetic stabilities of ammonia borane （AB, NH3BH3 ） and metal amidoboranes （MAB, MNH2BH3）, formed by substituting H atom in AB with one of main group metal atoms, have been investigated by density-functional theory and optimized at the B3LYP levels with 6-311G＋＋ （3dr, 3pd） basic set. Their structural parameters and infrared spectrum characteristic peaks have been predicted, which should be the criterion of a successfully synthesized material. Several parameters such as binding energies, vibrational frequencies, and the energy gaps between the HOMO and the LUMO have been adopted to characterize and evaluate their structure stabilities. It is also found that the binding energies and HOMO-LUMO energy gaps of the MAB obviously change with the substitution of the atoms. MgAB has the lowest binding energy and is easier to decompose than any other substitutional structures under same conditions, while CaAB has the highest chemical activity.     

Complex beryllium amidoboranes: Structures,stability, and evaluation of their potential as hydrogen storage materials

Complex beryllium amidoboranes M_x [Be(NH₂BH₃)_{x +2}] (M = Li‐Cs, x = 1,2) have been computationally studied at M06‐2X/def2‐TZVPPD//B3LYP/def2‐TZVPPD level of theory. Compounds are predicted to be stable at room temperature but release H₂ on heating. Agostic Be…H B bonds play an important role in stabilization of oligomeric beryllium imidoboranes. Polymeric imidoborane, hydrogen, and ammonia are expected as major thermal decomposition products of complex beryllium amidoboranes. Ammonia evolution is predicted to proceed at slightly higher temperatures than hydrogen evolution. Based on thermodynamic analysis, Li[Be(NH₂BH₃)₃] and Li₂[Be(NH₂BH₃)₄] are the most perspective synthetic targets. Synthetic approaches to these potentially efficient hydrogen storage materials have been proposed. © 2016 Wiley Periodicals, Inc.     

Metal Amidoboranes: Superior Double‐Hydrogen‐Transfer Agents in the Reduction of Ketones and Imines

Metal amidoboranes (MABs), such as lithium amidoborane (LiAB), show superior ability in reducing ketones and imines directly into their corresponding secondary alcohols and amines, respectively, at room temperature with high conversion and yields. A mechanistic study indicates that the reduction proceeds through a double‐hydrogen‐transfer process. Both protic H(N) and hydridic H(B) protons in the amidoborane participate in the reaction. Theoretical investigations show that the first (and rate‐determining) step of the reduction reaction is the elimination of LiH from LiAB, followed by the transfer of H(Li) to the C site of the unsaturated bond.     

Formation and Unimolecular Dehydrogenation of Gaseous Alkaline‐earth Metal Amidoboranes M(NH2BH3)2 (M = Be – Ba): Comparative Computational Study

Formation of alkaline‐earth metal amidoboranes M(NH₂BH₃)₂ (M = Be, Mg, Ca, Sr, Ba) and unimolecular dehydrogenation reactions were computationally studied at the B3LYP/def2‐TZVPPD level of theory. Formation of M(NH₂BH₃)₂ from ammonia borane and MH₂ is exergonic, but subsequent unimolecular dehydrogenation reactions are endergonic at room temperature. In contrast to alkali metal amidoboranes, for M(NH₂BH₃)₂ the nature of M significantly affects their reactivity. Activation energies for the dehydrogenation of first and second hydrogen molecules decrease from Be to Ba. In case of Be compounds, intramolecular M ··· H–B contacts play an important role, whereas for heavier analogs such contacts are much less pronounced.