New Syntheses and Structural Characterization of NH₃BH₂Cl and (BH₂NH₂)₃ and Thermal Decomposition Behavior of NH₃BH₂Cl

New convenient procedures for the preparation of ammonia monochloroborane (NH₃BH₂Cl) and cyclotriborazane [(BH₂NH₂)₃] are described. Crystal structures have been determined by single-crystal X-ray diffraction. Strong H···Cl···H bifurcated hydrogen bonding and weak N–H···H dihydrogen bonding are observed in the crystal structure of ammonia monochloroborane. When heated at 50 °C or under vacuum, ammonia monochloroborane decomposes to (NH₂BHCl)_<i>x</i>, which was characterized by NMR, elemental analysis, and powder X-ray diffraction. Redetermination of the crystal structure of cyclotriborazane at low temperature by single-crystal X-ray diffraction analysis provides accurate hydrogen positions. Similar to ammonia borane, cyclotriborazane shows extensive dihydrogen bonding of N–H···H and B–H···H bonds with H^δ+···H^δ− interactions in the range of 2.00–2.34 Å

Thermal Decomposition Behavior of Hydrated Magnesium Dodecahydrododecaborates

MgB₁₂H₁₂ is an intermediate in the hydrogen desorption and sorption processes of magnesium borohydride, which is an important candidate material for hydrogen storage. It is thus highly desirable to synthesize anhydrous MgB₁₂H₁₂ in order to study its properties and its role in the hydrogenation and dehydrogenation of magnesium borohydride. Contrary to the literature claim, we find that anhydrous MgB₁₂H₁₂ cannot be obtained from simple thermal decomposition of Mg(H₂O)₆B₁₂H₁₂·6H₂O (<b>1</b>) which has different thermal decomposition behavior from that of most hydrated alkali and alkaline earth salts of dodecahydrododecaborates. Thermal decomposition of <b>1</b> involves both dehydration and dehydrogenation processes in three steps, resulting in the formation of complexes Mg(H₂O)₆B₁₂H₁₂ (<b>2</b>), Mg(H₂O)₃B₁₂H₁₂ (<b>3</b>), and Mg(μ-OH)_<i>x</i>B₁₂H_{12−<i>x</i>} (<b>4</b>) that were characterized by XRD, IR, and ¹¹B NMR. Dehydrogenation was also confirmed by both the generation of hydrogen observed in TPD-MS spectra and the formation of polyhydroxylated complexes

Unusual Cationic Tris(Dimethylsulfide)-Substituted <i>closo</i>-Boranes: Preparation and Characterization of [1,7,9-(Me₂S)₃-B₁₂H₉] BF₄ and [1,2,10-(Me₂S)₃-B₁₀H₇] BF₄

Rational syntheses of trisubstituted sulfur-bearing closo-boranes are presented. In the development of these syntheses unusual cationic <i>closo</i>-boranes [1,7,9-(Me₂S)₃-B₁₂H₉]⁺ (<b>3</b>) and [1,2,10-(Me₂S)₃-B₁₀H₇]⁺ (<b>4</b>) have been identified. These were initially recognized to be intermediates in the formation of the neutral trisubstituted species 1,7-(Me₂S)₂-9-(MeS)-B₁₂H₉ (<b>1</b>) and 1,10-(Me₂S)₂-2-(MeS)-B₁₀H₇ (<b>2</b>), respectively. Stable tetrafluoroborate salts were prepared and isolated, and their structures are presented. They are believed to represent the first structural determinations of cationic borane clusters of any type

Elucidation of the Formation Mechanisms of the Octahydrotriborate Anion (B₃H₈^–) through the Nucleophilicity of the B–H Bond

Boron compounds are well-known electrophiles. Much less known are their nucleophilic properties. By recognition of the nucleophilicity of the B–H bond, the formation mechanism of octahydrotriborate (B₃H₈^–) was elucidated on the bases of both experimental and computational investigations. Two possible routes from the reaction of BH₄^– and THF·BH₃ to B₃H₈^– were proposed, both involving the B₂H₆ and BH₄^– intermediates. The two pathways consist of a set of complicated intermediates, which can convert to each other reversibly at room temperature and can be represented by a reaction circle. Only under reflux can the B₂H₆ and BH₄^– intermediates be converted to B₂H₅^– and BH₃(H₂) via a high energy barrier, from which H₂ elimination occurs to yield the B₃H₈^– final product. The formation of B₂H₆ from THF·BH₃ by nucleophilic substitution of the B–H bond was captured and identified, and the reaction of B₂H₆ with BH₄^– to produce B₃H₈^– was confirmed experimentally. On the bases of the formation mechanisms of B₃H₈^–, we have developed a facile synthetic method for MB₃H₈ (M = Li and Na) in high yields by directly reacting the corresponding MBH₄ salts with THF·BH₃. In the new synthetic method for MB₃H₈, no electron carriers are needed, allowing convenient preparation of MB₃H₈ in large scales and paving the way for their wide applications

Formation Mechanisms, Structure, Solution Behavior, and Reactivity of Aminodiborane

A facile synthesis of cyclic aminodiborane (NH₂B₂H₅, ADB) from ammonia borane (NH₃·BH₃, AB) and THF·BH₃ has made it possible to determine its important characteristics. Ammonia diborane (NH₃BH₂(μ-H)­BH₃, AaDB) and aminoborane (NH₂BH₂, AoB) were identified as key intermediates in the formation of ADB. Elimination of molecular hydrogen occurred from an ion pair, [H₂B­(NH₃) (THF)]⁺[BH₄]⁻. Protic-hydridic hydrogen scrambling was proved on the basis of analysis of the molecular hydrogen products, ADB and other reagents through ²H NMR and MS, and it was proposed that the scrambling occurred as the ion pair reversibly formed a BH₅-like intermediate, [(THF)­BH₂NH₂]­(η²-H₂)­BH₃. Loss of molecular hydrogen from the ion pair led to the formation of AoB, most of which was trapped by BH₃ to form ADB with a small amount oligomerizing to (NH₂BH₂)_<i>n</i>. Theoretical calculations showed the thermodynamic feasibility of the proposed intermediates and the activation processes. The structure of the ADB·THF complex was found from X-ray single crystal analysis to be a three-dimensional array of zigzag chains of ADB and THF, maintained by hydrogen and dihydrogen bonding. Room temperature exchange of terminal and bridge hydrogens in ADB was observed in THF solution, while such exchange was not observed in diethyl ether or toluene. Both experimental and theoretical results confirm that the B–H–B bridge in ADB is stronger than that in diborane (B₂H₆, DB). The B–H–B bridge is opened when ADB and NaH react to form sodium aminodiboronate, Na­[NH₂(BH₃)₂]. The structure of the sodium salt as its 18-crown-6 ether adduct was determined by X-ray single crystal analysis

Controllable Synthesis and Catalytic Performance of Nanocrystals of Rare-Earth-Polyoxometalates

Large-scale isolation of nanocrystals of rare-earth-polyoxometalates (RE-POMs) catalysts is important in fundamental research and applications. Here, we synthesized a family of monomeric RE-POMs by the self-assembly of Ta/W mixed-addendum POM {P₂W₁₅Ta₃O₆₂} and rare-earth (RE) ions. These RE-POMs with molecular formulas of [RE­(H₂O)₇]₃­P₂W₁₅Ta₃O₆₂­·<i>n</i>H₂O (RE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) are all electroneutral molecular clusters, insoluble in water and common organic solvents. The electronic structures, electrochemical properties, and catalytic activities of them have been investigated by experimental and computational methods. In particular, based on a mild and controllable synthetic process, a convenient and controllable approach to prepare nanocrystals and self-organized aggregates of these monomers has been developed. They exhibit remarkable heterogeneous catalytic activity for cyanosilylation. Both the increased Lewis acid strength of RE in the title compounds, as indicated by theoretical calculations, and the decreased particle size contribute to their high catalytic performances