vilrado lensar carley

[Timmy Tatel]

A borane is a compound with the formula BxHy or a related anion. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.[1]

The development of the chemistry of boranes led to innovations in synthetic methods as well as structure and bonding. First, new synthetic techniques were required to handle diborane and many of its derivatives, which are both pyrophoric and volatile. Alfred Stock invented the glass vacuum line for this purpose.[2]

The structure of diborane was correctly predicted in 1943 many years after its discovery.[3] The structures of the boron hydride clusters were determined beginning in 1948 with the characterization of decaborane. William Lipscomb was awarded the Nobel prize in Chemistry in 1976 for this and many subsequent crystallographic investigations. These investigations revealed the prevalence of deltahedral structures, i.e., networks of triangular arrays of BH centers.

The bonding of the clusters ushered in Polyhedral skeletal electron pair theory and Wade's rules, which can be used to predict the structures of boranes.[4] These rules were found to describe structures of many cluster compounds.

Interest in boranes increased during World War II due to the potential of uranium borohydride for enrichment of the uranium isotopes and as a source of hydrogen for inflating weather balloons. In the US, a team led by Schlesinger developed the basic chemistry of the anionic boron hydrides and the related aluminium hydrides. Schlesinger's work laid the foundation for a host of boron hydride reagents for organic synthesis, most of which were developed by his student Herbert C. Brown. Borane-based reagents are now widely used in organic synthesis. Brown was awarded the Nobel prize in Chemistry in 1979 for this work.[5]

The International Union of Pure and Applied Chemistry rules for systematic naming is based on a prefix denoting a class of compound, followed by the number of boron atoms and finally the number of hydrogen atoms in parentheses. Various details can be omitted if there is no ambiguity about the meaning, for example, if only one structural type is possible. Some examples of the structures are shown below.

The hydrogen count is specified first followed by the boron count. The -ate suffix is applied with anions. The ionic charge value is included in the chemical formula but not as part of the systematic name.

Although relatively rare, several multi-cluster boranes have been characterized. For example, reaction of a borane cluster with B2H6 (as a source of BH3) can lead to the formation of a conjuncto-borane species in which borane cluster sub-units are joined by the sharing of boron atoms.[10]

Other conjuncto-boranes, where the sub-units are joined by a B-B bond, can be made by ultra violet irradiation of nido-boranes. Some B-B coupled conjuncto-boranes can be produced using PtBr2 as catalyst.[11]

For the boron hydride chemist, one of the most important reactions is the building up process by which smaller boron hydride clusters add borane to give larger clusters. This approach also applies to the synthesis of metallaboranes,

Reminiscent of the behavior of diborane and its adducts, higher boranes participate in hydroboration. When boron hydrides add an alkyne, the carbon becomes incorporated into the cluster, producing carboranes, e.g. C2B10H12.[17]

Boranes have a high specific energy of combustion compared to hydrocarbons, making them potentially attractive as fuels or igniters. Intense research was carried out in the 1950s into their use as jet fuel additives, but the effort did not lead to practical results.

This document discusses metal cluster higher boranes. It begins with an introduction to boranes and their synthesis. It then describes the different types of bonds found in higher boranes, including terminal, direct, bridging, and triply bridging bonds. Specific examples of higher borane structures are examined, including diborane B2H6, tetraborane B4H10, and pentaborane B5H9. Finally, the document classifies higher boranes into closo, nido, and arachno boranes based on their skeletal structures and electron counts, according to Wade's rules. Methods for synthesizing higher boranes are also briefly mentioned.Read less

Hypergolic hybrid rockets have the potential of providing systems that are simple, reliable, have high performance, and allow for energy management. Such a propulsion system can be applied to fields that need a single tactical motor with flexible mission requirements of either high speed to target or extended loitering. They also provide the possibility for alternative fast response dynamic altitude control systems if ignition delays are sufficiently short. ^ Amines are the traditional fuel of choice when selecting a hypergolic combination as these tend to react readily with both nitric acid and dinitrogen tertroxide based oxidizers. It has been found that the addition of a borane adduct to an amine fuel tends to reduce the ignition delay by up to an order of magnitude with white fuming nitric acid (WFNA). The borane addition has resulted in fuels with very short ignition delays between 2-10 ms - the fastest times for an amine based fuel reacting with nitric acid based oxidizers. The incorporation of these amine-boranes, specifically ethylenediamine bisborane (EDBB), into various fuel binders has also been found to result in ignition delays between 3-10 ms - the fastest times again for amine based fuels. ^ It was found that the addition of a borane to an amine increased theoretical performance of the amine resulting in high performance fuels. The amine-borane/fuel binder combinations also produced higher theoretical performance values than previously used hypergolic hybrid rockets. Some of the theoretical values are on par or higher than the current toxic liquid hypergolic fuels, making amine boranes an attractive replacement. The higher performing amine-borane/fuel binder combinations also have higher performance values than the traditional rocket fuels, excluding liquid hydrogen. Thus, amine-borane based fuels have the potential to influence various area in the rocket field. ^ An EDBB/ferrocene/epoxy fuel was tested in a hypergolic hybrid with pure nitric acid as the oxidizer. Hypergolic ignition occurred repeatably and with short combustor pressurization times of under 100 ms. The regression rate of the fuel exhibited never before observed high pressure dependence regression rates. The presence of a foam like layer on the fuel surface provides an adequate explanation for the observed combustion behavior with a calculated regression rate that depends on pressure raised to the 2nd power. Extrapolation of this theory indicates that amine-borane based fuels could produce high regression rate fuels.

P.V. Ramachandran, professor of organic chemistry in the Department of Chemistry, and graduate assistant Ameya S. Kulkarni have discovered a way to produce amine-boranes in an open-air environment using cheaper and more plentiful chemicals that have not been used before. The result could provide a safer, cheaper and more plentiful compound, including those not made before, that has a multitude of uses.

The process and one of their applications in organic chemistry is detailed in separate articles by Ramachandran, which appear in a recent issue of Chemical Communications. Links to the articles are available here and here.

Diborane is a pyrophoric gas seldom used as is due to its toxicity and safety issues. Amines, on the other hand, are compounds consisting of a nitrogen atom, Ramachandran said. When amines are combined with borane, the result is non-toxic, air- and moisture-stable amine-boranes.

According to a 2006 report from the U.S. Department of Energy, amine-borane complexes have great potential as a component in fuel sources due to their high hydrogen content. Hydrogen can be used in fuel cells to power electric vehicles and other electronic devices and can be used to propel spacecraft.

The new method of producing the compound removes many of the dangerous forms of borane and substitutes them with sodium borohydride and sodium bicarbonate (or baking soda), which are less dangerous. That allows water to be used as a reagent, meaning the process can be done in an open-air environment, Ramachandran said.

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Here we present the synthesis, structures and polymorphic transitions of Ag2B10H10 and Ag2B12H12 along with fascinating properties such as semiconductivity, extreme room temperature ion conductivities and metallic filament growth.

These new silver closo-boranes are insoluble in water and can be handled both in water or air without hydrate formation or decomposition, unlike most other metal boranes, which are often hygroscopic or reactive6,34.

M.P. acknowledges financial support from The Danish Council for Independent Research for DFF Mobility 1325-00072. We are grateful to the Carlsberg Foundation and Danish Council for Independent Research, DFF 4181-00462 (HyNanoBorN), The Innovation Fund Denmark (HyFill-Fast), The European Marie Curie Actions under ECOSTORE grant agreement n 607040 and the Danish National Research Foundation, Center for Materials Crystallography (DNRF93). Parts of this research were carried out at the light source Petra III at DESY, a member of the Helmholtz Association (HGF), and at beamline I711, in the research laboratory MAXIV, MAX II synchrotron, Lund, Sweden. We are grateful to the Villum Foundation for funding the FEI Talos TEM.

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