Abstract

There are four classes of B–N compounds. Amine boranes, R₃B–NR¢₃, have the nitrogen atom which supplies both electrons in the B–N bond; these are isoelectronic with alkanes, R₃C–CR¢₃. Aminoboranes, , have a covalent bond between B and N. These are isoelectronic with alkenes, . Iminoboranes, , have a two-coordinate boron interacting with the nitrogen via a triple bond; these are isoelectronic with alkynes, . Borazines, , are cyclic compounds containing alternating tricoordinate boron and nitrogen atoms. Borazines are isoelectronic with benzene, C₆H₆. Amine–borane adducts have the general formula where , alkyl, etc, and , H, halogen, etc. These compounds, characterized by a coordinate covalent bond between boron and nitrogen, form a class of reducing agents having a broad spectrum of reduction potentials. Amine boranes are usually colorless, crystalline compounds which exhibit sharp melting points and thermal stability when pure. The aminoboranes are characterized by a normal covalent bond between boron and nitrogen in which an electron from each atom is shared. There exists a wide variety of aminoborane compounds; among those that have been thoroughly investigated are the monoaminoboranes X₂BNR₂, bisaminoboranes XB(NR₂)₂, and trisaminoboranes B(NR₂)₃. The substituents X and R may vary widely, but generally R is an alkyl or aryl group, or hydrogen, whereas X can represent a rather wide variety of atoms or groups. One of the more common routes for the synthesis of aminoboranes involves the aminolysis of the appropriate boron halide. Monoaminoboranes readily undergo association. The monomers are generally liquids or low melting solids, whereas the dimers and trimers are crystalline solids. Aminoboranes have been used as ligands in complexes with transition metals. The reduction of (alkylamino)haloboranes using hydride reagents can provide a convenient route to (alkylamino)boranes. Iminoboranes, SBNR, are isoelectronic with alkynes, XCCR. The polarity of the B–N bond makes iminoboranes much more reactive than the analogous SCCR species. Polar compounds add to iminoboranes, provided the addition proceeds more rapidly than oligomerization of . In an analogous fashion to the hydroboration reaction, a variety of boron-containing substrates react with iminoboranes. The largest and most extensively studied family of boron–nitrogen compounds is that of the borazines, characterized by a six-membered ring system containing alternating boron and nitrogen atoms. Because borazine is isoelectronic and isostructural with benzene, it has been called inorganic benzene. Borazines are liquids or crystalline solids depending on the substitution pattern. Most are sensitive to moisture and must be handled in an inert atmosphere. Borazines undergo addition reactions rather than electrophilic substitution reactions typical of benzene compounds. Borazine is slowly hydrolyzed by water at ambient or higher temperatures to boric acid. Miscellaneous reactions of borazines include photolysis and complex formation. Organic boron–nitrogen compounds have not found extensive usage, and therefore very few are manufactured on a large scale. Borazines, particularly polymeric compounds, have been extensively investigated as preceramic materials from which coatings and fibers of boron nitride can be produced upon thermolysis. B-Aryl and halogeno–amino borazines are reported to have seen use as fire retardants in cotton and nylon textiles. Other reported uses for borazines are as epoxy resin catalysts, polymerization inhibitors of unsaturated alcohols and esters, and catalysts for polymerization of alkenes.