Iminoboranes comprise a group of

organoboron compound Organoborane or organoboron compounds are chemical compounds of boron and carbon that are Organic compound, organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compoun ...

s with the formula RB=NR'. They are electronically related to

acetylenes \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no ...

but are usually more reactive due to the polarity.

Structure and bonding

The parent iminoborane, HB=NH, is produced by the

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

of H₃BNH₃. Bonding in iminoboranes can be described by two

resonance structures In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

: :R-\overset\overset-R <-> R-B=\ddot-R The stability is dramatically affected by bulky substituents. One isolable iminoborane is (''tert''-Bu)B=N(''tert''-Bu).

Synthesis

Elimination of fluoro- or chlorosilanes provides a well-tested route. Bulky substituents such as (Me₃Si)₃Si stabilize the iminoborane with respect to oligomerization: :(Me₃Si)₃SiB(F)-N(SiMe₃)₂ → (Me₃Si)₃Si-B=N-SiMe₃ + F-SiMe₃ Thermal decomposition of azidoboranes induces migration of R from boron to the nascent nitrene gives iminoboranes: :R₂B-N₃ → RB=NR + N₂

Reactivity

Oligomerization

Iminoboranes tend to oligomerize, often forming cyclic derivatives. Preventing this reaction is the purpose of bulky substituents. Five types of oligomerization product are produced: cyclodimers (1,3,2,4-diazadiboretidines, Di), cyclotrimers ( borazines, Tr), bicyclotrimers ( Dewar borazines, Tr′), cyclotetramers (octahydro-1,3,5,7-tetraza-2,4,6,8-tetraborocines, Te), and polymers (polyiminoboranes, Po); which are shown below. Which product is dominant depends on the structures of reactants and the reaction conditions. Some of the products can be interconverted. Figure product of iminboboranes oligomerization-1

Addition reactions

The addition of protic agents is fast and quantitive. Boration reaction of iminoboranes is the addition of B-X single bond to B≡N, where -X can be -Cl (chloro-boration), -N₃ (azido-boration), -SR (thio-boration), -NR2 (amino-boration) and R (alkyl-boration). One of these reactions are illustrated here. Chloro-boration of iminoborane

Some electron-rich iminoboranes form adducts with Lewis acids. Lewis_acid_addition_of_iminoboranes

Cycloaddition

The typical +3cycloaddition is the addition of B≡N and RN₃ to give a BN₄ ring. One of the widely investigated +2cycloadditions is the reaction of aldehydes and ketones.

Coordination to transition metals

Like alkynes, iminoboranes bind transition metals. Coordination of iminoboranes to metals

References

{{reflist Boranes Amines Boron–nitrogen compounds