Iminoborane

Structure of tBuN=B-tBu (tBu = tert-butyl). The B-N bond distance is 126 pm.

Iminoboranes comprise a group of organoboron compounds with the formula RB=NR'. They are electronically related to acetylenes but are usually more reactive due to the polarity.

Structure and bonding

The parent iminoborane, HB=NH, is produced by the photolysis of H₃BNH₃. Bonding in iminoboranes can be described by two resonance structures:

$\mathrm{R}-\stackrel{\ominus }{\mathrm{B}}\equiv \stackrel{\oplus }{\mathrm{N}}-\mathrm{R}⟷\mathrm{R}-\mathrm{B}=\ddot{N}-\mathrm{R}$

The stability is dramatically affected by bulky substituents. One isolable iminoborane is Template:Chem2.

Comparison of bond lengths in simple boron-nitrogen hydrides

Molecule	Ammonia borane	Aminoborane	Iminoborane
Formula	BNH6	BNH4	BNH2
Class	amine-borane	aminoborane	iminoborane
Analogous hydrocarbon	ethane	ethylene	acetylene
Analogous hydrocarbon class	alkane	alkene	alkyne
Structure
Ball-and-stick model
Hybridisation of boron and nitrogen	sp3	sp2	sp
B-N bond length	1.658 Å	1.391 Å	1.238 Å
Proportion of B-N single bond	100%	84%	75%
B-H bond length	1.216 Å	1.195 Å
N-H bond length	1.014 Å	1.004 Å
Structure determination method	microwave spectroscopy	microwave spectroscopy	infrared spectroscopy

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 (Template:Chem names, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Template:Prime), cyclotetramers (Template:Chem names, 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.

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.

Some electron-rich iminoboranes form adducts with Lewis acids.

Cycloaddition

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

Coordination to transition metals

Like alkynes, iminoboranes bind transition metals.