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.[2][3]
 Iminoborane (parent compound) | |
| Names | |
|---|---|
| Other names Boraneimine | |
| Identifiers | |
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3D model (JSmol) |
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PubChem CID |
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| Properties | |
| BH2N | |
| Molar mass | 26.83 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
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Structure and bonding
The parent iminoborane, HB=NH, is produced by the photolysis of H3BNH3.[4][5][6] Bonding in iminoboranes can be described by two resonance structures:[7]
The stability is dramatically affected by bulky substituents. One isolable iminoborane is (CH3)3C−B−≡N+−C(CH3)3.[1]
| Molecule | Ammonia borane[8] | Aminoborane[9] | Iminoborane[10] |
|---|---|---|---|
| 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:[11]
- (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:[12]
- 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-diaza-2,4-diboretidines,[1] Di[13]), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr′[14]), cyclotetramers (octahydro-1,3,5,7-tetraza-2,4,6,8-tetraborocines, Te[15]), and polymers (polyiminoboranes, Po); which are shown below.[16] Which product is dominant depends on the structures of reactants and the reaction conditions. Some of the products can be interconverted.[17]
Addition reactions
The addition of protic agents is fast and quantitive.[18] 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.[19]
Cycloaddition
The typical [2+3]-cycloaddition is the addition of B≡N and RN3 to give a BN4 ring.[1] 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.
