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Iminoborane

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Class of chemical compounds
Iminoborane

Iminoborane (parent compound)
Names
Other names Boraneimine
Identifiers
CAS Number
3D model (JSmol)
PubChem CID
InChI
  • R,R'=H: InChI=1S/BH2N/c1-2/h1-2HKey: LNLSXDSWJBUPHM-UHFFFAOYSA-N
SMILES
  • R,R'=H: B=N
Properties
Chemical formula BH2N
Molar mass 26.83 g·mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references
Chemical compound
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:

R − B ⊖ ≡ N ⊕ − R ⟷ R − B = N ¨ − R {\displaystyle {\ce {R-{\overset {\ominus }{B}}{\equiv }{\overset {\oplus }{N}}-R<->R-B={\ddot {N}}-R}}}

The stability is dramatically affected by bulky substituents. One isolable iminoborane is (CH3)3C−B≡N−C(CH3)3.

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 sp sp 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 (1,3-diaza-2,4-diboretidines, 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.

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 -cycloaddition is the addition of B≡N and RN3 to give a BN4 ring. One of the widely investigated -cycloadditions is the reaction of aldehydes and ketones.

Coordination to transition metals

Like alkynes, iminoboranes bind transition metals.

References

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