Iminoborane

Iminoboranes comprise a group of inorganic compounds with the generic formula containing triple bonds XB≡NY, where X, Y can be H, F, RO, R2N, R3C, etc, among which the simplest form is HB≡NH. They are electronically related to acetylenes but are usually more reactive due to the polarity.[1][2][3]

Structure and Bonding

Parent compound HB≡NH
Figure 1: Lewis Resonance Structure of HBNH.

For iminoboranes, two resonance structures mainly contribute to the final structure (Figure 1).[4]

Substituent effect

Because the substituents can donate their lone pairs into one of the B-N anti-bonding orbitals, the bonding scheme can be dramatically altered when substitution occurs, and the new bonding scheme highly depends on the nature of substituents and the site of substitution, which can be investigated theoretically.[5]

HBNH

HBNH is an intermediate from the photolysis of solid H3BNH3[6][7][8] Strong shielding effect of bulky substitutions can make the iminoboranes more stable and more resistant to oligomerization, which allows for the storage at room temperature.[9]

Thermal decomposition of azidoboranes and trimethyl-silyl(trimethylsilyloxy)aminoboranes can be used to access symmetric iminoboranes, especially those with bulky R groups.[10]

Reactivity

Oligomerization

Iminoboranes tend to oligomerize. Five types of oligomerization product are mainly produced: cyclodimers (1,3,2,4-diazadiboretidines, Di[11][12]), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr’[13]), cyclotetramers (octahydro-1,3,5,7-tetraza-2,4,6,8-tetraborocines, Te[14]), and polymers (polyiminoboranes, Po); which are shown below.[15] Which product is dominant depends on the structures of reactants and the reaction conditions. Some of the products can be interconverted.[16]

Polar Addition

Acid

If there is a π-electron donating group such as -NR2 connected to B, the neutral Lewis acids can be bonded to the N atom of the B≡N. The product shown in the following scheme is a famous diaminoboron cation [C9H18N=B=NR2]+, where the positive charge is balanced intramolecularly, not by a external anion.[17]

Base

The 11BNMR signals of iminoboranes can be altered when the iminoboranes are dissolved in liquid with Lewis base activity (tertiary amines, tetrahydrofuran). There may be interactions between the positive-charged B and the Lewis base, but the chemistry behind this still needs to be further investigated.[18]

Figure 7: addition of iminoboranes with protic agents. H-Y can be HCl, tBuOH, Et2NH, iPr2NH, tBuNH2, (Me3Si)2NH.

Addition of protic agents

The addition of protic agents is a quantitive reaction, fast even far below 0 ℃, while the addition of protic agents of acetylenes requires higher temperature, which also indicates that iminoboranes are more reactive towards polar addition than acetylenes.[19]

Boration

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.

Cycloadditions

Cycloaddition is the addition of π bond to B≡N, and can be categorized into [2+2]-cycloadditions, [2+3]-cycloadditions, [2+4]-cycloadditions etc.

One of the widely investigated [2+2]-cycloadditions is the reaction of aldehydes and ketones with iminoboranes, which has two possible pathways. Usually, the relatively stable iminoboranes which are less likely to undergo the total opening of B≡N, and the ketones containing enolic protons, prefer to undergo path b.[20]

The typical [2+3]-cycloaddition is the addition of B≡N and RN3,and the typical [2+4]-cycloaddition is the addition of B≡N and cyclopentadiene.

Coordination to transition metals

Like acetylenes, iminoboranes can also coordinate with transition metals.

gollark: Of the kind JS has about 12 million of.
gollark: Is there some kind of "bundler" for CC?
gollark: It has one for pointlessness reasons.
gollark: Maybe I should replace potatOS's very bad PRNG with that, then.
gollark: The other one is just that it seems pretty much unpointful.

References
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