# 6.10: Boron Compounds with Nitrogen Donors

The “B-N” unit is isoelectronic (3 + 5 valence electrons) to the “C-C” unit (4 + 4 valence electrons). The two moieties are also isolobal, and as such there are many of the compound types formed by carbon have analogous derivatives in the chemistry of boron-nitrogen.

Boron compounds, BX3, are strong Lewis acids and as such form stable addition compounds with Lewis bases, in particular those with nitrogen donor ligands.

$\text{BF}_3 \text{ + NMe}_3 \rightarrow \text{F}_3\text{B-NMe}_3$

In principle these Lewis acid-base complexes should be similar to their isolobal hydrocarbon analogs, however, whereas the dipole in ethane is zero (by symmetry) the dipole in H3NBH3 is 5.2 D as a consequence of the difference in the Pauling electronegativities (i.e., B = 2.04 and N = 3.04). It is this dipole that generally differentiates the B-N compounds from their C-C analogs.

Homolytic cleavage of the C-C bond in ethane will yield two neutral methyl radicals, (6.7.2). In contrast, heterolytic cleavage will result in the formation of two charged species, (6.7.3). Thus, the products either have a net spin, (6.7.2), or a net charge, (6.7.3). By contrast, cleavage of the B-N bond in H3N-BH3 either yields products with both spin and charge, (6.7.5), or neither, (6.7.4). Heterolytic cleavage of the B-N bond yields neutral compounds, (6.7.4), while hemolytic cleavage results in the formation of radical ions, (6.7.5).

$\text{H}_3\text{C-CH}_3 \rightarrow \cdot\text{CH}_3 \text{ + } \cdot\text{CH}_3$

$\text{H}_3\text{C-CH}_3 \rightarrow \text{CH}^+_3 \text{ + } \cdot\text{CH}^-_3$

$\text{H}_3\text{N-BH}_3 \rightarrow \text{NH}_3 \text{ + BH}_3$

$\text{H}_3\text{N-BH}_3 \rightarrow \text{NH}^+_3\cdot \text{ + BH}^-_3\cdot$

The difference in bond strength between H3N-BH3 and ethane is reflected in the difference in bond lengths (Table $$\PageIndex{1}$$).

 Compound Bond length (Å) Bond strength (kcal/mol) H3C-CH3 1.533 89 H3N-BH3 1.658 31

## Aminoboranes

The group R’2N-BR2 is isoelectronic and isolobal to the olefin sub-unit R’2C=CR2, and there is even appreciable π-bonding character (Figure $$\PageIndex{1}$$). A measure of the multiple bond character can be seen from a comparison of the calculated B-N bond in H2NBN2 (1.391 Å) as compared to a typical olefin (1.33 Å). It is interesting that a consideration of the possible resonance form (Figure $$\PageIndex{1}$$) suggest the dipole in the σ-bond is in the opposite direction of that of the π-bond.

Unlike olefins, borazines oligomerize to form dimmers and trimers (Figure $$\PageIndex{2}$$) in the absence of significant steric hindrance. Analogous structures are also observed for the other Group 13-15 homologs (R2AlNR’2, R2GaPR’2, etc.).

## Borazines

The condensation of boron hydride with ammonia results in the formation of a benzene analog: borazine, (6.7.6). Substituted derivatives are formed by the reaction with primary amines.

$\text{BH}_3 \text{ + NH}_3 \rightleftharpoons \text{H}_3\text{B-NH}_3 \xrightarrow[\text{- H}_2]{\Delta} \text{[H}_2\text{B-NH}_2\text{]}_n \xrightarrow[\text{- H}_2]{\Delta} \text{[HBNH]}_6$

Despite the cyclic structure (Figure $$\PageIndex{3}$$), borazine is not a true analog of benzene. Despite all the B-N bond distances being equal (1.44 Å) consistent with a delocalized structure, the difference in electronegativity of boron and nitrogen (2.04 and 3.04, respectively) results in a polarization of the bonds (i.e., Bδ+-Nδ-) and hence a limit to the delocalization. The molecular orbitals of the π-system in borazine are lumpy in appearance (Figure $$\PageIndex{4}$$ a) compared to benzene (Figure $$\PageIndex{4}$$ b). This uneven distribution makes borazine prone to addition reactions, making it as a molecule less stable than benzene.

### Iminoboranes: analogs of acetylene

Iminoboranes, RB=NR’, are analogs of alkynes, but like aminoboranes are only isolated as monomers with sterically hindered subsistent. In the absence of sufficient steric bulk oligomerization occurs, forming substituted benzene analogs.

## Boron nitrides: analogs of elemental carbon

The fusion of borax, Na2[B4O5(OH)4] with ammonium chloride (NH4Cl) results in the formation of hexagonal boron nitride (h-BN). Although h-BN has a planar, layered structure consisting of six-member rings similar to graphite (Figure $$\PageIndex{5}$$), it is a white solid. The difference in color is symptomatic of the more localized bonding in BN than in graphite.

As is found for its carbon analog, hexagonal boron nitride (h-BN or α-BN) is converted at high temperatures (600 – 2000 °C) and pressures (50 – 200 kbar) to a cubic phase (c-BN or β-BN). In a similar manner to diamond, cubic-BN is very hard being actually able to cut diamond, and as a consequence its main use is as an industrial grinding agent. The cubic form has the sphalerite crystal structure (Figure $$\PageIndex{6}$$). Finally, a wurtzite form of boron nitride (w-BN) is known that has similar structure as lonsdaleite, rare hexagonal polymorph of carbon. Table $$\PageIndex{2}$$ shows a comparison of the properties of the hexagonal and cubic phases of BN with their carbon analogs.

Table $$\PageIndex{2}$$: Comparison of structural and physical properties of carbon and boron nitride analogs.
Phase Carbon Boron nitride
Cubic Colorless, hard, mp = 3550 °C. C-C = 1.514 Å Colorless, hard, B-N = 1.56 Å
Hexagonal Black solid, planar layers, conductor, mp = 3652 – 3697 °C (sublimes), C-C = 1.415 Å White solid, planar layers, semiconductor (Eg = 5.2 eV), mp = 2973 °C (sublimes), B-N = 1.45 Å

The partly ionic structure of BN layers in h-BN reduces covalency and electrical conductivity, whereas the interlayer interaction increases resulting in higher hardness of h-BN relative to graphite.

## Bibliography

• K. M. Bissett and T. M. Gilbert, Organometallics, 2004, 23, 850.
• P. Paetzold, Adv. Inorg. Chem., 1987, 31, 123.
• L. R. Thorne, R. D. Suenram, and F. J. Lovas, J. Chem. Phys., 1983, 78, 167.