Chemistry 242 - Inorganic Chemistry II
Chapter 12 - Boron

Introduction

Manufacture and Properties of Boron

Boron is quite difficult to isolate, because it is refractory and reactive at high temperature and so it is difficult to contain:
  1. B2O3   +   3Mg      2B   +   MgO   (98% - Wash with NaOH, HCl and HF)

  2. 2BCl3   +   3Zn      3ZnCl2   +   2B   (900 oC)

  3. 2BX3   +   3H2      6HX   +   2B   (Tantalum catalyst)

Boron is rather inert in most forms which contain icosahedral cages. It is attacked by hot oxidizing acids.

Amorphous boron is more reactive, if white hot, and is attacked by NH3 to form boron nitride which is isomorphous with graphite.

Oxygen Compounds of Boron

See Figure 12-1.

The "anhydrous" borates involve the ions: BO33-, B3O63-, (BO2)nn-, and larger aggregates.

The hydrated ones feature: BO3 units which are planar and BO4 unitsd which are tetrahedral and formally have a -ve charge on the boron. The charge on the ion is equal to th number of these latter units. The structures without BO4 units hydrate readily.

Boric Acid

See Figure 12-2 for a summary.

Halides of Boron

Trihalides

BF3 is the most important and is used on an industrial scale. It is a gas boiling at -101 oC.

It is a very strong Lewis acid:

BF3   +   :F-      BF4-

BF3   +   :L      F3B:L

Unlike the others, BF3 is only partly hydrolysed:

4BF3   +   6H2O      3H3O+   +   3BF4-   +   B(OH)3

BF4-   +   H2O      BF3(OH)   +   HF

(BCl3   +   3H2O      B(OH)3   +   3HCl)

In synthetic organic chemistry it is used as follows:

The conversion of ethers or alcohols with acids to esters e.g:

H+   +   RCOOH      [RCOOH2]+

[RCOOH2]+   +   BF3      [RCO}+   +   F3B:OH2

[RCO]+   +   R'OH      RCOOR'   +   H+

Friedel-Crafts alkylations and acylations:

RX   +   BF3      R+   +   BF3X-

R+   +   PhH      PhR   +   H+

H+   +   BF3X-      BF3   +   HX

Fluoroboric acid: "HBF4" is sold as a 40% solution in water. It is a strong acid and source of BF4- ions useful for crystallizations where a coordinating anion is to be avoided.

Reactions of the Trihalides of Boron.

  1. Adduct formation - the main thing to remember that the order of acid strength is counter to naive expectations because of p-bonding effects whnich are strongest for B-F bonds and lead to BF3 being least willing to go from sp2 to sp3. hybridized.

  2. Halide exchange reactions:
    BCl3   +   BBr3      BCl2Br   +   BBr2Cl

    The exchange, presumably through a bridged intermediate, is very facile, so pure mixed compounds cannot be obtained.

  3. Elimination of halide - covers the various solvolyses in addition to hydrolysis:

    BCl3   +   3C2H5OH      B(OC2H5)3   +   3HCl

    BCl3   +   3NH(C2H5)2      B(N(C2H5)2)3   +   3HCl

    This will happen with any solvent with an exchangeable H.

Subhalides of Boron

They have a B:X ratio less than 1:3

The Hydrides of Boron - the Boranes

Table 12-1 lists the hydrides up to B10H14

Figure 12-4 shows some of their structures as perspective drawings. Note that the lines are intended to clarify the shape of the molecule and do not necessarily represent 2e- - 2-centre bonds.

Note also the nomenclature - the prefix gives the number of boron atoms and the number in parentheses the number of hydrogen atoms, e.g. pentaborane(9) is B5H9.

Synthesis

Diborane (b.p. -92.6 oC can be made by several methods:

3NaBH4   +   4BF3      2B2H6   +   3NaBF4
2NaBH4   +   I2      B2H6   +   2NaI   +   H2
BF3   +   6NaH      B2H6   +   6NaF
The last is the main industrial method. The higher boranes are made by thermolysis of diborane under various conditions.

Structures

The connectivities in the boranes cannot be explained using 2e- - 2-centre bonding only, that is the molecules are electron deficient. Valence bond theory has been "extended" by designating three types of 2e- - 3-centre bonding in addition to a normal 2e- - 2-centre B-H and B-B bonds:

An example of the use of this scheme for B10H14 is shown below:

Each hydrogen must have one bond ending at it and each boron must have a total of 4 bonds ending at it. If an atom is in the middle of a three-centre bond, the curved line passing through it counts as only one bond.

Thus, for example, B6 has one normal bond to a terminal hydrogen and one normal bond to B2, plus it is at the end of two three-centre bonds through the bridging hydrogens for a total of four bonds i.e eight electrons.

B2 has one normal bond from B6, one normal bond from its terminal hydrogen, is at the end of a "closed" three-centre bond from B1 and B2, and it is in the centre of an "open" three-centre bond from B5 to B7. This is also equivalent to four bonds.

In some cases, more than one "resonance" (canonical) structure can be formulated to account for the observed molecular shape.


Reactions of the boranes

Diborane

  1. With oxygen (explosive):

    B2H6   +   3O2      B2O3   +   3H2O
  2. With water:

    B2H6   +   3H2O      B(OH)3   +   6H2
    or alcohols:

    B2H6   +   3HOR      B(OR)3   +   6H2
  3. Substitution reactions:

    B2H6   +   HCl      B2H5Cl   +   H2
    B2H6   +   6Cl2      2BCl3   +   6HCl
  4. Cleavage with Lewis bases:

    Symmetrical:

    B2H6   +   N(CH3)3      2H3BN(CH3)3
    Unsymmetrical:

    B2H6   +   2NH3      [H2B(NH3)2]+[BH4]-
  5. Reduction:

    2B2H6   +   2Na      NaBH4   +   NaB3H8
    B2H6   +   NaBH4      NaB3H8   +   H2
    5B2H6   +   2NaBH4      Na2B12H12

Pentaborane(9)

This molecule illustrates two general trends: Attack by bases can remove the somewhat acidic bridging hydrogen:

B5H9   +   NaH      NaB5H8   +   H2
The pyramidal B5H9 loses one of its four bridging hydrogens and the resulting ion is "fluxional", that is, the location of the missing bridge is not stationary, and all the atoms in the base of the pyramid (four borons, four terminal hydrogens and three bridging hydrogens) appear equivalent on the time scale of nmr experiments which might otherwise have distinguished them.

Attack by electrophiles can lead to substitution at the apex of the pyramid:

B5H9   +   I2      B5H8I(apical)   +   HI

Decaborane(14)

Once again the bridging hydrogens can be removed by base:

B10H14   +   OH-      B10H13-   +   H2O

or converted to terminal hydrogens by reducing agents:

B10H14   +   2Na      Na2B10H14
In this reaction, the product has two bridging hydrogens between B1 and B5 and B7 and B8.

Other nucleophiles will add at B6 and B9 with loss of two bridging hydrogens. Again, the two that are left bridge between B1 and B5 and B7 and B8:

B10H14   +   2CH3CN      6,9-(CH3CN)2B10H12   +   H2
Electrophiles substitute terminal hydrogens at the bottom of the "basket" in the 1 and 3 or 2 and 4 positions, e.g.:

B10H14   +   I2      2,4-I2B10H12   +   2HI
There are two reactions that lead to a closed cage:

B10H14   +   2Et3NBH3      [Et3NH]+2[B12H12]2-
and

(SEt2)2B10H14   +   HCCH      B10C2H12   +   H2   +   2SEt2

Polyhedral Borane Anions and Carboranes

Realize that two carbon atoms can replace two B-'s in a closo-borane anion BnHn2-. Derived molecule or molecule ions are the nido structures, which are missing one vertex relative to the closo structure and the more open arachno structures which are missing two vertices. Figure 12-12 shows structures from B4 to B12. see also Figure 12-8.

Skip the chemistry of these species.

The Tetrahydroborate Ion (BH4-)

This is an important reducing agent, source of H-, and reagent to make other less ionic borohydrides.

NaBH4 is stable in dry air and alkaline aqueous solution. (It will react with water initially but the reaction stops as the concentration of the hydrolysis product, sodium borate, builds up.)

LiBH4 is similar to NaBH4 but more sensitive to water.

Al(BH4)3 is liquid which explodes with air or water. It probably has pairs of hydrogen bridges like diborane.

Zr(BH4)4 is a molecular solid with three bridging hydrogens connecting each boron to the zirconium.

Boron-Nitrogen Compounds

The following are really equivalent representations, but the text uses the right-hand one to indicate a weaker bond:

Amine Boranes

This is the class of amine - BH3 Lewis adducts. They contain the unit shown above. The safest synthesis is:

H3NRCl   +   LiBH4      H2RN:BH3   +   LiCl   +   H2

Aminoboranes

These have the structure:

The molecules are flat, and rotation about the B—N bond is restricted, therefore the left-hand structure must be is a significant contributor. It is perhaps not correct to represent them as canonical structures since the geometries would be so different. The cleanest synthetic route is, for example:

(CH3)2NH   +   BCl3      (CH3)2HN:BCl3
(CH3)2HN:BCl3      (CH3)2N:BCl2   +   HCl      (on heating)
(CH3)2N:BCl3   +   2RMgBr      (CH3)2N:BR2   +   2MgClBr

Borazine

This six membered ring can be synthesised by several routes:

Unlike benzene, borazine undergoes addition reactions:

Notice where the Hd+ and the Cld- end up: This illustrates how unrealistic the formal charges on the boron and nitrogen atoms really are!

Like benzene, borazine can form p-complexes with transition metals:

Wade's Rules

This section is not covered - skip it.