Group 13 Compounds

The compounds in this group are electron deficient species There are three available oxidation states for the group 13 compounds, represented by the basic formulae: R 3 M – where M(III) is any metal in the group. This is by far the most common organometallic species for group 13.

R 2 M-MR 2 – M(II) with a homonuclear bond. Not common.

RM: – M(I) accessible due to the difference in energy between the

n

s and

n

p orbital energies. Since this energy difference becomes more pronounced as

n

increases, this is more common for the heavier group 13 compounds (In, Tl). It allows for a Lewis base reactivity.

Inert Pair Effect

The s electrons are more tightly bound than the p electrons. As the principle quantum number is increased, this effect increases. Al Tl

n

3 6

Ionisation E (MJ/mol) 1st 2nd 3rd

0.58

0.59

1.82

1.97

2.74

2.88

The n = 6 electrons are moving at speeds much closer to the speed of light, thus they appear to have increased mass compared to other electrons. This leads to increased nuclear attraction and the 6s orbital contracts. Thus the 6p electrons are higher energy than the 6s electrons. The 6s pair appears inert.

Boron

The chemistry of boron is controlled by two major factors: The much higher charge density of boron compared to the rest of the group, so its s bonds are more covalent in nature. It also is very polarizing, which changes its interaction with carbon.

The diameter of boron is small, and so p interactions (particularly hyperconjugation) are much more important

Homoleptic Alkyls

The homoleptic (all the same R group) alkyls for group 13 are typically monomeric in solution and in the gas phase, with the exception of aluminium.

The trialkyl compounds are typically made by metathesis: BX 3 + 3 RMgX  BR 3 + 3 MgX 2 AlCl 3 + 3 MeLi  AlMe 3 + 3 LiCl Industrial preparation of alkylboranes exploits the oxophilicity of aluminium: B(OR) 3 B 2 O 3 + AlR 3 + AlR 3   BR 3 Al 2 O 3 + Al(OR) 3 + BR 3

Homoleptic Alkyls

Trimethyl boron is a gas (b.p. –22C) that is monomeric, pyrophoric but not rapidly hydrolyzed by water. Incorporation into heterocycles is common OH O + BR 3 B OH O R + 2RH

Homoleptic Alkyls

Aluminum Distinguishing feature is bridged dimeric structures via 3c2e bonds. Commercial synthesis of Al 2 Me 6 CH 3 Cl + AlCl 3 Me Me Al Cl Al Me Me + 6Na Cl Me Me Al Me Al Me Me + 2Al + 6NaCl Me Gallium Indium Thallium All succeeding members are monomeric in solution. Judging by hydrolytic stability the carbanionic character has the following order AlR>GaR>InR>TlR

Direct Reaction Of Sorts

Commercial synthesis of triethyl aluminum and higher homologues (direct reaction): Al + 3/2 H 2 + H 2 C=CHR  Al(CH 2 CH 2 R) 3 (100C, 100atm) This is actually a combination of direct reaction and 1,2-insertion (hydrometallation): Al + 3/2 H 3 Et 2 2 + 2 AlEt AlH + 3 H 2 3 C=CH  2 3 HAlEt 2  3 AlEt 3 “Vermehrung” (increase) step “Anlagerung” (attachment) step Hydrometallation is common for boron, aluminium, and gallium hydrides. These are reversible reactions that can be applied to synthetic applications.

Hydrometallation in Group 13

Hydrometallation goes through a transition state that uses a 3c2e bond: R R M H 3 H H R R H MH 2 H H R R H MH 2 H H anti-Markovnikov R R H H R R H H

H

H H H H H It is an anti-Markovnikov addition, as the metal will shift to the less sterically congested side, adding the hydride to the higher branched side.

Anti-Markovnikov Regioselectivity

The regioselectivity of hydrometallation in group 13 decreases down the group, thus hydroboration is the most important in synthetic chemistry.

A possible explanation is that the orbitals involved are more extensive as

n

increases, making the bond distance longer.

Thus the boron is more responsive to the steric environment than the rest of group 13.

As well, hyperconjugation means that hydroboration is much less reversible than hydrometallation in the rest of the group. This is an important factor in organic chemistry for yields.

Boron and Hydrometallation

Some interesting boron organometallic compounds can be made by hydroboration:

Hydroboration

Hydroboration is a versatile organic reaction: H 3 CRCH 2 H + BH 3 + H 2 Et 2 O, 25 o C=CRH 150 o C HRC H RCH 2 CH 2 OH H 2 O + OH HCrO 4 RCH 2 COOH CH 2 BH 2 Ag 2 O (RCH 2 CH 2 ) 2 IH 2 CRCH 2 I 2 /OH Et 2 NCl RCH 2 CH 2 Cl

Displacement: Reverse Hydrometallation

The reverse of hydrometallation, elimination of an alkene is possible in hydrometallation ( b -elimination): M-CH 2 CH 2 R  M-H + H 2 C=CHR This can be exploited synthetically since the metal (especially Al) will exchange alkene for less branched alkyls: M-H + H 2 C=CHR + H 2 C=CH 2  M-CH 2 CH 3 + H 2 C=CHR Given the choice, the metal will go to the lower branched system: H 2 C=CH 2 > H 2 C=CHR > H 2 C=CR 2 As a synthetic method, you would obviously use an excess of the alkene you wish to incorporate.

Aluminium and Hydrometallation

Aluminium is the key example of alkene exchange and reversible hydrometallation as a synthetic technique: Al + 3/2 H 2 + 3 H 2 C=CMe 2 (100 o C, 200 atm)  i Bu 3 Al i Bu 3 Al + H 2 C=CHMe  n Pr 3 Al + 3 H 2 C=CMe 2 n Pr 3 Al + 3 H 2 C=CH 2  Et 3 Al + 3 H 2 C=CHMe

BR

3

Hyperconjugation offers an explanation for monomeric nature. The empty p orbital on B is close to the s bonds in the a -carbon, due to its very small diameter. Stabilized by forming a partial p bond: C 2sp H C B R R B 2p The s bond is free to rotate, and this loss of stabilization is made up for by the hyperconjugation of the p orbital to the other s bonds on the carbon.

Hyperconjugation can occur to all three groups.

Another Example of Hyperconjugation

This same interaction explains why carbanions are less stable with small radii centers.

The carbanion can be thought of as an sp 3 hybridized center with a lone pair. This lone pair is stabilized by electron withdrawing groups. However, the closer these groups are to the carbon (i.e. the smaller their radii) the higher degree of hyperconjugation.

This leads to a less stable carbanion: C 2sp F F C F F 2p

Al 2 Me 6 Aluminium alkyls form dimers in non-donor solvents: Dimeric nature retained in the solid crystalline phase, but is a monomer in the gas phase and in donor solvents.

Note that with halide and alkoxide bridges the Al-E-Al angle approaches 90 degrees. These are 2c,2e bonds.

The heavier members of the group exhibit less electrophilicity due to a decreasing charge density and do not form 3c2e bonds.

Al

2

Me

6

The short interatomic Al-Al distance (261 pm in the dimer, compared to 238 pm as a covalent bond) suggests that the geometry of aluminium may be between the sp 2 and sp 3 hybridization: Al with an sp 2 hybridization Al with an sp 3 hybridization

Aluminium Alkyls

On aluminium, the size of the R group affects the degree of aggregation. This is a steric crowding effect.

Compound Solid

AlMe 3 Dimer AlEt 3 Dimer AlPr 3 Al i Bu 3 Dimer Dimer AlPh 3

Solution

Dimer Dimer Dimer Monomer Dimer Equilibrium

Gas

Equilibrium Monomer Monomer Monomer Monomer Degree of dissociation increases with steric bulk. For example at RT Keq for dissociation of Al 2 Me 6 is about 1.5x10

-8 while for Al 2 Bu 6 it is about 2 x 10 -4

Al

2

Ph

6

The internal angle of the phenyl ring and the orientation suggests that the bond between the ring and the two aluminium is more complex (e.g. 3c2e bond and a 3c4e bond).

The 3c4e bond destroy the aromaticity at the bonding carbon by incorporating the p -system p orbital in an sp 3 hybridization:

Hydrides of Group 13 “Ate” Complexes

Lithium hydride is relatively insoluble and reacts slowly because of the very unreactive Li-H bond.

It is a polymer in the solid state and thus has solubility issues.

Activation of this material is possible by reacting it with MR 3 , typically AlH 3 BH 3 : or LiH + MH 3  LiMH 4 The resulting material is soluble in a larger range of solvents, and is a more reactive hydride source.

The electropositive nature of the group 13 metal center is the driving force behind complex formation.

Reducing Agents

Most hydrides are used as reducing agents. The most popular in order of reactivity are: NaBH 4 : The hydrogen bond is less polar in boron, so this is a less harsh hydride. It is possible to use it in alcohol solutions and even in water, if the reaction is fast. The sodium cation helps solubility in alcohols and water.

LiAlH(OR) 3 : These are intermediate reducing agents that aren’t as indiscriminate as LiAlH 4 but more reactive than NaBH 4 . (e.g. react with acid chlorides but not aldehydes) LiAlH 4 : very reactive with water and alcohol due to the more polar Al-H bond. This hydride is soluble in toluene, and ethers. It is much stronger.

Mixed Alkyl Halides of Group 13 Metals

The most common method of making mixed alkyl/halide compounds in group 13 is reproportionation: 2x MR 3 + (6-2x) MX 3  6 MR x X 3-x (x = 1, 2) This can be controlled by stoichiometry and conditions.

In many cases a mixture of RMX 2 and R 2 MX can result.

Subsequent reaction usually drives the system to stoichiometrically controlled point, but isolation of a pure product can be difficult.

Thus, there are better ways to go about this.

Reaction with HCl

The metal alkyls (particularly B, Al, Ga) react with one equivalent of hydrogen chloride to eliminate an alkane and form the monochloride: MR 3 + HCl  R 2 MCl + RH This reactivity is particularly labile and must be controlled (low T, introducing HCl slowly).

The second replacement can be more difficult (especially with boron) and results in mixed (R 2 MCl, RMCl 2 ) systems.

Reaction with R’Cl

Aluminium and gallium can undergo an alkyl-halogen exchange: R 3 Al + R’Cl  R 2 AlCl + R’R The metals can be stabilized as dialkyl cations when R can be a stable anion.

Inorg. Chem.

1996

,

35

(15), 4277.

This reactivity requires a base to stabilize the aluminium cation This chemistry is similar for gallium, but not seen for boron because of the strength of B-C bonds.

Bond Strengths

Hyperconjugation and a high charge density lead to stronger bonds for B when compared to Al. As a result R 2 B + with R’Cl.

is not in any significant concentration to allow reaction

Bond D (kJ/mol) Bond D (kJ/mol)

B-C 365 Al-C 254 B-F B-Cl 757 536 Al-F Al-Cl 664 511

Stabilization of Aluminium Cations

The stability of an aluminium cation requires either coordinative saturation (i.e. four groups around aluminium) as per the previous example, or by steric protection of the aluminium centre.

J. Am. Chem. Soc.

1999

,

121

(37), 8673.

Stabilization of Aluminium Cations

Icosahedral carboranes of the type CB 11 H 6 X 6 (X =halide) outperform fluorinated tetraphenylborates in sustaining Brønsted acidity Weak coordination to Br/Cl C-Al-C 130.0 and 136.6 –suggesting that Al approaching sp hybridization

J. AM. CHEM. SOC. 2002

,

124

, 7662-7663

Stabilization of Aluminium Cations

Reaction with ethene - Alkylated benzene and butanes. Unaltered Al-Et groups – no aufbau

J. AM. CHEM. SOC. 2002

,

124

, 7662-7663

Mixed Ligand Systems via Friedel-Crafts Reaction

BCl 3 + AlCl 3 H BCl 2 + Cl-AlCl 3 The Friedel-Crafts Reaction works only with BCl 2 alkyl: to produce a boron ArH + BCl 3 (AlCl 3 )  HCl + ArBCl 2 The mechanism relies on aluminium being both electropositive enough to form a tetrahedral anion and big enough to accommodate 4 chloride in its coordination sphere.

BCl 2 + HCl + AlCl 3

Other Mixed Ligand Systems

The homoleptic alkyl compounds of group 13 are reactive with proton-bearing species (including water).

Manipulating these compounds requires exclusion of oxygen and water, and the reactions are similar for Al, Ga, In: (R 2 M-H) H 2 , pressure (R 2 M-OR) HOR MR 3 HPR 2 (R 2 M-PR 2 ) HNR 2 (R 2 M-NR 2 )n

Mixed Ligand Systems

These compounds require a four-coordinate intermediate, and thus is not common for boron: MeH NHMe 2 Me Me Ga Me Me 2 N H Me Me Ga Me 1/2 Me Me Ga Me 2 N Ga N Me 2 Me Me

Mixed Ligand Systems

Mixed BR n X 3-n trialkyls compounds are more reactive than the simple Halide easily displace by either protonolysis with ROH, R 2 NH or reaction with anionic reagent (e.g. LiR, LiNR 2 ) Boron phosphides can be made by reaction with R 2 BX: R 2 BX + HPR 2 + NEt 3  R 2 B-PR 2 + NHEt 3 + X This reaction with phosphorous is again due to boron’s polarizing power. Other mixed ligand systems of boron are made by metathesis.

Mixed Ligand Compounds

It is also common to make these through a metathesis reaction: R 2 AlCl + LiNMe 2  R 2 Al-NMe 2 + LiCl The mixed ligand compounds usually oligomerize.

Boron again sets itself apart with a tendency toward hyperconjugation and planar compounds, while the rest of the group tends toward three dimensional structures.

Mixed Ligand Compounds of Indium

Due to the size of indium, it can easily form a trigonal bipyramidal geometry.

This allows it to form insoluble, low-reactivity polymers: Me Cl In Me Cl In Me Me Cl In Me Me

Boracycles

Due to boron’s ability to participate in p systems, it has been incorporated in many aromatic compounds: C C B C C B C C B This penchant for hydride (3c2e) bridging and p system participation gives way to cluster chemistry: boranes and carboranes