Group 14 General Features -

Electron precise species

four coordination - greater steric congestion

lack of low energy LUMO - hydrolytically resistant

much less susceptible to nucleophilic attack vs. group 13

These elements have electronegativity closer to C

low polarity to the E-C

Decreasing E-C strength down the group

Pb-R homolysis on heating about 100ºC.

bond enthalpy of approx 150kJ/mol vs. Si-C of approx 320 kJ/mol

Element

C 2.55Si 1.9Ge 2.01Sn 1.96Pb 2.33

Group 14General

Carbon is a special member of the group. For example, bonding much more important to this element than to other members.

Homoleptic alkyls are possible in both the +2 (ER2) and +4 (ER4) oxidation states

ER4 are generally very stable.

The inert pair effect leads to higher prevalence of R2E down the group.

SynthesisOne of the first organometallic compounds (Frankland, J. Chem Soc. 1849, 2, 263) :

Sn + 2EtI Et2SnI2

Followed up by reactions with Et2Zn:

2 ZnR2 + SnCl4 SnR4 + 2 ZnCl2ZnR2 + SnCl2 SnR4 + ZnCl2

With the discovery of Grignard, this synthetic method replaced by RMgX metathesis.

SynthesisThe homoleptic ER4 compounds can be made by metathesis, hydrometallation, and coupling reactions :

SnCl4 + LiR4 SnR4 + 4 LiClSiH4 + H2C=CH2 SiEt4

Coupling reactions combine direct and metathesis reactions:

GeCl4 + 4 RX + 8 Na GeR4 + 4 NaCl + 4 NaX

4 RX + 8 Na 4 NaX + 4 NaR4 NaR + GeCl4 GeR4 + 4 NaCl

Industrial preparation of tetrabutyl tin3 SnCl4 + 4 R3Al 3 SnR4 + AlCl3

SynthesisAn interesting reaction for Sn is the transmetallation with Li reagents :

R3SnR’ + LiR” R3SnR” + LiR’

This reaction is particularly useful when R’ = allyl or benzyl.

In this case it is difficult to directly make LiR’ due to the reaction of LiR’ with R’X:

Li + R’X LiR’ + LiX

LiR’ + R’X R’-R’ + LiX

Structures and Stability of ER4 Compounds

These compounds typically exist as monomers.

The coordinative saturation and low electrophilicity prevent dimerization or interaction with a donor solvent.

The reactivity of these compounds is not enhanced by the addition of a donor solvent or atom.

With the exception of lead compounds, these organometallics are stable in air at room temperature. Bond strengths determine their lability to thermolysis..

The inert pair effect leads to more stable +2 oxidation state which also increases the lability of R4E down the group.

PbEt4 Synthesis

“Leaded Gas” contains the antiknock agent PbEt4.

It is produced by the disproportionation of Pb(II) acetate in the presence of Et3Al:

6 Pb(OAc)2 + 4 AlEt3 3 Pb + 3 PbEt4 + 4 Al(OAc)3

Two key features of organolead compounds

toxic (but 1/10 that of Pd!)much weaker M-C – thermal and light.

Organoleads will decompose by Pb-C bond homolysis and -hydrogen elimination/reductive elimination to produce lead, H2, alkanes and alkenes.

For R4Pb, the stability follows:

Me > Et > iPr

Synthesis of the Mixed Alkyl HalidesMixed halo alkyl species are synthetically useful and more reactive than homoleptic alkyls

Redistribution reactions of the homoleptic alkyls are employed commercially to prepare halosilanes (AlCl3 Lewis acid catalyst)

n R4M + (4-n) MX4 4 MRnX4-n

Often non-statistical product distribution - subtle bonding and steric effects can favor particular product.

Metathesis reactions can be employed using LiR or Grignard:

SiCl4 + x LiR SiR4-xClx + x LiCl

Transmetallation can be used with divalent species(Sn(II)/(IV) with Hg(II/0))

SnCl2 + Ph2Hg Ph2SnCl2 + Hg

Structures of the Group 14 Alkyl Halides

Presence of halide leads to rich structural chemistry for Sn and Pb owing to M-X-M bridges The tendency toward aggregation increases with diameter.

Silicon and germanium compounds are typically monomers.

Alkyl monohalides of Sn and Pb show polymeric aggregation in the solid phase, bridging through the halides:

Ph3PbX

Me3SnF

tbp

Sterics of R play a key role

Alkyl Dihalides of Sn and PbDihalo derivatives tend to display octahedral centers.

All of these are monomers in the gas and solution phases.

Me2SnF2

Me2SnCl2

Reactivity of Si-X

Protonolysis

Convenient route to Si-O bonds (siloxanes), Si-N (silazanes from N-H), Si-S (from S-H)

Note that the less polar E-C are not as reactive

For the reaction with water –

Initial reaction is formation of silanol but a strong tendency to form Si-O-Si linkages leads to H2O elimination and formation of siloxanes

dihalosilane – rings and chains

RSiCl3 can lead to more elaborate three-dimensional structures.

 

Siloxanes undergo redistribution to yield silicones.

Reactivity of R3ECl

It is common for silicon to dehydrate and couple (less so for Sn and Ge):

2 R3SiOH H2O + (R3Si)2O2 R3SiSH H2S + (R3Si)2S

2 R3SiNH2 H3N + (R3Si)2NH

R3ECl

R'Li

R3ER'

LiAlH4

R3EH

H2O

R'OH

Li or Na

R3E-ER3

or NaSnR3

R3EOH

R3EOR'

(R3E)3N

L-ER3Cl L

Li3N

(R3E)2O

Reactivity of R3ECl

Mixed species are particularly useful starting materials for metathesis reactions.

The mechanism of these reactions appear to be associative and second order overall with a dependence of k on identity of entering group.

Intermediate is likely a five-coordinate species

Stereochemical studies indicate that both inversion and retention of E configuration can be observed

Lewis Acid Character

These compounds are still have enough electrophilic behaviour to accept nucleophiles and produce anions in solution:

Bu3SnCl + Cl- Bu3SnCl2- Me2SnCl2 + 2 Cl- Me2SnCl42-

Me2SnCl2 + 2 O=SMe2 Me2SnCl2.2 O=SMe

This amphoteric behaviour allows these compounds to eliminate a halide and form anions when reduced with sodium.

Rochow Process (Direct)

Organosilicon dichlorides (specifically) can be made by the Rochow process:

2 RCl + E/Cu R2ECl2 + Cu

generalizes over Si, Ge, and Sn

Cu is a necessary catalyst in this process – shuttles between Cu/CuCl and transfers Cl and Me to Si.

This process allowed access to silicones.

Polysiloxanes (Silicones)

Once organosilicon chlorides were widely available, allowed large-scale silicone production

Silicones are made by the hydrolysis of organosilicon chlorides and subsequent dehydration and redistribution :

R4-xSiClx + x H2O x HCl + R4-xSi(OH)x [R4-xSi-O]n + n H2O

Depending on the number of chlorides, the resulting silicone can be linear or highly branched.

They are strong, flexible polymers

Stability of Si-O-Si

Si-O-Si exhibits low Lewis basicity and large angle suggest a role for p-d-p bonding () or overlap with the * on Si

Increased flexibility of this linkage due to decreased directionality of the Si-O bonds

Planarity in N(SiH3)3 has also been explained by similar delocalization of the N

lone pair (weakly basic)

A related observation is the relative ease of deprotonation of SiCH3 by strong bases (carbanions) - conjugate base stabilization via delocalization to Si

R3Si O SiR3

Polystannoxanes

Due to the weaker Sn-O bond, polystannoxanes don’t show as strong a backbone, and thus as unreactive a polymer as the silicon analogue:

R2SnCl2 (OH-, H2O) R2Sn(OH)2 (-H2O) [R2Sn-O-]n

This willingness to datively bond to another polymer chain is due to the weaker p interactions due to tin’s size and low charge density.

Reactivity of Polystannoxanes

Polystannoxanes are subject to decomposition by acids and bases, unlike silicones:

Again, the weak Sn-O-Sn backbone allows attack of a nucleophile at the tin, or by protons at the oxygen linkage to dehydrate.

AnionsThe monohalides of group 14 can be reduced with electropositive metals to produce anions:

Ph3SiCl + 2 Li Ph3SiLi + LiCl

Tin and lead need to be reduced in liquid ammonia, due to their amphoteric behaviour:

R3EX + 2 Na (NH3, -78oC) NaER3 + NaX (E = Sn, Pb)

This is analogous to similar carbon chemistry, and tertiary group 14 compounds with electron-withdrawing moieties stabilizes these anions.

Commercial uses of Organotin Compounds

Catalysis, stabilizers, biocidal agents: Bu3SnOAc (Bu3SnCl and NaOAc) – antifouling agent and applications to

catalysis (polymerization) Bu2SnOAc2 PVC stabilizer

 (cyclo-C6H11)3SnOAc – insecticide in orchards and vineyards

 Bu3SnOSnBu3 (hydrolysis of Bu3SnCl) – algicide and antifouling

 Ph3SnOSnPh3 - antifouling

 (Bu2SnS)3 from Bu2SnCl2 with Na2S – PVC stabilizer

 

Electronegativity - HydridesElement

C 2.55Si 1.9Ge 2.01Sn 1.96Pb 2.33

Because hydrogen has an electronegativity of 2.20, Si, Ge, and Sn hydrides can react in a “hydridic” fashion.

Note that since germanium and hydrogen are very close in electronegativity, electron-donating groups can allow the germaninum hydride compound to react to release a proton:

R3GeH + RLi RH + R3GeLi

"hydride" DH (kcal/mol)

CH4 105

Me3SnH 95

Bu3GeH 88.6

Bu3SnH 78.6

Hydrides - Silane Synthesis

Silane is made by the reaction of lithium aluminium hydride with a polychlorosilane.

It requires a silicon chloride species, which can be made by direct reaction:

n Si + (n+1) Cl2 SinCl2n+2 (n = 1-6)

This step goes through SiCl4 and subsequently reacts with additional silicon. The amount of excess silicon determines n.

SinCl2n+2 + xs LiAlH4 SinH2n+2

This step produces some AlCl3, but it is very difficult to get all of the hydrides on LiAlH4 to exchange.

Hydrosilation

Hydrosilation is an important organic reaction. Regioselective to anti-Markovnikov products.

Unlike hydroboration, it can selectively reduce a carbonyl group:

CO

R

R

HR3Si

CO

R

R

HSiR3

CO

R

R

HR3Si

Here the hydrogen is the nucleophile due to the low electronegativity of silicon. Thus, this is a hydride transfer.

Hydrogermylation and hydrostannylation are also known.

Ge/Sn Hydrides

Radical based reduction reactions:

e.g. R3SnH + R’X (h) R3SnX + R’H

Due to the weakness of the Sn-H bond (300 kJ/mol), it is possible to break organotin hydrides into radical species:

R3SnH (h) R3Sn• + H•

Bu3GeH is about 10x slower than Bu3SnH

(interestingly (Me3Si)3SiH is comparable to Bu3SnH in many cases)

Used to prepare GeCl2 from GeCl4

Lead Hydride

Lead hydride decomposes at room temperature due to the weakness of its Pb-H bond (~205 kJ/mol) .

It will form R3Pb-PbR3 compounds and H2 from a radical coupling reaction.

Hydroplumbation adds readily at low temperature to alkenes and alkynes to gve stable Pb(IV) compounds.

Aside from reaction with lithium aluminium hydride, it is common to synthesize lead hydride (at low temperature) by metathesis using another group 14 hydride:

nBu3PbX + Ph3SnH (-78oC) nBu3PbH + Ph3SnX

Use in Organic ReactionsGroup 14 organic compounds are commonly used in C-C bond forming reactions in organic chemistry.

Si-C reacts as a carbanion equivalent

Mukiyama aldol

Hosomi-Sakurai

Hiyama coupling

Sn used in Pd catalyzed coupling reactions

Stille coupling

Use in Organic ReactionsSilyl enolates:

silyl enol ethers as an enolate equivalent in Lewis acid-catalyzed aldol additions

Trichlorosilyl enolate-offers a route free of catalysts

Hosomi-Sakurai Reaction Lewis acid-promoted allylation of various electrophiles with allyltrimethysilane. Activation by Lewis acids is critical for an efficient allylation to take place.

Only catalytic amounts of Lewis acid are needed in the newer protocols (allylsilyl chlorides instead of allyltrimethylsilane)

Initial step of proposed mechanism:

H. M. Zerth, N. M. Leonard, R. S. Mohan, Org. Lett., 2003, 5, 55-57.

Stille Coupling The Stille Coupling is a versatile C-C bond forming reaction between stannanes and halides or pseudohalides, with very few limitations on the R-groups.

The main drawback is the toxicity of the tin compounds used, and their low polarity, which makes them poorly soluble in water. Stannanes are stable, but boronic acids and their derivatives undergo much the same chemistry in what is known as the Suzuki Coupling.