Carbenes and Analogues

Carbenes are 6-electron carbon centres, typically -bonded to two other groups, and with two lone electrons.

These can be spin-paired (singlet) or spin-non-paired (triplet):

triplet singlet

C C

R

R

R

R

Heavier Group 14 Analogues

In the singlet state these compounds can be Lewis bases or Lewis acids.

When R is a “small” group these species oligomerize.

If R is large or is there is a site for intermolecular coordination to the E center (increases the coordination number up to three or four) then these species can be stabilized.

ERR

Silylene

Silylene is the silicon analogue of the carbene, and is similarly unstable.

They cannot be isolated by simple reduction of the dihalide:

Me2SiCl2 + 2 K 2 KCl + (Me2Si)n + Me(Me2Si)mMen = 5, 6, 7; m < 100

But photolysis or thermolysis of these species afford silylenes:

c-(Me2Si)6 Me2Si: + c-(Me2Si)5

Interestingly, thermolysis either in air (300oC) or under nitrogen (1330oC) produces SiC, a popular industial abrasive.

Silylenes are also highly reactive and thus transient species

Thermal Routes to Silylenes

Me2C

Me2CSiMe2 Me2C CMe2 + :SiMe2

Elimination of a silylene from hexamethylsilirane occurs at 80oC.

It is an equilibrium reaction:

The evolved silylene adds into the three-membered silirane ring to alleviate ring strain:

+ :SiMe2

Me2C

Me2CSiMe2

Me2C

Me2C SiMe2

SiMe2

Formation of Si=Si - DisilenesBefore coming back to silylenes we need to realize that one reaction path might be dimerization and formation of compounds containing Si=Si.

Such species are not widespread – Why?

Closer to the top of the group, the s and p orbital energies are closer together.

Si=Si is weaker than two Si-Si bonds and the double bond is not usually kinetically stable (C=C is also weaker than two C-C, but kinetics helps)

As a result silylenes tend to aggregate into rings and clusters.

6 R2Si: (R2Si)6

bulky ligands might protect the double bond

Formation of Si=Si

The first isolable disilene (Si=Si formation) was generated in 1981 by the elimination of silane to make the disilene:

Mes2Si(SiMe3)2 (h) Me6Si2 + Mes2Si=SiMes2

Employs bulky mesityl groups

Structure is trans bent:

Most are planar with Si-Si about 2.15A

E ER

R

RR

18 deg

Double Bonds Between E

Small out of plane angle

Si-Si distance 0.2 Å shorter than that of a single bond

No tendency to dissociate to monomers in hydrocarbon

To date approx 30 stable disilenes have been isolated and characterized (Si=Si 2.14-2.25 Å, approximately planar core geometries)

Formation of Si=Si

Another example:

Formation of Si=SiPhotolysis:

J. Am. Chem. Soc. 1993, 115, 10428.Key to formation is protection of the Si=Si with bulky groups

Reduction:

=

Thermal Routes to Silylenes

Thermolysis of a Si=Si bond gives a silylene.

Note that only due to the tremendous steric hinderance is the silylene around long enough to form a Lewis base adduct

J. Am. Chem. Soc. 1997, 119(6), 1456.

Theoretical Ph2SiCNPh

Although the compound was not structurally characterized, calculations show the silicon to be sp3, as would make sense with a silylene-Lewis base adduct.

Reactivity of Silylenes

Silylenes insert into bonds to silicon.

(Note that SiMe2 is really hexamethylsilirane heated up to ~80oC to generate dimethylsilylene in situ)

R3 SiH + :SiMe2 R3Si-SiMe2HR3 SiOR + :SiMe2 R3Si-SiMe2OR

Me3SiC CSiMe3 + :SiMes2

Me3SiC

SiMes2

CSiMe3

1

Interestingly, 1 is so stabilized by the mesityl groups (steric) that this compound can be precipitated from methanol.

Germylenes, Stanylenes, Plumbylenes

As we go down the periodic table, the stability of the carbene derivatives goes up.

With bulky R groups, these can be stable monomers:

EX2 + 2LiHCMeSi

Me Me

Me

Si

MeMe

ECH

Me

SiMe

Me

MeSi

MeMe

CH

Me

Si

MeMe

MeSi

MeMe

EX2 + 2 LiR 2 LiX + :ER2 (R = CH(SiMe3)2)

ERR

Germylenes, Stanylenes, Plumbylenes

Most common way to make transient germylenes (photo- or thermoloysis):

Other examples of stable monomeric species:

GeR2

Ph

Ph Ph

Ph

Ph

Ph

Ph

Ph

GeR2+ R = Me, Et, Ph etc

MMes

Mes

Mes

Mes

M = Ge, Sn, Pb(via metathesis) M

F3C

F3C

CF3

CF3

F3C

F3C

M = Sn, Pb(via metathesis)

Germylenes, Stannylenes, Plumbylenes

The two-coordinate species are considered to be sp2 hybridized with a vacant p orbital. They tend to be colored due to the n to p transitions

Three coordinate species viewed as sp3 (Td) and four-coordinate as dsp3 (tbp)

SnOHMe

OMe

OMe

MeO N

N

Sn

H

Me3Si

SiMe3

H

SnH

Trip

Trip

Ar

SnH

Ar

Sn

H

chloride analogue also known to dimerize

NtBu

tBu

Sn

NN

Sn

N

2

Formation of Ge=Ge and Sn=SnIf the steric bulk isn’t great enough, these compounds will dimerize - Do not have planar geometry

E ER

R

RR

E ER

R

RR

The lone pairs are in the sp2 orbital on “E”, and it is energetically more favorable to overlap with the empty p orbital rather than promote the electron pair into the p orbital for a more classic double bond.

E E RRR

R

Double Bonds Between E

out of plane angle 41° Sn-Sn distance 2.768(1) Å, which is slightly shorter than that of a single bonddissociates to (SnR2) monomers in hydrocarbon

Back to Disilenes: Bonding

Si=Si bond is much weaker than C=C bond consequently the HOMO LUMO gap is much lower leading to absorption in the visible

Bonding in Disilaethenes - Isomerization

Isomerization of disilenes is much easier than for alkenes because the Si=Si bond is weaker

Initially produced photochemically Thermally isomerized at room temp! (Note strain relief)

Reactivity Disilenes1,2 - Additions

The reactivity of disilenes has some things in common with alkenes including 1,2 addition processes, but they tend to be more reactive!

This would not go without a catalyst for an alkene

Reactivity of Disilenes: 2+2 cycloadditions

Unlike simple alkenes, disilaethenes readily

undergo a variety of 2+2 cycloadditions

Formation of Ge=Ge and Sn=Sn

An electron transfer method (reduction) can be used for formation of Ge=Ge (and Sn=Sn) bonds. (This also occurs for Si)

Ar2ECl2 + LiNap

Ar2E EAr2

Ar2E Ar2E EAr2h+ naphthalene

Another method is to exchange ligands with a stable germylene. If the new ligand is less bulky, formation of the dimer occurs:

((Me3Si)2N)2Ge: + LiR R2Ge=GeR2 + LiN(SiMe3)2

Reactivity of Disilylenes

Although the relatively weak Si=Si bond can be easily opened to form rings and disubstitutions, disilylenes never react like silylenes (insertion into bonds:

R2Si=SiR2 + X2 R2XSi-SiXR2

R2Si=SiR2 + HX R2XSi-SiHR2

The closest reactivity to insertion is the reaction with oxygen, which goes by a coordination pathway:

R2Si SiR2 + O2

R2Si SiR2

O

O

R2Si SiR2

O OR2Si

O

O

SiR2

Double Bonds to Carbon

From a synthetic standpoint, it is interesting to have double bond to carbon that could be exploited for C-C bond formation.

The first work was done by gas phase thermolysis to produce a highly reactive species.

Me2Si

H2C

CH2

CH2 H2C CH2 H2C SiMe2+

Me2Si

H2C

SiMe2

CH2

Because of the difference in electronegativity, it is common to consider a silaethene in equilibrium with a charge separated species

H2C SiMe2 H2C SiMe2

Stable Silaethenes

Stable silaethenes can be made by exploiting the electron-withdrawing power of the carbon substituent, and by increasing steric bulk, to prevent dimerization:

O

R(Me3Si)3Sih

OSiMe3

(Me3Si)2Si R

Si

Si

Me3Si

R

Me3Si

OSiMe3

R

Me3SiOSiMe3

Me3Si

This head-to-head dimerization occurs when R = Me, but not when R = CEt3, adamantyl

Stable Silaethenes

The silaethene Me2SiC(SiMe3)(SiMetBu2) is made from an intermolecular metathesis:

Me2Si

LiCl

SiMetBu2

SiMe3

Me2Si

SiMe3

SiMetBu2

+LiCl

Si-C = 1.702 ATwist of 1.6 deg.

Stable Silaethenes

This is stabilized by methyl exchange in a four-centered methyl exchange:

SiMetBu2

SiMe3Me2Si

SiMetBu2

SiMe2Me2SiMe

SiMetBu2

Me3Si SiMe2

Of course, this exchange can happen with the SiMetBu2 moiety as well, but is less stable:

SiMetBu2

SiMe3Me2Si

SiMetBu2

Me3Si SiMe2

SitBu2

Me3Si SiMe3

Stable Silaethenes

A similar route to silaallene:

SiMes*(Ad)FiPr

MeO

Pri

Br

iPr

OMe

iPr2 BuLi

-LiBr, LiF CSi

Ad

*Mes

Pri

Pri

OMe

iPr

OMe

iPr

GermaethenesGe=C bonds can also be stabilized by lithium halide elimination:

Me2Ge

LiX

SiMe3SiMe3

Me2Ge

SiMe3

SiMe3

+ LX

The germaethene is more stable than the silaethene due to the difference in electronegativity, but is more labile due to the weaker double bond.

This gives it a broader range of reactivity.

Element- Element Double BondsTrans-bent configurationThe angle increase and the H-E-H angle decreases with increasing atomic numberThe ratio of the double to single bond length also increases with increasing atomic number.

Types of R4E2 Structures

All Sn and Pb along with some Ge and Si dissociate into type F- the inert pair effect

Energetic differences between C and D are small esp. for Si

Kinetic and thermodynamic stabilization through substituent design

Structural types of R4E2

Transition Metal-E Double BondsAnalogues of carbenes – variation of substituents and metal offers wide range of species

Description of bonding

Transition Metal-E Double Bonds

• short tungsten-germylene bond of 2.4590(16) Å vs. 2.667(3) Å• planar geometry around the germanium atom of the germylene ligand suggesting sp2 hybridization

Organometallics 1999, 18, 4468-4470

Transition Metal-E Double Bonds

Description of bonding

Several complexes with K structure are known – supports zwitterionic formalism (structure H)

The First Stable CyclotrisileneThe silicon analogue of cyclopropene:

3 in 65% yield together with tris(tertbutyldimethylsilyl)chlorosilane as the sole byproduct

Iwamoto, J. Am. Chem. Soc. 1999, 121, 886

A Si Bow-TieNo carbon analogue to spiropentasiladiene:

Dark redThermally stable to 215C!Two rings slightly twisted from 90Non-planar Si=Si

Iwamoto, SCIENCE 290, 2000, 505.