Chemistry of carbenes, silylenes, carbones and silylones

For more details refer chapter 5 and 6 of the book‘The Chemistry of the p-Block elements: Syntheses, reactions and applications

Chemistry of carbenes and carbones

Carbenes are divalent carbon compounds

which are generally highly reactive organic

intermediates with six valence electrons

with the general formula R2C or R1R2C.

Carbenes traditionally were so reactive that

they were only considered as reaction

intermediates or transition states and were

only indirectly studied, often by trapping

them in the presence of suitable reagents.

The initial emphasis was also in

making stable metal complexes of

carbenes such as Fischer and Schrock

carbenes

Carbenes are generally classified as belonging to two types—triplet and singlet. In

singlet carbenes the two electrons are paired and the molecule is diamagnetic, while for

triplet carbenes the two electrons are unpaired and the molecule is paramagnetic.

Carbenes can be either linear or bent. Linear geometry indicates sp hybridisation with

two nonbonding and degenerate px and py orbitals while bent geometry indicates sp2

hybridisation where the py is unchanged and is called pπ and the px gains stability and

some s character and is termed . Most carbenes are bent and their frontier orbitals are

and pπ. The four possible electronic states for carbenes are given below

Singlet versus triplet carbenes

Between singlet and triplet carbenes, singlet carbenes are relatively more stable and some of them

have been isolated and stored under inert atmosphere for indefinite periods of time.

Persistent triplet carbenes

Diazo compounds are usually used as precursors for preparing triplet carbenes by photolysis

with UV radiation or by using a XeCl excimer laser. The diradical nature of these

compounds makes them extremely reactive and difficult to isolate. They can be trapped with

oxygen to convert them to ketones. Triplet carbenes are generated at low temperature and are

characterised by spectral techniques which identify radicals such as ESR spectroscopy. They

are generally very short-lived with lifetimes of a few seconds especially in solutions, but

some of them are stable for extended periods in the solid phase. Example shown below is a

rare example stable for about a week in dilute benzene solution.

Unlike singlet carbenes, these species do not react with carbon–halogen bonds and their

stability was found to increase when they have bromo or trifluoromethyl groups in the

vicinity of the carbene carbon.

Persistent singlet carbenes: N-Heterocyclic carbenes

In 1960 H-W Wanzlick proposed that carbenes obtained from dihydroimidazol-

2-ylidene can be generated by the vacuum pyrolysis of 2- trichloromethyl

dihydroimidazole derivatives with the release of chloroform. He showed that the

free carbene reacted with electrophiles but believed that they will dimerize and

an equilibrium exist between the monomer and the dimer with dimer being more

stable. However two such dimers with different N substituents when heated

together were not found to exchange the individual carbene units. Thus

Wanzlick did not get the credit for making the first stable carbene. Later it was

shown that Wanzlik was right.

Anthony J Arduengo, in 1991 prepared an isolable and bottleable NHC, 1,3-

Diadamantyl imidazol-2-ylidene which was the first in a series of such stable N-

heterocyclic carbenes. This crystalline compound was obtained in near

quantitative yield by the deprotonation of 1,3-di-1-adamantylimidazolium

chloride with sodium or potassium hydride in the presence of catalytic amounts

of a base such as potassium tert-butoxide or the dimethyl sulfoxide anion.

Hans-Werner Wanzlick

Anthony J

Arduengo III

NHCs are in general defined as heterocycles containing a carbene carbon with at

least one nitrogen atom as part of the ring. They generally have sterically hindered

substituents on the atom adjacent to the carbene carbon which helps to provide

kinetic stability. In the traditional NHCs, such as the one prepared by Arduengo, the

HOMO has been described as a formally sp2 hybridised lone pair and the LUMO

has been described as an unoccupied p orbital on the carbene carbon. The nitrogen

atoms of this heterocycle are -electron withdrawing and -electron donating thus

providing inductive and mesomeric stabilisation for the carbene. The cyclic

structure also stabilises and favours a singlet ground state by forcing the carbene

carbon into a bent sp2-like arrangement. The stability of singlet carbenes results

mostly from electronic effects [mesomeric (+M) as well as inductive (–I) effects],

while steric bulkiness of the substituents also contributes significantly

Property N-Heterocyclic carbenes Phosphines

Stability towards water Hydrolyses rapidly Not sensitive to degradationin deoxygenated water,especially aryl phosphines

Stability towards oxygen Does not react directlywith O2; requires specialreagents for oxidation

Highly reactive towardsoxidation especially with O2

Donation of electrons Better -donors thaneven alkyl phosphines

Some alkyl phosphines aregood -donors

Range of electron donorability

Narrow range Wide range

Steric bulkiness based on%Vbur

Generally stericallyhindered

Steric bulkiness depends onsubstituents on phosphorus

Shape as ligand in themetal- bound form

Fan like Cone shaped

Metal–ligand bondstrength

Stronger bonds, noteasily displaced

Not as strong as NHCs; canbe displaced during reactions

Ligand stability inoxidation reactions

Stable Unstable—phosphine ligandsare prone to oxidation

In 2010 Nolan made a comprehensive comparison using around 700 X-ray crystallographic data of phosphines and NHC complexes especially using AuCl as the metal unit and compared the steric properties. He defined %Vbur simply as the percent of the total volume of a sphere occupied by a ligand when bound to a metal. The volume of this sphere represents the potential coordination sphere space around the metal occupied by a ligand/ligand fragment. The sphere has a defined radius (which was fixed at 3.50 Å

Ipr* : Me groups of Ipr replaced by Ph groups

The two most well known types of diamino carbenes are—those with unsaturation in the

five-membered ring (imidazol-ylidenes) and those with a saturated five-membered ring

(imidazolidin-ylidenes). While NHCs with saturated rings (imidazolidinylidenes) are

easily dimerised in the absence of sterically hindered substituents, those with

unsaturation in the ring (imidazolylidenes) do not undergo dimerisation even in the

absence of sterically hindered groups.

Stable against dimerization

The efficiency of many existing catalysts was improved by introducing NHCs as ligands.

For example, Grubbs 2nd generation catalysts for olefin metathesis and PEPPSI catalyst for

cross coupling reactions are better than other related catalysts due to the fact that the better

-donor ability of the NHCs over phosphines and amines helps in the labilisation of

ligands trans to them (trans effect). In addition, the steric bulk of the NHCs provide kinetic

stability to the metal centre making it easy to handle the complex under air and moisture.

Many examples of Ag(I) and Au(I) NHC complexes have also shown promise as

antibacterial and anticancer agents.

NHCs are found to be extremely useful in stabilising unusually low oxidation

states of many main group elements such as phosphorus, silicon and boron. They

are also used for stabilising hitherto unknown multiple bonds between p-block

elements. They are also found to be useful as organocatalysts.

New types of singlet carbenes

After the development of saturated and unsaturated NHCs possessing two ring nitrogens,

chemists expanded the scope of NHCs by introducing more nitrogens, other main group

elements and carbon as part of the five-membered heterocycles as well as by bringing in

acyclic carbenes

Among the new type of carbenes, cyclic alkyl amino carbenes (cAACs) introduced first by

Bertrand in 2005 have shown better donor properties than NHCs. cAACs are carbenes

where one of the σ-withdrawing and π-donating nitrogen atoms of the NHC has been

replaced by a σ-donating quaternary carbon atom with a -donating alkyl group. By this

replacement, the carbene center has been made relatively more nucleophilic (-donating)

and electrophilic (π-accepting) compared to the NHCs.

A comparison of the CO stretching frequency of the rhodium carbonyl complexes of

(NHC)Rh(CO)2Cl (2039 cm–1) and (cAAC)Rh(CO)2Cl (2036 cm–1) indicate the relatively better -

donating capability of the cAACs. Computational studies have also indicated that the HOMOs of

the cAACs are slightly higher in energy and the singlet–triplet gap is smaller than that of the

NHCs. Similar properties have been reported for acyclic alkyl amino carbenes (aAACs) as well.

Abnormal NHCs (aNHC) where the carbene carbon is no longer located between two nitrogens

have also been predicted to be more basic than normal NHCs based on computational studies

Carbodicarbenes (Carbones) or bent allenes ?

A classical allene Recent theoretical and experimental studies have

revealed a class of divalent carbon(0) compounds

CL2 that exhibit peculiar bonding and chemical

reactivity clearly distinguishable from carbenes CR2.

These compounds have been designated as carbones

and may be viewed as donor–acceptor

complexes L→C←L between a bare carbon atom in

the excited 1D state and two σ‐donor ligands L. In

contrast, carbenes involve two electron‐sharing bonds

between the substituents R and a ground‐state 3P carbon

Gernot Frenking

Dative Bonds in Main-Group Compounds: A Case for More Arrows!” G. Frenking, Angew. Chem. Int. Ed. 53, 6040 (2014)

Carbodiphosphorane

1960 - 2017

A changeover from a dative bond in NHC→SiCl2 to electron sharing (covalent) bond in

SiCl2(cAAC)2 which means that the complex has converted from a singlet silylene to a

stable biradical. The presence of a biradical was ascertained by magnetic studies and ESR

spectroscopy. The SiCl2 bound biradical on reduction with KC8 was found to result in a

silylone.

Experimental proof for showing that silylones have silicon in zero oxidation state

Stable silylenes [silicon (II) compounds]

A significant difference between silylenes and germylenes on one hand and traditional

methylenes on the other hand is the difference in the multiplicity of their ground states.

Methylene is a ground state triplet while silylene and germylene are ground state singlets. In

the case of methylene its singlet state lies 9.0 kcal/mol higher in energy. This reversal in the

multiplicity of the ground state on going from methylene to silylene has a significant effect on

the chemistry of silylenes compared to methylenes. The HOMO–LUMO gap is also larger in

SiH2 (52 kcal/mol) than in the case of CH2 (27 kcal/mol). For SiH2, the energy gained in

forming a triplet configuration is not sufficient to compensate for the large HOMO–LUMO gap

and therefore SiH2 favors singlet configuration.

A different story for silicon bromides

Silanonecompounds prepared recently having Si=O but no three coordinate silicon

Silanones are some of the most sought after compounds of silicon. Unlike ketones,silanones are very highly unstable and reactive. This is due to (a) a weak bondhaving unfavorable overlap between p (Si) and p(O) orbitals and (b) a stronglypolarized Si +--O- bond.

A stable silanone [Cr-(Si=O)-C type ]with a three co-ordinate silicon atom:

Road to an ideal silanone [C-(Si=O)-C type ] : where have we reached so far (till 2015)?