Chemistry of carbenes, silylenes, carbones and silylonesweb.iitd.ac.in/~elias/links/CML 738...
Transcript of Chemistry of carbenes, silylenes, carbones and silylonesweb.iitd.ac.in/~elias/links/CML 738...
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)?