The Nature of N - Heterocyclic Carbenes COPYRIGHTED MATERIAL · 2020-02-16 · The Nature of N-Heterocyclic Carbenes 13 N N S R R N N R R: S Cl Cl NHR NHR KC8 S H2N H2N O OH R Figure

Functionalised N-Heterocyclic Carbene Complexes. Olaf Kühl

© 2010 John Wiley & Sons, Ltd.

1 The Nature of N - Heterocyclic

Carbenes

There are essentially three different types of transition metal carbene complexes featur-

ing three different types of carbene ligands. They have all been named after their fi rst

discoverers: Fischer carbenes [ 27 – 29 ], Schrock carbenes [ 30 , 31 ] and Wanzlick – Arduengo

carbenes (see Figure 1.1 ). The latter, also known as N - heterocyclic carbenes (NHC), should

actually be named after three people: Ö fele [ 2 ] and Wanzlick [ 3 ], who independently syn-

thesised their fi rst transition metal complexes in 1968, and Arduengo [ 1 ] who reported

the fi rst free and stable NHC in 1991. Fischer carbene complexes have an electrophilic

carbene carbon atom [ 32 ] that can be attacked by a Lewis base. The Schrock carbene

complex has a reversed reactivity. The Schrock carbene complex is usually employed in

olefi n metathesis (Grubbs ’ catalyst) or as an alternative to phosphorus ylides in the Wittig

reaction [ 33 ].

The NHC are different from both other types. They form transition metal complexes

that are essentially inert, although exceptions are known. Their exceptional stability

derives from their intrinsic stabilisation from the N - C p π - p

π bonding interaction of the

nitrogen atoms fl anking the carbene carbon atom. The p orbital of the carbon is thus

partially fi lled and no longer available for nucleophilic attack. Also, it does not have a

lone pair of its own with which it could possibly engage in a nucleophilic interaction of

its own. Hence, it remains unreactive taking the middle ground between the opposing

reactivities of Fischer and Schrock carbenes. It is this apparent lack of reactivity that

meant that the transition metal NHC complexes received little attention in their early

history.

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COPYRIG

HTED M

ATERIAL

8 Functionalised N-Heterocyclic Carbene Complexes

1.1 Synthesis

The easiest way to arrive at a NHC or its complexes is to use an appropriately substituted

azolium salt as the starting material [ 18 ]. As many of these imidazolium salts may not be

commercially available or may prove to be too expensive, diffi cult to store or too dry, it is

best to be familiar with their synthesis.

Although arguably the most popular route to NHC is via the azolium salt, there are

many other options for their synthesis. An excellent and comprehensive overview of the

synthesis of NHC and some of their complexes can be found in Hahn [ 34 ]. An interesting

route to tri - and tetracarbenes is provided by the template controlled cyclisation of isocya-

nides, a method pionered by Lappert and coworkers [ 35 , 36 ] (see Figure 1.2 ).

1.1.1 Synthesis of the Imidazolium Salts

There are two general methods to generate the required imidazolium salt: (i) substitution

on the imidazole ring; and (ii) synthesis of the imidazole ring with the substituents already

in place [ 18 ].

R

R MLn

Fischer

N

N

MLn

X

X

Wanzlick-Arduengo

R

R MLn

Schrock

Figure 1.1 Bonding in Fischer, Wanzlick – Arduengo and Schrock carbene transition metal complexes .

M

N

N

RH

H

EtOOEt

H

H

M

NH

N

R

4

4

MX2

H2N

OEt

OEt

C N R H

�EtOH

��

� ��

Figure 1.2 Template synthesis of transition metal NHC complexes using isocyanides .

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The Nature of N-Heterocyclic Carbenes 9

(i) Substitution on the imidazole ring

The fi rst substituent is usually introduced on the nitrogen atom by reacting potassium

imidazolide with an alkyl halide to obtain the 1 - alkylimidazole [ 37 ]. The second substitu-

ent is subsequently introduced by reacting the 1 - alkylimidazole with a second equivalent

of a (different) primary alkyl halide (see Figure 1.3 ).

Advantage : Two different substituents can be introduced and thus unsymmetrically substituted NHC can be obtained using this route .

Disadvantage : The method is limited to primary alkyl halides as secondary and tertiary alkyl halides are subject to unwanted elimination reactions .

(ii) Synthesis of the imidazole ring

When substituents other than primary alkyls are required on the nitrogen atoms of the

imidazole ring, the imidazole ring has to be synthesised from glyoxal, formaldehyde and

a primary amine in the presence of an acid [ 38 ] (see Figure 1.4 ). Now, all substituents

are available as long as pendant functional groups do not interfere with the ring forming

reaction.

Advantage : Most symmetrically N,N ’ - disubstituted imidazolium salts can be synthe-sised using this method .

N

KN

RX�KX

N

N

R

XCH2R'

N

N

R

R'

H�

X�

Figure 1.3 Synthesis of an imidazolium salt starting from imidazole .

O O

O

RH2N

N

N

R

H

R

RNH2 HX

X�

�

Figure 1.4 Synthesis of an imidazolium salt via the assembly route .

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Advantage : By changing the acid, the anion of the imidazolium salt can be chosen [ 39 ]. Thus, halides can be avoided or iodide introduced depending on further requirements .

Disadvantage : Only symmetrically N,N ’ - disubstituted imidazolium salts are accessible .

For the synthesis of unsymmetrically N,N ’ - disubstituted imidazolium salts, method

(ii) has to be modifi ed. This is achieved by using 1 equiv. of ammonium chloride and 1 equiv.

of the primary amine resulting in the formation of N - substituted imidazole. Subsequent

reaction with a primary alkyl halide [method (i)] yields the desired unsymmetrically N,N ’ -

disubstituted imidazolium salt [ 40 , 41 ] (see Figure 1.5 ).

Note : This method is limited in its substitution pattern regarding the second substitution by the same disadvantage as method (i) .

With the knowledge of the two principle methods of synthesis, we can now turn our atten-

tion towards modifi cations of the ring itself and the substituents on it. There are a range of

different NHC available that fall within the range of carbenes covered by this book. These

include saturated and unsaturated, annulated and chiral NHC and NHC whose carbene

carbon atom is not part of a fi ve - membered ring [ 36 ] as well as NHC with heteroatoms in

positions 4 and 5 of the imidazole ring [ 42 ] (see Figure 1.6 ).

1.1.2 Closing the Ring

A very important and versatile method en route to a NHC is the ring closure reaction with

ortho - formate [ 18 , 34 , 36 , 43 ] (see Figure 1.7 ). Here a suitable diamine is reacted with a

triester of formic acid resulting in a N=CH - N group that can be deprotonated to form

a carbene.

Advantage : Carbenes based on four - , fi ve - and six - membered rings become available .

Advantage : A broad range of substitution patterns on the ring are accessible .

The method is particularly suitable for NHC that have ‘ uncommon ’ substituents in the

4,5 - position [ 44 ], an unusual scaffold [ 45 , 46 ], annulated rings [ 47 – 49 ], are saturated [ 50 ]

or have readily available diamino precursors [ 51 , 52 ].

O O

O

NH4XR

H2N

N

N

R

H

N

N

R

H

C H2R '

X

R'CH2X

X�

�

Figure 1.5 Synthesis of an unsymmetrically substituted imidazolium salt via the imidazole assembly route.

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1.1.3 Synthesis of the Free Carbenes

The free carbene is usually generated from the imidazolium salt precursor by reaction

with a strong Br ø nstedt base like potassium (sodium) hydride [ 53 ], potassium tert - butylate

(with catalytic amounts of DMSO) [ 54 ], MN(SiMe 3 )

2 (M = Li, Na, K) [ 55 ] or BuLi [ 56 ].

An elegant method to generate the free carbene is the reaction of the imidazolium salt

precursor with sodium hydride in liquid ammonia. In contrast to other solvents, liquid

ammonia dissolves both, the imidazolium salt and the base (NaH) providing a medium

for smooth, effi cient deprotonation and carbene formation in high yields [ 57 , 58 ]

(see Figure 1.8 ).

N

N

R

R

:

N

N

R

R

:

N

N

R

R

:

N

N

R

R

:

N

N

:

R'

R'

*

*

*

*

R'

R

R'

R

B

BN

N

R

R

:

N

N

R2N

R2N

R

R

:

Figure 1.6 Examples for different NHC frameworks.

NH

PPh2

PPh2

boc

N

PPh2

boc

HN Mes

PPh2

H2NNH2 Mes

NN

Mes

BrCH2CH2NHMes

HCl

HC(OEt)3

��

�

Figure 1.7 Ring closure to an imidazolium salt with triethyl - orthoformate .

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Advantage : Many transition metal carbene complexes can only be synthesised using the free carbene. The method is not limited by suitable anions or oxidation states associ-ated with the transition metal compound used as starting material .

Disadvantage : The strong bases used are incompatible with many functional groups in the pendant sidechains .

Disadvantage : Several transition metal carbene complexes can be synthesised directly from the imidazolium salts without the need to isolate the free carbene .

An alternative to the imidazolium salts as starting materials is the elimination of an alco-

hol from 2 - alkoxy - 1,2 - dihydro - 1 H - imidazoles that are accessible by reaction of vicinal

diamines with ortho - esters of formic acid [ 59 – 61 ] (see Figure 1.9 ).

N

N

R

R

N

N

R

R

:H

X

NH3 (1)/NaH

�NaX/H2

�

�

Figure 1.8 Deprotonation of an imidazolium salt with sodium hydride in liquid ammonia.

NH

NH

R

R

HC(OEt)3

N

N

R

R

N

N

R

R

N

N

R

R

H

OMeN

N

R

R

:

NaOMeMeOH ΔT

�

�

Figure 1.9 HNC via alcohol thermolysis – a similar method to triethyl - orthoformate.

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N

N

S

R

R

N

N

R

R

:

S

Cl

Cl

NHR

NHR

�

KC8

S

H2N

H2N

O

OH

R

R

�

Figure 1.10 Synthesis of a free NHC via the thione route.

Disadvantage : Thermal decomposition of the 2 - alkoxy - 1,2 - dihydro - 1H - imidazole often leads to dimerisation of the NHC and the corresponding tetraaminoethylenes are formed instead .

There are of course a range of methods of less general application. One that is of con-

siderable importance is the reduction of imidazol - 2 - thiones, accessible from the reaction of

thioureas and α - hydroxyketones or alternatively o - phenylene diamines and thiophosgene

[ 62 ] (see Figure 1.10 ).

Disadvantage : The reduction of the thione often fails or the C=S group is directly reduced to a methylene carbon rather than a carbene .

1.1.4 Synthesis of Transition Metal Complexes of NHC

Today, transition metal complexes of NHC are mainly formed using four methods:

(i) reaction of a transition metal complex with a free carbene (preformed or generated in situ );

(ii) reaction of an imidazolium salt with a transition metal complex possessing a basic

anion or entity; (iii) by using a carbene transfer agent; and (iv) by reacting an imidazolium

salt with a transition metal salt in the presence of a weak base (see Figure 1.11 ).

(i) Reaction of a transition metal complex with a free carbene

Deprotonation of an azolium salt with a strong base renders the free carbene which

can then be reacted with a suitable transition metal complex to yield the transition metal

carbene complex. In many cases, the NHC is not isolated, but prepared in situ prior to add-

ing the metal complex.

Advantage : The choice of transition metal complexes as starting materials is far greater than with any other method, save that using carbene transfer agents .

Disadvantage : The choice of carbene is limited by the tolerance of functional groups towards strong bases .

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(ii) Reaction of an imidazolium salt with a transition metal complex possessing a basic

anion or ligand

The deprotonation of an azolium salt to form a carbene requires a base. This base can

be supplied as the anion of a transition metal compound, in which case the azolium salt is

deprotonated in situ and the carbene formed coordinates to the metal generating the NHC

transition metal complex. This method works best if a coordinating anion, such as bromide

or iodide, is supplied with the azolium salt.

This method was successfully applied using acetates [ 64 , 65 ], acetylacetonates [ 66 , 67 ],

alkoxides [ 16 , 68 , 69 ] and [Pd 2 (dba)

3 ] [ 70 ]. In fact, the fi rst NHC transition metal com-

plex reported by Wanzlick and Sch ö nherr in 1968 [ 3 ] used mercury(II) acetate and 1,3 -

diphenylimidazolium perchlorate as the starting materials.

An interesting variant is the in situ preparation of transition metal alkoxides from

the corresponding halogenides and subsequent reaction with an azolium salt to form the

NHC transition metal complex [ 69 ]. This works particularly well with rhodium, iridium

and ruthenium where [( η 4 - cod)MCl] 2 (M = Rh, Ir) and [Cp*RuCl]

2 are readily available

[ 57 , 58 , 71 ].

Advantage : A mild method to prepare NHC transition metal complexes in high yields .

Advantage : Transition metal complexes with chelating bis - carbenes featuring acidic methylene protons in the linker unit can be readily prepared, when the isolation of the free carbene using strong bases would be far more diffi cult .

Pd

I

I N

NN

N

R

R R

R

N

N

R

R

:2� [Pd(cod)I2]

N

N

R

R

� Pd(OAc)22

N

N

R

R

2� Ag2O

N

N

R

R

AgI

[Pd(cod)I2]

2N

N

R

R

2

PdI2K2CO3

�

�

�

Figure 1.11 Synthesis of transition metal NHC complexes using basic transition metal complexes.

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Disadvantage : When the azolium salt does not contain a coordinating anion, this anion has to be provided by adding LiI or a similar salt .

Synthesis using a transition metal hydride

One of the fi rst publications of a NHC transition metal complex, published by Ö fele in

1968, was the reaction of an imidazolium salt with [HCr(CO) 5 ] to form [Cr(Me

2 Im)(CO)

5 ]

with the formation of hydrogen [ 2 ]. The method can also be used with other transition

metal hydrides, such as [IrH 5 (PPh

3 )

2 ] [ 63 ].

Advantage : There are no purifi cation problems as the elimination product, hydrogen, is a gas .

Disadvantage : There is only a limited number of transition metal hydride complexes available as starting material .

Disadvantage : The method leads to the formation of a salt, with a cationic metal carbene complex, unless the anion of the azolium salt can coordinate to the metal .

This is a disadvantage because cationic transition metal NHC complexes are subject

to carbene decomposition pathways, unless they are stabilised as a chelating bis - carbene

[ 21 , 22 ].

The silver(I) oxide method

One of the most generally used methods to prepare a NHC transition metal complex

is the reaction of an azolium salt with silver oxide to form the silver carbene complex

[ 24 , 25 , 72 ]. It is so general that it has its own name, the silver(I) oxide (Ag 2 O) method

[ 25 , 26 ]. Of course, oxide is a base and thus it falls under the heading of reactions with basic

transition metal compounds, but the silver carbene complexes are usually only synthesised

because the silver atom coordinates only weakly and can easily be replaced by another

metal of choice. It is therefore known as a carbene transfer agent.

Advantage : The silver carbene complex can usually be synthesised using undried solvents and with exposure to air .

Advantage : The method is compatible with most functional groups .

(iii) Synthesis using a carbene transfer agent

Silver carbene complexes are the most commonly used carbene transfer complexes [ 83 ].

Other carbene transfer agents include lithium adducts [ 56 ], potassium complexes [ 53 ], molyb-

denum carbene complexes [ 83 , 84 ] or chromium carbene complexes [ 85 ].

(iv) Reaction of an imidazolium salt with a transition metal salt in the presence of a

weak base

The concept of Wanzlick (and Ö fele) to react the imidazolium salt with a basic transition

metal complex can be modifi ed – and generalised – by separating the base and the transition

metal complex. In this case, an equilibrium between the imidazolium salt and its deproto-

nated form, the carbene, is established. Although the equilibrium is very much on the side

of the imidazolium salt, by far the weaker conjugate acid, the reaction is shifted towards

the carbene by strong coordination of the NHC ligand to the transition metal. Examples for

weak bases used in this context include Na 2 CO

3 [ 73 ], K

2 CO

3 [ 74 – 77 ], Cs

2 CO

3 [ 78 ], NEt

3

[ 79 , 80 ], pyridine [ 76 ] and NaOAc [ 81 , 82 ].

(v) Other Methods

There are two standard methods to generate transition metal NHC complexes that origi-

nate from the times when stable free carbenes were not accessible and thus the NHC had

to be generated within the coordination sphere of the metal. These are the reaction of suit-

able transition metal complexes with electron - rich tetraaminoethylenes [ 4 , 18 , 86 , 87 ] and a

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template reaction using isocyanides as the building block for the imidazole ring [ 88 – 90 ]

(see Figure 1.12 ). Today, they are all but displaced by modern alternatives, although for

cyclic tetracarbenes the isocyanide route is still the method of choice [ 35 , 91 , 92 ].

1.2 Properties of NHC

1.2.1 The Internal Electronic Structure

The stability of NHC is often described in terms of the singlet – triplet gap which is the

energy difference between the singlet ground state and the triplet excited state [ 16 ]

(see Figure 1.13 ). When this difference is small, then there is a strong tendency for the

NHC to dimerise forming tetraaminoethylenes. The stable NHC have typically singlet –

triplet gaps above 65 kcal mol – 1 [ 11 , 93 ]. What is the reason for this stability and how is the

carbene centre stabilised electronically?

The fi rst stable NHC ever isolated was 1,3 - diadamantyl - 1 H - imidazol - 2 - ylidene, an

unsaturated carbene with bulky substituents on the nitrogen atoms [ 1 ] (see Figure 1.14 ). It

N

NN

N

R R

RR

� [Fe(CO)5]Fe

OC CO

COCO

N

N

NNRR

R

R�2CO

CrOC CO

CO

CO

CO

HN

NHN

But

Ph

Ph

PhCHO�[PhNH3]Cl�ButNC�[NEt4][(NC)Cr(CO)5]�H2O�[NEt4]Cl

�

Figure 1.12 Transition metal NHC complexes via thermolysis of electron - rich olefi ns or iso-cyanide template synthesis .

N

N

N

N

�E

Singlet Triplet

Figure 1.13 The singlet – triplet gap.

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was thought that steric and electronic factors contributed to its stability and its reluctance

to dimerise. It is easy to imagine that the bulky adamantyl substituents would prevent the

two carbene carbon atoms from approaching each other and forming a C=C double bond.

It is equally easy to count the 6 π - electrons (two from the C=C double bond and four

from the two nitrogen lone pairs) and assume that there is aromaticity due to a delocalised

6 π - electron system [ 10 , 11 , 94 ].

However, the isolation of N,N ’ - dimethyl substituted NHC [ 54 ] and the fi rst satu-

rated NHC, 1,3 - dimethyl - 1 H - imidazolin - 2 - ylidene by Arduengo et al . in 1995 [ 95 ]

cast serious doubt as to the validity of the need for steric or aromatic stabilisation in

these carbenes. Two independent theoretical studies by Boehme and Frenking [ 10 ] and

Heinemann et al . [ 11 ] in 1996 as well as a later one by Tafi polski et al . [ 94 ] gave an

excellent account of the stabilising factors and the differences between saturated and

unsaturated NHC.

The fi rst question that needed answering was whether the unsaturated NHC are indeed

aromatic. The H ü ckel rule states that aromatic systems are monocyclic, homonuclear, pla-

nar and possess a delocalised 4n+2 π - electron system [ 96 ]. Unsaturated NHC fulfi l all

the criteria except homonuclearity, which would make them heteroaromatic compounds.

However, there was doubt as to the delocalisation of the 6 π - electron system. Although

there are 6 π - electrons, their distribution is extremely unequal, the backbone carbon atoms

C 4 and C 5 contribute one π - electron each, the nitrogen atoms provide a lone pair each,

but the carbene carbon atom C 2 gives none. Can there be a true delocalisation in such

circumstances?

N

N

:

N

N

:

N

N

:

Figure 1.14 A series of the fi rst three frameworks of free carbenes: steric protection, sterically unprotected unsaturated NHC, sterically and electronically unprotected, saturated NHC.

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Delocalisation of the 6 π - electron system in unsaturated NHC would have consequences

in the thermodynamic, structural and magnetic data and the charge density within the

p orbitals of the ring atoms [ 11 ].

From thermodynamic considerations one can learn that the main contribution to the

stability of the NHC comes from a combination of the large electron - withdrawing effect of

the electronegative nitrogen atom on the σ - electrons of the C - N single bond paired with a

π N - π

C backdonation via the p orbitals [ 97 ] (see Figure 1.15 ). The presence of a C=C double

bond in the backbone provides additional thermodynamic stabilisation of at least 20 kcal

mol – 1 [ 11 ].

The structural analysis shows that in the unsaturated NHC the C 4 - C 5 and N 3 - C 4 bonds

are shorter but the C 2 - N bonds longer than in the unsaturated NHC, as is expected in a

delocalised 6 π - electron system. The N 1 - C 2 - N 3 bond angle at the carbene carbon atom

is larger in saturated than in unsaturated NHC corresponding to a higher percentage of

s character for the C 2 - N bonds of saturated compared with unsaturated NHC [ 10 ]. Of

course, the differences in the C 4 - C 5 and N 3 - C 4 bond lengths are as expected for the intro-

duction of a formal C=C double bond, but the relative lengths of the C 2 - N bonds need

further explanation. The difference does not lie in a stronger p π - p

π contribution for the satu-

rated carbene, but in the fact that the larger s character in the hybrid orbitals of the carbene

carbon atom of the saturated NHC results in a larger Coulomb attraction between N and

C 2 [ 10 ].

The anisotropy of the magnetic susceptibility Δ χ is regarded as a criterion for an

aromatic ring current [ 11 ] and it was found that magnetic anisotropy for unsaturated

NHC is only slightly lower than for imidazole, but much greater than for saturated NHC

[ 10 ]. The effect is signifi cantly smaller even for unsaturated NHC than for typical aro-

matic compounds like benzene, but shows that unsaturated NHC possess partial aromatic

character.

A charge density analysis reveals that the carbene carbon atom of the unsaturated

carbene has a higher absolute and relative p π

(C 2 ) occupancy than the saturated counterpart

[ 10 ] indicating a larger π delocalisation for the unsaturated NHC.

Note : The π interactions ‘ undoubtedly enhance the stability of the unsaturated compounds over the saturated analogues ’ [ 95 ].

N

N

N

N

N

N

�C-�N electron withdrawal �N-�C back donation �C-�N electron withdrawal�N-�C back donation

Figure 1.15 Electronic stabilisation of NHC .

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Note : ‘ The electron donation of the neighbouring lone pairs is suffi cient (but necessary and still dominating) and ... the delocalisation in [unsaturated NHC] provides merely an additional but not a necessary thermodynamic stabilisation ’ [ 10 ].

Note : The stability of NHC is electronic rather than steric in nature .

We now know what a monocyclic carbene on the basis of an imidazole ring looks like

electronically. We can safely assume that carbenes derived from triazolium and even

tetrazolium salts have similar principal characteristics. However, does this hold true for

carbenes synthesised from benzimidazolium salts and other annulated NHC? Could it be

that a benza imidazol - 2 - ylidene has an electronic structure that consists of a benzene ring

and a N - C - N allyl system with 4 π - electrons (see Figure 1.16 ). The answer is not a clear

‘ yes ’ or ‘ no ’ decision. The truth is that one trend for annulated carbenes goes in the direc-

tion of saturated or even acyclic carbenes [ 98–100 ]. Annulation usually results in the desta-

bilisation of the carbene centre and it becomes increasingly more diffi cult to isolate the free

carbene [ 47 , 50 ] if the annulated ring system is extended from benzene to naphthene and

phenanthrene [ 101 , 102 ]. The incorporation of nitrogen atoms in the annulated ring system

usually enhances the destabilising effect.

The destabilisation of the carbene by the effect of annulation results in a smaller

singlet – triplet gap. 1,3 - Dineopentyl - 1 H - benzimidazol - 2 - ylidene, an annulated carbene

with intermediate singlet – triplet gap, shows an interesting equilibrium between the mono-

meric free carbene and the dimer form, a tetraaminoethylene [ 103 ] (see Figure 1.17 ).

10-� electron system 6-� electron system � N-C-N allyl system

N

N

N

N

Figure 1.16 6 (10) π - electron system or 2 (6) π - electron + N - C - N allyl system .

ButBut

But But

ButBut

But But

::N

NN

NN

NN

N

Figure 1.17 Equilibrium between electron - rich olefi n and free NHC .

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Photoelectron spectroscopy reveals that the highest occupied molecular orbital

(HOMO) in NHC is the lone pair in the sp 2 hybrid orbital of the carbene carbon atom (see

Figure 1.18 ). However, a molecular orbital with π symmetry centred around the N - C - N

heteroallyl system is of almost equal energy. This gives the carbene the ability to engage

in π - face donor interactions with the transition metal fragment. For unsaturated NHC these

π - face donor interactions are more favourable than for saturated ones [ 104 ].

1.2.2 Basicity of NHC

Knowing that the free carbenes can be synthesised from the azolium salts by abstraction of

the H 2 - proton with a strong base, it is not a surprise that the NHC themselves are very strong

Br ø nstedt acids [ 105 – 110 ]. This behaviour follows the well - known rule that a weak acid (the

azolium cation) has a strong conjugate base (the carbene). Just how strong a base these carbenes

are may not be that easy to determine. A classical Br ø nstedt base is defi ned within the medium

water where the pH value is restricted by the autoprotolysis equilibrium of water itself, limiting

the pH range to values of 0 – 14. Water will protonate a carbene to the azolium salt [ 111 ], but the

hydroxide ion leads to decomposition reactions under ring opening [ 108 , 112 ].

How, then, can we determine the actual basicity of these carbenes? One method

was proposed by Alder et al . [ 105 ] who reacted 1,3 - diisopropyl - 4,5 - dimethylimidazol -

2 - ylidene with acidic hydrocarbons with known p K a values in (CD

3 )

2 SO and monitored the

reaction by 1 H - NMR spectroscopy. Whereas indene (p K a 20.1) was completely deproto-

nated, the weaker acids 9 - phenylxanthene (p K a 27.7) and triphenylmethane (p K

a 30.6) were

not deprotonated at all. With fl uorene (p K a 22.9) and 2,3 - benzofl uorene (p K

a 23.5) mix-

tures of the two acid/conjugate base couples carbene/imidazolium and hydrocarbon/anion

were obtained. Integration of the signals in these spectra led to the calculation of the p K a

value of 24.0 for 1,3 - diisopropyl - 4,5 - dimethylimidazol - 2 - ylidene in DMSO - d 6 .

Denk and Rodezno found that the carbene 1,3 - di - tert - butylimidazol - 2 - ylidene reacts

rapidly with DMSO - d 6 under H/D exchange of the ring protons. The deuterated carbene

Figure 1.18 Abbreviated molecular orbital scheme of an annulated NHC ligand . Copyright Wiley-VCH Verlag GmbH & Co KGaA. Reproduced with permission.


c01.indd 20c01.indd 20 12/28/09 10:18:17 AM12/28/09 10:18:17 AM


1,3 - di - tert - butyl - 4,5 - dideutero - imidazol - 2 - ylidene was isolated from the reaction mixture

[ 108 ] (see Figure 1.19 ).

Given that DMSO is not an inert solvent for NHC, although 1,3 - diisopropyl - 4,

5 - dimethylimidazol - 2 - ylidene, the carbene used by Alder et al ., has no ring protons and

thus is not subject to H/D exchange in DMSO - d 6 , Kim and Streitwieser have investigated

the basicity of NHC in THF using UV - Vis spectroscopy [ 106 ]. They used three different

hydrocarbon indicators, 9 - phenyl - 2,3 - benzafl uorene, 3,4 - benzafl uorene and 9 - benzylfl u-

orene, all of which have slightly different UV - Vis spectra for the neutral and the anionic

species. Like Denk and Rodezno, they used 1,3 - di - tert - butylimidazol - 2 - ylidene as the

carbene and found a p K a value of 20.0 in THF for this carbene. As 1,3 - diisopropyl - 4,5 -

dimethylimidazol - 2 - ylidene, the carbene used by Allen et al ., is actually an isomer of 1,3 -

di - tert - butylimidazol - 2 - ylidene, the basicity of the two NHC should be roughly identical

within experimental error. The difference of four pH units is a clear indication that the

measured basicity is dependent on experimental conditions, in this case mainly the choice

of solvent. This was already observed by Alder et al . when they reacted 1,3 - di - tert - butylimidazol - 2 - ylidene with 9 - phenylfl uorene (p K

a 18.5 in THF), fl uorene and indene in

THF [ 105 ] (see Figure 1.20 ). Whereas the deprotonation of fl uorene failed, 9 - phenylfl uorene

and indene were deprotonated giving a p K a value upwards of 18.5 in accordance with the

fi ndings of Kim and Streitwieser.

We conclude that it is easier to generate the free carbene from the imidazolium salt

in THF rather than in DMSO. However, Arduengo et al . tell us that it is advantageous to

add a catalytic amount of DMSO in the reaction between imidazolium salts and KOBu t

in THF to facilitate the removal of the proton [ 54 ]. The anion of DMSO, generated in

small amounts, is a better proton scavenger for imidazolium salts than the potassium tert - butoxylate itself.

Looking at the reaction of an imidazolium salt with mercury(II) acetate to form the

[Hg(NHC) 2 ] complex, the original reaction of Wanzlick and Sch ö nherr [ 3 ], we are left with

a small mystery. Why could the weak conjugate base acetate (p K a 4.75 in water) deproto-

nate the much weaker acid imidazolium?

N

N

But

But

But

But

But

But

But

But

But

But

But

But

But

ButBut

But

H

H

:

N

NH

H

N

ND

DN

ND

N

ND

H

N

N

HN

ND

H

N

ND

H

:

DD

�HD

DD

�D

�H D

DD

D

D

�

�

�

�

�

�

�

��

��

��

�

�

Figure 1.19 H/D exchange in an imidazolium/NHC system .

c01.indd 21c01.indd 21 12/28/09 10:18:19 AM12/28/09 10:18:19 AM

9-phenyl-2,3-benzafluorene 3,4-benzafluorene 9-benzylfluorene

Figure 1.20 Three annulated aromatic compounds used as indicators to measure the basicity of NHC.


Note : The strong M - NHC bond facilitates deprotonation of the azolium salt enabling the use of weak anionic bases like acetate .

The exceptionally strong basicity of the NHC is refl ected in their equally great nucle-

ophilicity. A convenient way to measure the electronic properties of ligands is through the

A 1 carbonyl stretching frequencies of the corresponding transition metal carbonyl com-

plexes [ 113 ]. The standard is known as the Tolman Electronic Parameter (TEP) and is

derived from the [Ni(CO) 3 L] complex of the ligand. Many different transition metal carbo-

nyl complexes have been used and conversion tables as well as theoretical methods for the

computation of TEP values are available [ 113 , 114 ].

In general, NHC have lower TEP values than phosphorus - containing ligands

[ 19 ,107, 115 ], even lower than 2056 cm – 1 , the TEP value of the most basic trialkyl phos-

phane PBu t 3 [ 115 ]. The accepted interpretation is that these carbene ligands are very strong

σ - donors and weak π - acceptors [ 107 ]. However, π - donation of NHC ligands towards

transition metals is also discussed [ 104 , 109 ], a property that is generally not attributed to

phosphanes.

An interesting aspect is the relative net electron donicities of carbene ligands. In a series

of N - stabilised carbenes [C(NPr i 2 ), SIMes, Imes], Herrmann et al . showed that unsaturated

NHC are better net donors than saturated NHC and sometimes can even outperform the

acyclic and more basic carbene ligand C(NPr i 2 )

2 [ 57 ,104, 107 ] (see Figure 1.21 ). A similar

observation was made by Dorta et al . [ 19 ]. However, the absolute differences between the

net donicities of these different carbene ligands are very small and are unlikely to play a

major role in catalyst performance [ 19 ].

Note : There are no signifi cant differences in the electronic properties of NHC ligands deriving from N - substitution or indeed unsaturation versus saturation issues [ 19 ].

Note : The electronic properties of NHC ligands are affected by annulation [ 50 ].

The same conclusion concerning the M - NHC bond structure and strength can be drawn

from bond parameters derived from X - ray structure determinations of transition metal

NHC complexes [ 109 ,116, 117 ].

c01.indd 22c01.indd 22 12/28/09 10:18:20 AM12/28/09 10:18:20 AM


The interesting electronic properties of NHC and their azolium precursors can be seen

in the 1 H - and 13 C - NMR spectra for the H 2 and C 2 atoms. Since the H 2 atom of an azo-

lium salt is essentially acidic, the corresponding chemical shift will be observed downfi eld,

typically at δ = 8 – 11 ppm (see Figure 1.22 ). There is a correlation between the proton

chemical shift and the ease of deprotonation [ 50 ]. The precursor of the acyclic carbene bis -

(diisopropylamino)carbene, N , N , N ’ , N ’ ’ - tetraisopropylformamidinium chloride has a

proton chemical shift of δ = 7.60 ppm [ 118 ], signifi cantly upfi eld of the normal range for

azolium salts.

The carbon chemical shifts of the azolium salts can be found at the downfi eld end of

the aromatic range at δ = 140 – 160 ppm and the carbenes themselves about Δ δ = 100 ppm

downfi eld of the imidazolium salts. Coordination to transition metals brings the carbon

chemical shift upfi eld from the value of the free carbene. Whereas the C 2 resonance in

[Cp*Ru(NHC)Cl] complexes are typically around δ = 200 ppm [ 116 ], the same signal in

[Ag(NHC)Cl] complexes can be found at δ = 170 – 190 ppm [ 50 ] (see Figure 1.23 ).

The dependence of the C 2 chemical shift of a coordinated carbene on the nature of

the carbene is given in Denk et al. [ 107 ] for a series of rhodium NHC complexes as

δ = 211.9 ppm for a saturated NHC, δ = 195.9 ppm for a benzannulated NHC and

δ = 182.6 ppm for an unsaturated NHC. The corresponding bis - (diisopropylamino)carbene

complex is given as δ = 233.8 ppm, the most downfi eld and Δ δ = 21.7 ppm upfi eld from

the free carbene [ 118 ].

RhCl CO

CON

N

RhCl CO

CON

N

RhCl CO

CON

N

R

R

R

R

Pri

Pri

PriPri

� (CO) � 2057 2081 2076 cm�1

Figure 1.21 Evaluation of NHC σ - donicity using IR spectroscopy .

N

N

But

But

N

N

But

But

N

N

N

N

But

But

(1H) 9.71 9.92 10.50 ppm

� � �

Figure 1.22 The infl uence of annulation in 1 H - NMR spectroscopy .

c01.indd 23c01.indd 23 12/28/09 10:18:21 AM12/28/09 10:18:21 AM


Note : The carbene carbon atom of a NHC is observed as a characteristic downfi eld signal in the 13 C - NMR spectrum .

Note : The carbene carbon atom of a coordinated NHC is observed upfi eld from the corresponding free carbene in the 13 C - NMR spectrum .

Note : Silver complexes of NHC typically experience the largest coordination chemical shift in the 13 C - NMR spectrum .

1.2.3 Steric Properties

The description of the steric properties of phosphanes using the Tolman cone angle [ 113 ]

proved to be an excellent concept capable of explaining many phenomena in the coordina-

tion chemistry of phosphanes and their applications, especially in homogenous catalysis.

That there is a steric infl uence connected with NHC was noticed very early, in fact it was

thought that the steric hindrance introduced by the N - mesityl substituents was a contribut-

ing factor in the isolation of the fi rst stable carbene in 1991 as opposed to dimerisation to

the known tetraaminoethylenes [ 1 ].

It is comparatively easy to divide the N - substituents into bulky and nonbulky ones, but

it is far more diffi cult to justify the decision in borderline cases and even more diffi cult

to quantify the fi ndings in a similar way to the Tolman cone angle, which gives a single

value that can be calculated as the sum of the three contributing substituents on phospho-

rus. In short, the Tolman cone angle is valid for symmetrical and unsymmetrical (tertiary)

phosphanes [ 113 ].

RhCl

N

N

RhCl

N

N

RhCl

N

N

RhCl

N

N

R

R

R

R

R

R

Pri

Pri

PriPri

� 233.79 211.9

195.9 182.6 ppm

Figure 1.23 The infl uence of annulation in 13 C - NMR spectroscopy .

c01.indd 24c01.indd 24 12/28/09 10:18:22 AM12/28/09 10:18:22 AM


NHC are not cone - shaped, but can be described as wedges or fences. They usually have

a fl at centre and spreading wings – often the N - substituents are referred to as wingtips

[ 19 ,116,119 – 122 ]. A NHC has two relevant angles in the coordination sphere of a metal,

one connected with the wingspan and one connected with the effective ‘ height ’ of the

carbene, a parameter largely determined by the wings as the N - substituents protrude beyond

the fl atness of the imidazol centre.

A rather facile way to divide NHC ligands into bulky and nonbulky is by look-

ing at their trans - [M(NHC) 2 X

2 ] (L = Ni, Pd, Pt; X = Cl, Br, I) complexes. Sterically

undemanding NHC ligands arrange coplanar and perpendicular to the MC 2 X

2 plane,

whereas sterically demanding NHC show substantial deviations from coplanarity

[ 123 ].

As early as 1999, Nolan proposed a set of two angles to describe the steric impact of

NHC on the coordination sphere of a transition metal, A H (for the height) and A

L (for

the length) of the carbene ligand described as a ‘ fence ’ [ 116 ] (see Figure 1.24 ). Nolan

et al . developed this concept on a series of [Cp*Ru(L)Cl] (L = NHC, PCy 3 , P i Pr

3 ) com-

plexes and felt the need to point out that A L depends on the value of the Ru - NHC bond

length, although Δ ̊ < 1 ̊ for Δ Ru - C < 5 pm. It should also be pointed out that the arithmetic

average of A H and A

L , at least for the bulkier carbenes, is roughly equal to the Tolman

cone angle of the two phosphane ligands used in the study. It is also interesting to note

that for N - aryl substituents the A L value largely depends on the para - substituent and the

A H value on the ortho - substituent ( o - tolyl would have the same A

H value but a smaller A

L

value than mesityl).

These parameters did not gain widespread recognition within the carbene community

and in 2003 Nolan et al . introduced a modifi ed parameter, termed % V burr

[ 19 ,120, 121 ]. The

parameter % V burr

combines the two angles A H and A

L into one parameter that merely gives

what ‘ amount of volume of a sphere centred on the metal (is) buried by overlap with atoms

of the NHC ligand ’ [ 121 ] (see Figure 1.25 ).

Advantage : All the steric impact is compacted into one simple number .

Advantage : % V burr

values for phosphanes are easily computed .

Disadvantage : NHC with different steric properties can have the same % V burr

value .

P

N

N

Wedge shaped NHCCone shaped phosphane

Figure 1.24 The angular dependance of the steric infl uence: phosphane and NHC ligands .

c01.indd 25c01.indd 25 12/28/09 10:18:24 AM12/28/09 10:18:24 AM


Consider the two phenanthrene annulated NHC in Figure 1.26 , they differ only in the

position of the methyl group in the tolyl N - substituent. They have the same % V burr

value,

but the o - tolyl substituted one is a monomeric NHC whereas the p - tolyl isomer is a dimeric

tetraaminoethylene at ambient temperature [ 124 ]. The A H , A

L nomenclature is able to dis-

tinguish between the two and explain the monomeric or dimeric structure. The greater A H

value introduced by the o - methyl group is sterically active, whereas the greater A L value

introduced by the p - methyl group is sterically inactive.

It is worth mentioning that M ü ller and Vogt have recently reintroduced the A H , A

L con-

cept for phosphinine ligands that have similar steric characteristics to NHC [ 119 ]. They call

the two different angles the occupancy angles α and β , but the defi nitions are almost identi-

cal and they point out that the arithmetic average of α and β is very close to the Tolman

cone angle Θ for tertiary phosphanes.

Advantage : The two dimensions that are subject to steric infl uence by the NHC ligand are well separated .

Disadvantage : The occupancy angles are dependent on the M - C bond length and thus different for different metal atoms .

N

N

N

N

p-Tol

:

N

N

p-Tol

p-Tol p-Tol

Figure 1.26 Signifi cant structural differences in a pair of NHC with the same % V burr value .

NN

M

300 pm

Figure 1.25 A graphical description of the steric demand – like an umbrella in the midday sun .

c01.indd 26c01.indd 26 12/28/09 10:18:25 AM12/28/09 10:18:25 AM


The occupancy angle shares this disadvantage with other geometry - based ligand param-

eters such as the natural bite angle introduced for chelating bisphosphane ligands [ 125 – 127 ]

and of course the % V burr

value. The reason lies in the defi nition of these parameters. They

all depend to some degree on the bond length between the metal and the ligating atom. The

longer this bond length, the smaller % V burr

and the smaller the occupancy angles α and β .

Therefore, metal atoms with larger radius, like Mo, W and Ru, have a smaller % V burr

and

occupancy angles for the same ligands than metals with a smaller radius, like Cu and Ni.

1.2.4 The Carbene - Metal Bond

Two types of transition metal carbene compounds are traditionally referred to by the names

of the scientists who fi rst made them, namely E. O. Fischer [ 27 ] and R. R. Schrock [ 30 ].

The discovery of the fi rst transition metal NHC complexes by Ö fele [ 2 ] and Wanzlick [ 3 ]

falls in between the other two, but did not receive the same amount of recognition at the

time (see Figure 1.27 ).

The structures and reactivities of Fischer and Schrock carbenes can be explained

by interactions of singlet and triplet carbenes with suitable metal d orbitals without

any stabilisation from neighbouring nitrogen atoms at the carbene carbon atoms

[ 14 ,128 – 132 ].

In this model, Fischer carbenes can be described as resulting from a σ - donor interaction

of the singlet carbene lone pair into the empty d z 2

orbital of the metal. The metal than uses

its full d xz

orbital for a π - backdonation into the empty p orbital of the carbene carbon atom

[ 129 ]. Characteristically, Fischer carbenes are found with low valent (late) transition met-

als and a carbene ligand where at least one of the two substituents carries a π - donor group

[ 14 , 132 ], usually a heteroatom or phenyl substituent [ 131 ]. Typical representatives are

18 - electron species like [Cr(CO) 5 {=C(OMe)Ph}] [ 130 , 131 ].

In contrast, Schrock carbenes are electron defi cient [ 10 to 16 valence electrons (VE)]

early transition metal complexes with the metal atom in a high oxidation state and carbene

substituents that are limited to alkyl groups and hydrogen [ 131 ]. Their bonding situation

can be described in terms of the interaction of a triplet carbene with a triplet metal fragment

resulting in a covalent double bond [ 132 ]. Tantalum complexes like [(np) 3 Ta=CHBu t ] and

[Cp 2 (Me)Ta=CH

2 ] are representative of Schrock carbenes.

Ta

CH2

CH3 CrOC

OC CO

COCO

EtO PhN

N

Mes

Mes

Au Cl

Schrock Fischer Wanzlick/Arduengo

Figure 1.27 Three different transition metal carbene complexes .

c01.indd 27c01.indd 27 12/28/09 10:18:26 AM12/28/09 10:18:26 AM


Fischer carbenes possess electrophilic carbene carbon centres (they react with nucle-

ophiles) whereas Schrock carbenes show opposite reactivity, and have nucleophilic carbene

centres (that react with electrophiles) [ 14 , 131 ].

What then is the nature of the metal (M) - NHC bond? As NHC are internally stabi-

lised by N → C carbene

π - donations, a simple answer would be that they are pure σ - donors and

π - backdonation from the transition metal is negligible. The d π - p

π M→ C backdonation

would have to be delivered into an already (partially) fi lled p orbital at the carbene car-

bon atom. Indeed, for a relatively long time (in the short history of NHC transition metal

complexes), Wanzlick (Arduengo) carbenes were considered to be pure σ - donor ligands

[ 14 , 132 ], but more recently, a somewhat larger degree of metal → carbene π - backdonation

[ 129 ] has begun to emerge. Today, it is widely accepted that metal → carbene π - backdona-

tion can be responsible for up to 20 – 30% of total M → C bond strength in transition metal

NHC complexes [ 73 ,132, 133 ].

Note : NHC can engage in metal → carbene π - backbonding to some degree, but it is not necessary for the exceptional stability of the M - NHC bond .

That this is so can be seen from the investigation of some beryllium NHC complexes [ 13 , 134 ]

where the carbene was seen to replace bridging chloride ligands and coordinate to the Be 2+

cation despite the lack of suitable electrons for π - backbonding on the beryllium atom.

Corresponding to the low degree of metal → carbene π - backbonding in transition metal

NHC complexes, the M - C bond in these compounds is very long and has to be regarded as

a single bond [ 115 ] despite a π - backbonding degree of up to 20 – 30%.

Occasionally, an additional π - donor bond from the carbene to the metal is discussed

[ 104 ]. Such a bond is principally possible since NHC possess an occupied orbital of

π - symmetry immediately below the sp 2 - hybridised HOMO lone pair. This would contrib-

ute to the great ( σ / π ) donor strength of the NHC ligands.

1.2.5 Decomposition Pathways

Originally, it was believed that NHC behave strictly as donor spectator ligands [ 16 , 18 ]

and do not react either with the substrate, the metal or any other substituent or ligand on

the metal they are bonded to. However, it has been emerging for some years that NHC are

not as straightforward a ligand class as many chemists wished to believe [ 22 ]. In 2001,

McGuinness et al . showed transition metals (calculated for Pd) carrying a NHC ligand and

an alkyl or aryl group cis to it can undergo concertive reductive elimination with the forma-

tion of C 2 - substituted imidazolium salts [ 21 ]. The reaction is exothermic (about – 4 kcal

mol – 1 ) and originates from a four coordinate palladium(II) complex where the C 2 - p π orbital

is perpendicular to the plane of the metal and thus correctly orientated to interact with the

methyl (aryl) group and the d xy

orbital of the metal. After formation of the C 2 - C R bond

(R = alkyl, aryl) the imidazolium ring separates from the metal.

Of course, the opposite has also been observed whereby an imidazolium salt oxidatively

adds to a transition metal complex. [ 135 , 136 ]. This was fi rst noticed in catalytic reac-

tions involving transition metal phosphane complexes as catalysts and imidazolium based

ionic liquids. Unexpected improvements in catalytic performance prompted investigations

to fi nd out whether the phosphane ligands had been replaced by more electron - rich NHC

c01.indd 28c01.indd 28 12/28/09 10:18:26 AM12/28/09 10:18:26 AM


[ 137 ]. In a theoretical and experimental study, McGuinness et al . described the oxidative

addition of an imidalium cation to zerovalent d 10 metals [ 23 ] and came to the conclusion

that addition on chelated Pd(0) is favoured over linear [Pd(PR 3 )

2 ] complexes and that C 2 - X

(X = I, Br, I) activation on the imidazolium cation is much easier than C 2 - H and C 2 - Me

activation with the methyl group beeing the least reactive.

Disadvantage : Deactivation of catalyst due to concertive reductive elimination of the NHC ligand and the catalytically active alkyl or aryl group .

Note : Carrying out the reaction in an imidazolium - based ionic liquid is believed to mini-mise the effect of reductive elimination as oxidative addition of the solvent is favoured instead [ 23 ].

A similar mechanism might operate in the activation of an azolium salt by a transition

metal compound forming the metal carbene complex. However, since a basic substituent

on the metal (acetate, alkoxide, hydride) usually reacts with the H 2 - proton, the proton is

removed from the reaction as the conjugate acid and reductive elimination does not occur.

In many catalytic reactions, a methyl or hydride substituent on the metal increases the

reactivity of the catalytic system and thus the presence of such a substituent is normally

highly desired. When this methyl or hydride group is cis to the NHC ligand or can move

into the cis position relative to the NHC ligand during the course of the reaction, reductive

elimination might occur in a facile manner [ 22 ].

Note : The presence of a positive charge on the metal facilitates reductive elimination .

Note : Electron - rich ligands decrease the rate of reductive elimination. This is in line with the observation that neutral complexes are more stable than cationic ones .

Note : Sterically bulky ligands accelerate reductive elimination .

Not surprisingly, chelating carbene ligands show greater stability with regard to reduc-

tive elimination than monodentate carbene ligands [ 22 ]. However, even transition metal

complexes with bis - carbene ligands can suffer degradation by reductive elimination

(see Figure 1.28 ).

Note : Chelating carbenes are signifi cantly more stable towards reductive elimination than monodentate NHC .

Note : It is possible to prepare stable cis - dimethyl compounds of nickel(II) and palladium(II) with chelating bis - NHC ligands [ 138 , 139 ].

This is an important observation, since functionalised carbenes usually possess a sidechain

with a coordinating functional group. Thus, functionalisation normally introduces added

stability to the catalyst.

There are two possible mechanisms to this observed elimination, simple reductive

elimination and a migratory insertion/decomplexation sequence [ 22 ]. Normally, a sim-

ple reductive elimination mechanism is observed [ 140 ], but occasionally a migratory

c01.indd 29c01.indd 29 12/28/09 10:18:27 AM12/28/09 10:18:27 AM


insertion/decomplexation sequence could be proven [ 141 ]. This migratory/decomplexation

mechanism follows closely the process observed in CO/ethylene copolymerisation cataly-

sis. This is hardly surprising, since NHC are isoelectronic to CO and isonitriles.

A second, very important decomposition pathway involves the activation of C - H bonds

on the N - alkyl [ 142 – 147 ] or N - aryl [ 148 – 151 ] sidechains. Occassionally, even C - C acti-

vation in the sidechain is observed [ 152 ]. Similar C - H activation is observed in transition

metal phosphides, especially when the phosphorus ligand has a SMes substituent [ 153 ].

The activation of C - H bonds on the N - aryl substituents in transition metal NHC

complexes was observed by Lappert et al . in 1979 when they reacted aryl substituted

tetraaminoethylenes with the ruthenium complexes [RuCl 2 (PPh

3 )

3 ], [RuCl(NO)(PPh

3 )

2 ] and

[RuCl 3 (NO)(PPh

3 )

2 ] in xylene at 140 ̊ C [ 149 ]. That elevated temperatures are not required

for the activation of C - H bonds on N - aryl substituents in the case of ruthenium was shown

by Enders et al . [ 151 , 154 ] when they reacted 2 - phenyl - 5 R - triazolium perchlorate with

[( η 6 - cymene)RuCl 2 ]

2 at ambient temperatures and observed ortho - metallation on

the phenyl ring. A similar reaction was reported by Baratta et al . using trans , cis -

[RuHCl(PPh 3 )

2 (ampy)] and 1,3,4 - triphenyl - 4,5 - dihydro - 1 H - 1,2,4 - triazol - 5 - ylidene [ 148 ]

in refl uxing toluene.

Ruthenium not only activates C - H bonds in the N - sidechain of the NHC ligand, but

also C - O [ 146 ], C - Cl [C 12 ] and C - C bonds [ 152 ]. Reaction of [( η 6 - cymene)RuCl 2 ]

2 with

N

Pd

N

NN

N

N

Pd

DMSON

NN

N

DMSO

150C

Pd

NCMe

MeCN

NN

N

N

Mes

MesNN

N

N

Mes

Mes

R

R

�T

C2H4/CO

� Pd

�

�

�

Figure 1.28 Reductive elimination and oxidative addition .

c01.indd 30c01.indd 30 12/28/09 10:18:27 AM12/28/09 10:18:27 AM

the functionalised imidazolium salts [MesICH 2 CH

2 X]Cl (X = Cl, OMe) results in the

loss of the functional group X and formation of a RuCNCC - metallacycle [ 146 ]. Whereas

activation of the C - Cl and C - O bonds were facile and could be achieved at ambient

temperature, the activation of the C - C bond required a reaction time of 16 days in refl uxing

benzene (80 ̊ C) [ 152 ].

In a very recent computational study, Diggle et al . have calculated the activation barri-

ers for C(aryl) - X activation (X = H, F, OH, NH 2 , CH

3 ) as 0 (H), 9 (F), 12 (OH), 20 (NH

2 )

and 21.3 kcal mol – 1 (CH 3 ), respectively [ 155 ]. In comparison, the activation barrier for

C(sp 3 ) - H is 6.6 kcal mol –1 [ 156 ]. C - X activation occurs under reaction conditions rele-

vant for homogenous catalysis [ 157 ], but does not always result in decomposition as C - H

activation is often reversible and can be exploited in catalytic transfer hydrogenations

involving alcohols [ 156 ].

Note : The possibility of C - X activation is extremely important, when functionalised NHC are used as ligands, especially with late transition metals .

It is interesting to note that C - H activation on ruthenium NHC complexes is not limited

to intramolecular protons located in the N - sidechain of the carbene, but occurs inter-

molecularly as well. Leitner et al . reacted [MesIRuH 4 PCy

3 ] with toluene - d

8 at ambient

temperature and observed a rapid H/D exchange reaction involving the four hydride hydro-

gen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene

ligand and the deuterium atoms on the meta positions of toluene - d 8 . The ortho - , para - and

methyl - deuterium atoms of the solvent did not participate [ 145 ].

This C - H activation is not limited to ruthenium, but occurs frequently in correspond-

ing iridium, rhodium, nickel, palladium and platinum complexes [ 147 ] and was recently

reported for a ytterbium complex [ 144 ]. Nolan et al . have even reported double C - H activa-

tion in a rhodium(III) complex carrying two NHC ligands [ 142 , 143 ]. The starting material

is a rhodium(I) species, [Rh(coe) 2 Cl]

2 that undergoes mono or double C - H activation after

reaction with IBu t depending on reaction conditions. The double C - H activated, square

pyramidal, 16 VE complex [Ru(I ’ Bu t ) 2 Cl] (I ’ Bu t = IBu t with one hydrogen missing) is

obtained in benzene and can be transformed into the corresponding square planar, 14 VE

complex [Ru(I ’ Bu t ) 2 ]PF 6 by chloride abstraction with AgPF

6 in methylene chloride [ 142 ].

Note : Activation of C - H bonds in the N - sidechains of NHC ligands is a frequently occur-ring phenomenon .

Note : Most instances of C - H, and C - X (X = C, O, Cl), activation have been observed with late transition metals, i.e. those frequently used in homogenous catalysis .

An interesting observation was made by Morris et al . [ 158 , 159 ] who reported that the

electron - rich system [Ru(IBu t )(PPh 3 )

2 H] showed no inclination for intramolecular C - H

activation. It seems that the same electronic preferences are working for reductive elimina-

tion and C - X activation.

A third decomposition pathway is decomplexation. In principle, NHC ligands can disso-

ciate from the metal complex, although the M - NHC bond is signifi cantly stronger than the

M - phosphane bond [ 116 , 160 ]. Well known examples for decomplexation of NHC ligands


c01.indd 31c01.indd 31 12/28/09 10:18:28 AM12/28/09 10:18:28 AM


are a special family of second generation Grubbs ’ catalysts fi rst published by Weskamp

et al . [ 161 ]. During the initiation period one of the two NHC ligands dissociates and thus

activates the catalysts. An improvement in catalyst design was then introduced by the

Grubbs group with a mixed IMes/PCy 3 ruthenium catalyst for olefi n metathesis whereby

only the phosphane ligand dissociates [ 162 ] (see Figure 1.29 ). However, not all NHC lig-

ands are equally stable as an example from the Grubbs group shows, whereby a triazolium

substituted ruthenium catalysed decomposed yielding a bisphosphane ruthenium complex

and a triazolium salt [ 163 ] even at room temperature.

The fi rst decomplexation reaction involving substitution of a ruthenium coordinated

NHC ligand by trimethyl phosphite was reported by Lappert et al . [ 87 ] in the reaction

between [Ru(SIMe) 4 Cl

2 ] with trimethyl phosphite yielding [Ru(SIMe)

4 {P(OMe)

3 }

2 Cl

2 ].

Displacement of a coordinated NHC ligand by phosphanes was also reported for rhodium

complexes in the reaction between [Rh(IMes)(PPh 3 )

2 Cl] and PPh

3 in dichloroethane [ 164 ]

with subsequent alkylation of the carbene at C 2 (see Figure 1.30 ). A similar reaction occurs

with bis - diphenylphosphinoethane (dppe) as the phosphane in refl uxing xylene [ 165 ].

Ru

Cl

Cl

N

N

N

N

Ph

Cy

Cy

Cy

Cy

Ru

Cl

Cl

N

N

Ph

Cy

Cy

�T

initiation

catalysis

Ru

Cl

Cl

PCy3

N

N

Ph

Cy

Cy

�T

initiation

Ru

Cy3P

Cl PCy3

Cl

Ph

Ru

Cy3P

Cl

Cl

Ph

�T

initiation

catalysis

Figure 1.29 The second generation Grubbs ’ catalyst .

c01.indd 32c01.indd 32 12/28/09 10:18:29 AM12/28/09 10:18:29 AM

The picture becomes even more complex when a series of [Pd(NHC) 2 ] and

[Pd(NHC)PR 3 ] (R = Cy, tolyl) complexes [ 166 ] are considered. Reaction of [Pd(IBu t )

2 ]

with PTol 3 yields the mixed complex [Pd(IBu t ) PTol

3 ] after dissociation of a carbene

ligand even at ambient temperature. The more basic phosphine ligand PCy 3 reacts even

faster resulting in the monophosphane complex within 15 min at room temperature.

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The Nature of N - Heterocyclic Carbenes COPYRIGHTED MATERIAL · 2020-02-16 · The Nature of N-Heterocyclic Carbenes 13 N N S R R N N R R: S Cl Cl NHR NHR KC8 S H2N H2N O OH R Figure