Vrije Universiteit Amsterdam 1... · Chapter 1 2 1.111..111.1. ... IIIIntroductionntroductionntroduction The activation of small molecules like H 2, CO 2, NH 3, P 4 and N 2O among

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Small Molecule Activation and Capture by Preorganized Frustrated Lewis Pairs

Bertini, F.

2013

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citation for published version (APA)Bertini, F. (2013). Small Molecule Activation and Capture by Preorganized Frustrated Lewis Pairs.

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Chapter 1

Small Molecule Activation by Main

Group Compounds Federica Bertini, J. Chris Slootweg, Koop Lammertsma

Abstract: This introductory chapter describes recent spectacular discoveries

with respect to the new and fascinating field of small molecule activation by

main group compounds.

Chapter 1

2

1.11.11.11.1.... IIIIntroductionntroductionntroductionntroduction

The activation of small molecules like H2, CO2, NH3, P4 and N2O among others, and their

subsequent utilization for synthetic purposes, is of fundamental importance in chemistry.

Creation of new synthetic methodologies based on small molecule activation would create

new opportunities for material development from cheap, readily available and renewable

feedstock. For example, there is great interest in understanding how to use carbon dioxide

as a feedstock in green processes and as a carbon source for the production of more

complex molecules,1 while the development of novel strategies for the transformation of

white phosphorus (P4) is of great importance owing to the high demand of

organophosphorus compounds and to environmental concerns.2 Small molecules are

generally quite stable thermodynamically and key to their successful utilization is to

provide low-barrier reaction pathways, which can be achieved through binding and

activation processes typically mediated by transition metal ions. A great amount of

fundamental chemistry research has therefore been aimed to understand how metal

complexes coordinate to such small and often rather inert molecules, how they modulate

their reactivity and how to use the gained knowledge for the development of new catalytic

processes.3 This work has led to a deep understanding that has significantly impacted the

fields of organometallic chemistry and catalysis.

The terrific success of transition metal-based catalysts for the activation of small

molecules perhaps has overshadowed for many years the possibility of the utilization of

main group elements for the same purpose. In recent years, there has been an intense

drive towards developing “green” chemical processes using more environmentally benign

chemicals, reagents, solvents and catalysts.4 A part of this drive is to avoid or minimize

the use of transition metals in chemical reactions, as these are often toxic and difficult to

dispose properly in large quantities. Moreover, the difficulty of their separation leaves a

chance of their contamination of the product. The presence of a metal, even at the lowest

level, in pharmaceutical products is closely regulated. Thus, a transition metal-free process

is desired as a part of the requirements for the chemical industry as well as clean

environment.4 Hence, the development of metal-free systems that can replace the use of

transition metals is highly desired.

The past decade witnessed extraordinary discoveries in the field of small molecule

activation by main group species. The first major breakthrough occurred in 2005, when

Power and co-workers discovered that the germanium species ArGeΞGeAr (Ar = C6H3-

2,6(2,6-diisopropylphenyl)2) reacts with molecular H2 under mild conditions to give the

hydrogenated products Ar(H)Ge=Ge(H)Ar, Ar(H)2GeGe(H)2Ar and Ge(H)3Ar.5 This finding

Small Molecule Activation by Main Group Compounds

3

gained enormous attention in the scientific community since H2 activation had long been

known to occur at transition metal centers, but the reaction of H2 with a main group

compound under mild conditions was unprecedented.

In 2006, the group of D. W. Stephan reported on the reversible activation of the

dihydrogen molecule under ambient conditions, using a unimolecular phosphine-borane

Lewis pair.6 The ability of such phosphine-borane pair to activate H2 was attributed to the

contemporary presence of unquenched Lewis basic and Lewis acidic centers, which could

synergically interact with the dihydrogen molecule, leading to the heterolytic splitting of

the H―H bond. Mutual quenching of the reactive phosphorus and boron atoms was

prevented by bulky substituents, which inhibited Lewis adduct formation due to steric

hindrance. This finding opened a window of new opportunities in the field of metal-free

small molecule activation and catalysis and was the starting point from which the

chemistry of the so called “frustrated Lewis pairs” (FLPs) rapidly developed.7

In the same years, it was increasingly recognized that also low valent main group

species such as carbenes8 and their heavier congeners9 posses the ability to activate small

molecules. As in the case of frustrated Lewis pairs, the ability of carbene-like species to

split strong bonds heterolytically is due to the to the contemporary presence of an

electrophilic vacant orbital and a nucleophilic lone pair of electrons. Consequently to these

discoveries, the ability of such main group low valent species to activate small molecules

has been extensively studied.

This chapter illustrates recent developments with respect to an emergent and

fascinating topic in main group chemistry: transition metal-free small molecule activation.

This chapter is not intended to furnish a comprehensive treatment of the literature on such

a broad topic, but rather to illustrate the most remarkable discoveries, with emphasis on

those main group species which gain their ability to activate small molecules owing to the

contemporary presence of acidic empty orbitals and basic lone pairs of electrons. We will

first describe a few selected examples of small molecule activation by low valent main

group species. In particular, we will describe how carbenes, as well as their heavier

congeners, can mimic the behavior of transition metals owing to their ambiphilic nature,

and we will briefly comment on the chemistry of the related nitrenes and phosphinidenes,

which developed differently. Additionally, a few remarkable examples of small molecule

activation by phosphenium ions will be also described. Finally, we will illustrate the

chemistry of the newly discovered frustrated Lewis Pairs (FLPs), which is the main focus of

this PhD thesis.

Chapter 1

4

1.21.21.21.2.... Singlet Singlet Singlet Singlet CCCCarbenesarbenesarbenesarbenes

Carbenes are a family of organic molecules composed of a neutral divalent carbon

atom with a sextet of electrons and two substituents.10 They can be regarded as singlet or

triplet species depending upon their electronic structure, which is dictated by the nature of

the substituents on the carbene carbon atom.11 Triplet carbenes react as diradicals and are

therefore less interesting from a synthetic viewpoint since their reactions are non-

stereospecific.12 In contrast, singlet carbenes are closed shell species which possess a lone

pair of electrons and an empty orbital and react stereospecifically participating in

cheletropic reactions which occur in a single step, as exemplified by the widely

documented 1,2-cycloaddition to double bonds to yield cyclopropanes.10-13

Carbenes have long been known as very reactive and short lived species that could not

be isolated and were usually studied by observing the reactions they undergo. However,

strategies for the isolation of persistent carbenes have been developed and nowadays

stable carbenes, such as the Arduengo-type14,15 or the Bertrand-type,16,17 are very well

known. Key to their stabilization is the presence of one or two heteroatoms (such as

nitrogen or phosphorus) adjacent to the carbene carbon atom, which decrease the electron

deficiency of the empty orbital by donating electron density through resonance, while

stabilizing the lone pair on the carbon by inductively withdrawing electron density. Bulky

substituents on the heteroatom(s) provide additional stabilization by steric protection of

the reactive carbon atom. Despite the provided stabilization, such persistent carbenes

remain fairly reactive species.

In 2006, the group of Bertrand discovered that acyclic and cyclic

(alkyl)(amino)carbenes (aAACs and cAACs) react with CO to afford the corresponding

ketenes18 (as exemplified by aAAC 1 that reacts with CO to form 2; Scheme 1). This led to

the question of whether singlet carbenes could mimic the chemical behavior of transition

metals, which appeared reasonable since singlet carbenes possess both a lone pair of

electrons and an accessible vacant orbital and therefore resemble, at least to some extent,

transition metal centers. Consequently to this discovery, the ability of stable singlet

carbenes to activate other small molecules was explored.

In 2007, Bertrand reported that cAACs and aAACs are also capable of heterolytic H–H

splitting under mild conditions (as illustrated by the reaction of 1 with H2, which results in

the formation of 3; Scheme 1), a reaction that has long been known to occur with

transition metals only.19 In contrast to transition metals that act as electrophiles toward

dihydrogen, these carbenes primarily behave as nucleophiles. A computational analysis on

the mode of H2 activation by cAACs and aAACs showed that the H–H bond activation arises


5

from the primary interaction of the carbene`s lone pair with the antibonding orbital of H2,

creating a hydride-like hydrogen atom which then attacks the carbon centre. This

nucleophilic behavior allows these carbenes to activate the N–H bond of NH3 (resulting in

1,2-addition products, like 4; Scheme 1), which is a difficult task even for transition

metals because their typically electrophilic character favors the formation of Werner-type

LnM–NH3 complexes rather than the coordination to the metal via the N–H bond.19 In fact,

examples of N–H bond activation through N–H oxidative addition by transition metals are

rare20 to such extent that the first example of NH3 activation to afford a product containing

both M–H and M–NH2 groups was reported only in 2005.21 More recently, Bertrand and co-

workers have also demonstrated that AACs can activate enthalpically strong bonds such as

Si−H, B−H and P−H bonds.22 The more popular N-heterocyclic carbenes (NHCs), which are

well established organocatalysts,23 are currently known to be inert towards CO, H2 and

NH3. The higher reactivity of cAACs and aAACs with respect to NHCs arises from the fact

that for (alkyl)(amino)carbenes the singlet-triplet gap is much smaller, and the HOMO

much higher in energy than for the NHCs and consequently, they are more nucleophilic but

also more electrophilic, which confers them an increased reactivity.19 Interestingly,

Bielawsky reported that also cyclic di(amido)carbenes (cDACs), despite their higher

electrophilicity with respects to cAACs and aAACs, can activate NH324 and can reversibly

react with CO.25

Scheme 1. Reaction of aAAC 1 with CO, H2 and NH3.

Remarkably, the carbene mediated activation of white phosphorus (P4) recently

became an active research topic.26 White phosphorus is of industrial interest as it is the

typical starting material for the large-scale preparation of organophosphorus compounds.27

Typically, P4 is treated with Cl2 gas to make PCl3 or PCl5, from which the chlorine atoms are

subsequently substituted with organic substrates. Consequently to the increasing demand

in phosphorus derivatives and the increasingly stringent environmental regulations, new

tBu

(iPr)2N

CO tBu

(iPr)2N

C O

H2tBu

(iPr)2NH

H

NH3tBu

(iPr)2NNH2

H

1

2

3

4

Chapter 1

6

processes using white phosphorus, but avoiding chlorine, are highly desirable. While many

examples of P4 activation by transition metals have been reported,2 the catalytic

conversion of white phosphorus to useful products remains elusive, although important

steps forward towards the achievement of this goal have been made.2

The first example of P4 activation by a stable singlet carbene dates to 2007, when

Bertrand reported that cAAC 5 can open the P4 cluster and stabilizes the resulting acyclic

P4 species affording product 7 (Scheme 2; Dipp = 2,6-diisopropylphenyl).28 Since 7

features a diphosphene and two phosphalkene fragments, which are highly reactive

functional groups, it can be used further for the construction of more complex molecules,

as exemplified by the diastereoselective [4+2] cycloaddition between the diphosphene

moiety and 2,3-dimethylbutadiene, to give organophosphorus compound 9.28 The

intermediacy of triphosphirene 6 in the formation of 7, predicted by calculations,28 was

demonstrated by trapping the transient species with 2,3-dimethylbutadiene, which lead to

the desired [4+2] cycloaddition product 8 (Scheme 2).28 Shortly after, also NHCs proved

to be reactive towards P4. For example, NHC 10 reacts with P4 similarly to cAAC 5, forming

the P4 degradation product 12 (Scheme 3).29 Additionally, the NHC-stabilized P12 cluster

13, obtained upon heating 12, was isolated in high yield (Scheme 3).29 The lower thermal

stability of 12 with respect to 7, which allowed the isolation of cluster 13, was attributed

to the fact that NHCs are less basic than cAACs and therefore better leaving groups,

favoring the formation of 13. On the other hand, the higher electrophilicity of cAACs

provides higher stabilization to the acyclic P4 fragment in 7, by strengthening the P−C

bonds.30 The intermediacy of the [triangle+1]-form of P4 (analogous to 6) in the formation

of 12 was again demonstrated by trapping this transient species with 2,3-

dimethylbutadiene (forming 11; Scheme 3).30 Interestingly, the [triangle+1]-form of P4

could be isolated with aAAC 1430 (15; Scheme 4), which is strongly basic, but also one of

the most electrophilic carbenes known.31 In fact, aAAC 14 is sufficiently nucleophilic to

open the P4 tetrahedron, as well as so electrophilic that it undergoes a cyclopropanation

reaction with the [triangle+1]-form of P4 (to give 15), rather than inducing its ring-

opening as in the case of 5 and 10.30 With carbenes 16 and 19, P2 and P1 bis(carbene)

adducts 17 (Scheme 5) and 20 (Scheme 6) were isolated, which are of great interest

since most synthetically useful organophosphorus derivatives contain only one or two

phosphorus atoms, and therefore it is of primary importance to induce the fragmentation

of P4 into smaller Pn (n = 1,2) units.30 The few examples described above nicely illustrate

the diversity of the modes of P4 activation that could be achieved by simply varying the

electronic properties of the carbene used, offering new transition metal free strategies for

the synthesis of organophosphorus compounds.32


7

Scheme 2. Activation of P4 by cAAC 5.

Scheme 3. Activation of P4 by NHC 10.

N

N

Dipp

Dipp

N

N

Dipp

PP P

PN

N

Dipp

10

12

Dipp

Dipp

P PP

P

P

PP

PP

P P

N

NP

N

N

Dipp

Dipp

Dipp

Dipp

13

70 °C16h

11

N

N

Dipp

P

P P

P

Dipp

P4

P4

Chapter 1

8

Scheme 4. Isolation of a carbene stabilized [triangle+1]-form of P4 (15).

Scheme 5. Isolation of a P2 bis(carbene) adduct (17).

Scheme 6. Isolation of a P1 bis(carbene) adduct (20).

Singlet carbenes are well known to be extremely versatile ligands for transitions

metals33 and even well established organocatalysts23 and consequently are of great

importance in catalysis. In addition, the recent developments described herein proved

their ability to activate small molecules and robust chemical bonds, further increasing the

potential use of such low valent carbon species in catalysis. We note that very recently

another remarkable achievement has been reported, namely, the N-heterocyclic carbene

complexation of the greenhouse gas N2O.34


9

1.31.31.31.3.... Heavier Heavier Heavier Heavier GGGGroup XIV roup XIV roup XIV roup XIV CCCCarbene arbene arbene arbene AAAAnaloguesnaloguesnaloguesnalogues

The reactivity of the heavier group XIV carbene analogues, in particular silylenes,

germylenes and stannylenes, has also been extensively explored. As heavier analogues of

carbenes, they generally posses a singlet ground state, consequently to a larger singlet-

triplet energy gap,35 which makes them valuable candidates for the activation of chemical

bonds.

1.3.1. Silylenes

While until two decades ago silylenes were considered to be extremely elusive

species,35 since the isolation in 1994 of the first stable N-heterocyclic silylene (NHSi),36

new types of NHSis have been developed and their reactivity has been subject to several

reviews.37 In the past few years, important developments in silylene chemistry have been

made in connection with the emerging field of metal-free small molecule activation, which

demonstrated that the intriguing electronic properties of certain silylenes offer new

opportunities for the metal-free activation of C–H, C–X, Si–X, N–H , P–H, As–H, O–H, S–H,

P–P, C–O, N–O, O–O, S–S, Se–Se, Te–Te bonds.37

Outstanding results, in this context, were obtained with silylene 2138 (Dipp = 2,6-

diisopropylphenyl) and its derivatives. Due to its peculiar ylide-like zwitterionic structure

(Scheme 7), its chemistry is remarkably different from that of other known NHSis. In fact,

21 exhibits an electron-rich butadiene moiety in the backbone, with the exocyclic

methylene group that can behave as an additional nucleophilic group and cooperates with

the lone pair and empty orbital of the silicon centre making it more reactive towards both

nucleophiles and electrophiles. Thus, NHSi 21 features three reactive sites instead of two:

the basic lone pair on the Si(II) centre, a formally empty acidic orbital at silicon, and a

basic butadiene moiety. For this reason, shortly after its isolation,38 its ability to activate

small molecules gained significant attention.37a

Scheme 7. Silylene 21 and its ylide-like resonance structure 21`.

Chapter 1

10

Thanks to the unsaturated Si atom, 21 manifests a carbene-like behavior towards

unsaturated organic molecules undergoing a variety of cycloaddition reactions,39 offering

new synthetic methodologies for the synthesis of organosilanes. For example, with

terminal alkynes both C–H activation (22 and 23) and 1,2-cycloaddition to the CΞC triple

bond (24) were observed at different reaction temperatures (Scheme 8).40

Scheme 8. Reactivity of 21 towards terminal alkynes.

Like cAACs and aAACs, 21 can activate the N–H bond of NH3 (25; Scheme 9) and also

of hydrazine and methylhydrazine, forming the 1,1 N–H insertion products.41 In a similar

fashion, 21 reacts with PH3 (forming 26; Scheme 9),42 even though longer reaction times

and excess of PH3 are required to achieve complete conversion. By contrast, the reaction

with AsH3 is again very fast, due to the greater Brønsted acidity of AsH3. However, the 1,1

As–H activation product 27 could only be detected in solution, since it tautomerizes to 28,

due to the presence of the additional nucleophilic centre (Scheme 10).42

Scheme 9. Activation of NH3 and PH3 by 21.

N

Si

N

Dipp

Dipp

21

N

Si

N

Dipp

Dipp

N

Si

N

Dipp

Dipp

H

R

H

R

R = H or Ph

H R

N

Si

N

Dipp

Dipp

H

N

Si

N

Dipp

Dipp

H

-78 °C

RT

RTR = H

21

22 23

24


11

Scheme 10. Activation of AsH3 by 21.

The reaction of 21 with H2S, which resulted in the isolation of 29 (Scheme 11), nicely

illustrates the unusual ambivalent reactivity of 21 by combining two different types of

reactivity involving S–H bond activation: 1,4- and 1,1- addition.43 In this case, all three

reactive sites of 21 have been involved in a single reaction.

Other examples that illustrate the reactivity of 21, involve the activation of C–H bonds

(pentafluorobenzene and trifluorobenzene),44 C–F bonds (hexafluorobenzene,

pentafluoropyridine and octafluorotoluene),44 C–X bonds (X = Cl, Br, I)45 and Si–X bonds

(HSiCl3 and MeSiCl3).45

Scheme 11. Activation of H2S by 21.

In 2007, Driess and co-workers reported that 21 reacts with P4 forming two different

activation products, namely 30 (Scheme 12), which results from the Si(II) insertion into a

P–P bond of the P4 tetrahedron, and 31 (Scheme 12), which results from the insertion of a

second equivalent of 21.46 Interestingly, such mode of activation, which is well known for

transition metal/P4 chemistry,2 was calculated to be the most stable for both parent

silylene :SiH247 and methylene :CH248, but has not yet been observed with carbenes.

Notably, unlike 21, the germylene analogue of 2149 is resistant towards P4 even in

boiling toluene, owing to the lower reduction potential of Ge(II) versus Si(II). By contrast,

an analogous mode of P4 activation has been reported for related carbene-like aluminum

and gallium derivatives 32 and 33 (Scheme 13). In fact, the aluminum derivative 34 was

reported by Roesky already in 2004 as the first main-group complex containing the [P4]4-

Chapter 1

12

fragment (Scheme 13),50 while the related gallium derivative 35 was reported several

years later.51

Scheme 12. Activation of P4 by 21.

Scheme 13. Activation of P4 by carbene-like Al and Ga species 33 and 35.

More recent work on P4 activation by silylenes lead to the isolation of a P4 chain (37;

Scheme 14) and a P4 cage (39; Scheme 15).52 Reacting the three coordinated Si(II)

bis(trimethylsilyl)amide 36 with P4 in toluene at −10 °C, resulted in the isolation of 37,

which possesses a Z-diphosphene unit, as revealed by X-ray diffraction. The reaction of P4

with disilene 38, which is known to exist in equilibrium with the corresponding silylene

38` in solution,53,54 in a 1:1 molar ratio in toluene at ambient temperature, yielded 39,

with a triply opened P4 tetrahedron (Scheme 15).

Scheme 14. Activation of P4 by three coordinated Si(II) species 36.


13

Scheme 15. Activation of P4 by bis(silylene) 38.

Interestingly, the nucleophilicity of the Si(II) centre of silylenes can be increased by

stabilization of the Lewis acidic orbital of the silylene with a strong donor (D). This strategy

to enhance the nucleophilicity of the Si(II) atom broadened significantly the applicability of

silylenes for small molecule activation. For example, although silylene 21 is inert towards

the greenhouse gas N2O, its Lewis base stabilized analogues 21a,55 21b55 and 21c56 react

with N2O affording the corresponding silanones (40a, 40b and 40c; Scheme 16).55-57

Additionally, when DMAP is used as the stabilizing group, the resulting silanone (40c) can

activate NH3 under mild conditions,56 after replacement of the DMAP ligand (forming 41

and 42; Scheme 16), while the reaction with H2S gave 43.

Scheme 16. Reduction of N2O by 21a-c and activation of NH3 and H2S by 40c.

N

Si

N

Dipp

Dipp

a: D = 1,3,4,5-tetramethylimidazol-2-ylidine

D

N

Si

N

Dipp

Dipp

D

O

N2O

N

Si

N

Dipp

Dipp

NH2

OHN

Si

N

Dipp

Dipp

NH2

O

N

Si

N

Dipp

Dipp

S

OH

H2S

DMAP

40a-c

43

41 42

b: D = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidine

c: D = 4-dimethylaminopyridine (DMAP)

21a-c

(D = DMAP)

NH3

(-N2)

Chapter 1

14

While activation of CO2 by the Lewis base stabilized derivatives of 21 described above

(21a-c) was unsuccessful, this could be achieved with siloxy silylene 44, which is obtained

from 21 upon reaction with H2O.58 In fact, 44 is remarkably nucleophilic and proved to be

able to activate both greenhouse gasses CO2 and N2O forming the same silanoic silyl ester

45 upon liberation of N2 or CO (Scheme 17).59

Scheme 17. Activation of CO2 and N2O by 44.

Very recently, also two examples of CO2 and N2O activation by bis(silylenes) were

reported: the group of Baceiredo reported that bis(silylene) 46 reacts with 4 equivalents

of CO2 to form the carboxylato bis-silicate 47 (Scheme 18),60 while Roesky reported that

bis(silylene) 4861 reacts with 6 equivalents of N2O forming the siloxy compound 49 upon

liberation of N2 (Scheme 19).62

Scheme 18. Activation of CO2 by bis(silylene) 46.


15

Scheme 19. Activation of N2O by bis(silylene) 48.

Other examples of silylene-mediated bond activation include the activation of calcogens 57,63 (S, Se, Te) and a large variety of reactions with selected unsaturated organic

functional groups,64 which highlight the highly versatile chemistry of such low valent silicon

species.

Chapter 1

16

1.3.2. Germylenes and Stannylenes

Like silylenes, also germylenes and stannylenes, as well as related Ge(II) and Sn(II)

species, gained significant attention for their ability to activate chemical bonds. In the past

decade, Holl and co-workers reported a considerable amount of work on the indirect

activation of different types of C–H bonds by germylenes and stannylenes.65-66 Initially,

germylene 50 (Scheme 20) was observed to insert into α C–H bonds of organic nitriles,

when employing THF solutions of salts such as LiCl, MgCl2 or LiBr.65 Subsequently, Holl

reported examples of selective C–H activations of ethers and alkanes mediated by

germylenes 50 and 51 (Scheme 20) in the presence of in the presence of aryl halides

(Ph–X, X = I, Br, Cl).66a Benzene, derived from the aryl halide, is produced along with the

C–H activation products (R2GeXR’). The authors proposed a radical mechanism for the C–H

activation, involving free phenyl radical, while oxidative addition of aryl halide to the

germylene was observed as a concentration dependent side reaction. High yields in the C–

H activation products were obtained through the use of high-dilution techniques.66a In

2006, this chemistry was extended to the acyclic N,N-disubstituted stannylene 52

(Scheme 20), which was employed for the C−H activation of alkanes and ethers66b and in

2008, to the cyclic (alkyl)stannylene 53 (Scheme 20), for the C–H bond activation of

alkynes66c and alkenes.66d Additionally, the products obtained from the reactions of 52 with

alkanes and ethers were used in subsequent Stille-type C(sp3)–C(sp2) cross-coupling

reactions to form new C–C bonds.66e

In 2009, Holl also reported on the reactivity of NHSi 54 (Scheme 20) to activate the C–

H bond of alkanes and ethers with Ph−X and further extended this chemistry for NHSi 54

and germylene 50 to alkyl amines.66f Similarly, the reaction of stannylene 53 and Ar−I

with alkanes, alkenes, alkynes and ethers resulted in the corresponding C–H activation

products.66g

Scheme 20. Germylenes, stannylenes and silylene used for the activation of C–H bonds in

the presence of aryl halides.


17

Noteworthy, is also the reactivity of related Ge(II) and Sn(II) hydrides, which were

studied for their ability to activate small molecules.67 In fact, it was observed that, unlike

the tetravalent group XIV hydrides, the Ge(II) and Sn(II) hydrides can activate a large

number of small molecules in the absence of any added catalyst. For example, Ge(II) and

Sn(II) hydrides (55a,b; Scheme 21) react with CO2 to give the Ge(II) and Sn(II) esters of

formic acid (56a,b; Scheme 21).68,69 Similarly, Ge(II) and Sn(II) hydrides react with

carbonyl groups, alkynes, C=N bonds, and also with azo- and diazo-compounds and

azides.68-70,71,72 All these reactions proceed via the insertion of the small molecule into the

Ge–H or Sn–H. The activation of small molecules by such Ge(II) and Sn(II) hydrides, and

by related Si(II) hydrides, has been the subject of a recent review.67

Scheme 21. Activation of CO2 by Ge(II) and Sn(II) hydrides.

Remarkably, certain low valent silicon and germanium species, in analogy to carbenes,

are also able to split the H–H bond of H2. As we have mentioned in the introductory

paragraph (1.1.), the first example of H2 activation by a main group compound under mild

conditions was reported in 2005, when Power discovered that the germanium species

ArGeΞGeAr (57; Ar = C6H3-2,6-(2,6-diisopropylphenyl)2) reacts with molecular H2 forming

a mixture of hydrogenated species (58−60; Scheme 22).5 By contrast, the Sn analogue

(61; Ar = C6H3-2,6-(2,6-diisopropylphenyl)2) reacts with dihydrogen to give only ArSn(µ–

H)2SnAr (62; Scheme 23).73

Scheme 22. The first example of H2 activation by a main group compound.

More recently, also germylenes and stannylenes were found to be able to react with H2

under relatively mild conditions.74-75 In 2008, Power reported on the ability of stannylene

Chapter 1

18

SnAr2 (63: Ar = C6H3-2,6-(2,6-diisopropylphenyl)2) to activate H2 and even NH3, affording

dimeric products bearing two –H (62) or –NH2 (64) bridging ligands (Scheme 23), with

concurrent elimination of ArH. The corresponding lack of reactivity towards H2 and NH3

observed for SnAr#2 (Ar# = C6H3-2,6(Mes)2) and Sn[N(TMS)2]2 (53), suggested that the

higher reactivity of SnAr2 (63) may be attributed to the increased triplet character of 63 in

the ground state, accordingly to the wide C–Sn–C angle (117.6 °).75 Interestingly, related

examples of N–H and H−H bond activation have been reported for a low valent aryl

gallium(I) species (65/65`), which react with both NH3 and H2 forming gallium species 66

and 67 under ambient conditions (Scheme 24).76

Scheme 23. Activation of H2 and NH3 by stannylenes.

Scheme 24. Activation of H2 and NH3 by Ga(I) species.


19

In contrast, extension of this chemistry to germylenes showed the formation of the

tetravalent monomeric products. Germylenes 68 and 69 react with H2 to give germanes

70 and 71, and with NH3 forming germanes 72 and 73 (Scheme 25). The different

reactivity of 68 towards H2 most probably results from the increased steric bulk on the

germanium atom, which favors arene elimination and addition of a second equivalent of H2

to form 70.77 The germanium analogue of silylene 21 (74), also reacts with NH3, but in

contrast to the Si analogue 21, which undergoes formal oxidative addition to the N–H

bond forming the 1,1-addition product (25), with 74 the 1,4-addition product 75 is

observed (Scheme 26). Primary amines react with 74 in a similar fashion.78

Scheme 25. Activation of H2 and NH3 by germylenes.

Scheme 26. Activation of NH3 by germylene 74.

In 2009, Power reported the first example of room temperature reactions of CO with

heavier carbene analogues. Interestingly, while carbenes are known to react with CO to

give ketenes18 (par. 1.2.) germylenes 76 and 77 reacted with CO, to give germanium

oxides 78 and 79 (Scheme 27).79

Ge

Ar

ArGe

H

Ar

H

H

70

(-ArH)

H2

68

Ge

Ar

Ar

NH2

H

72

NH3

Ge

Ar#

Ar#

Ge

Ar#

Ar#

H

H

71

H2

69

Ge

Ar#

Ar#

NH2

H

73

NH3

Ar = C6H3-2,6-(2,6-diidopropylphenyl)2

Ar# = C6H3-2,6-Mes2

Chapter 1

20

Scheme 27. Activation of CO by germylenes.

In addition, the first example of gentle activation of P4 by a low valent Sn compound

has been reported recently.80 The reaction of P4 with the novel, unsymmetrically

coordinated, bis(stannylene) 80 afforded the butterfly-like

bicyclo[1.1.0]tetraphosphabutane derivative 81, by insertion of one P4 tetrahedron into

the Sn−Sn bond of 80 (Scheme 28). Unlike in the case of 80, in 81 both Sn atoms are

four-coordinated, as revealed by single crystal X-ray diffraction studies. We note that while

a few examples of P4 activation by carbenes28-30 and silylenes46,52 have recently appeared in

the literature, this is the first example of P4 activation by a low valent Sn compound, while

no gentle activation of P4 by low valent Ge species has yet been reported. Furthermore, as

we mentioned in the previous paragraph, Driess reported that the germylene analogue of

silylene 21, in contrast to the silicon species (21), is unreactive towards P4.49

Consequently, the search for the P4 activation by low valent Ge species appears to be an

attractive target, to fill in the gap in this series, in the hope that these interesting

compounds will pave the way for the development of a transition metal free and

environmentally friendly P4 activation for industrial processes.

Scheme 28. First example of P4 activation by a low valent Sn(I) species.


21

1.1.1.1.4444.... Group XV Group XV Group XV Group XV CCCCarbene arbene arbene arbene AAAAnalogues: nalogues: nalogues: nalogues: PPPPhosphinideneshosphinideneshosphinideneshosphinidenes and and and and

PPPPhosphenium hosphenium hosphenium hosphenium IIIIonsonsonsons

Having highlighted the ability of stable singlet carbenes and their heavier group XIV

analogues to react with small molecules, the chemistry of their group XV analogues

deserves also attention. Phosphenium ions (R2P+),81 in particular, are of interest for the

metal-free activation of small molecules because, due to the positive charge on the two

coordinate phosphorus atom, they are highly electrophilic82 and are known as singlet

species.83 In contrast, free phosphinidenes (RP) generally possess a triplet ground state84

and consequently they reacts as diradicals, which makes them inadequate for synthetic

applications. So far, singlet phosphinidenes are accessible only upon metal complexation,

which allowed a substantial body of fundamental research on the reactivity of singlet P(I)

species.85,86 In case of nitrenes,87 which also posses a triplet ground state, the singlet-

triplet gap is generally even larger than for phosphinidenes, as for the parent systems

imidogen H−N88 (36 kcal·mol-1) and phosphinidene H−P89 (22 kcal·mol-1). Additionally, for

nitrenes, the lowest energy singlet state is typically predicted to be open shell. In this

paragraph, we will limit our discussion to singlet phosphinidenes and phosphenium ions.

1.4.1. Singlet Phosphinidenes

While strategies for the isolation of stable singlet carbenes, silylenes, germylenes and

stannylenes have been developed, metal-free phosphinidenes are known as transient

species, whose existence has been inferred mainly by trapping reactions and

spectroscopically, which explains why their chemistry developed rather differently than the

one of their group XIV relatives. Substituents that have δ-type lone pair electrons (i.e.

−NX2, −PX2, −OX, −SX) are predicted to lower the singlet-triplet energy gap or even favor

the singlet state.84 While the most stabilized singlet ground states were predicted for P−SF

and P−SCl, from a practical point of view, amino (P–NR2) and phosphino (P–PR2)

derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for

preparing phosphinidenes possessing a closed shell singlet ground state. However,

appropriate precursors for such species have not yet been developed.90 So far, close shell

singlet phosphinidenes, which are suited for synthetic applications, are accessible only

upon complexation with a transition metal stabilizing group which confers them either a

nucleophilic or an electrophilic character.91

Chapter 1

22

Nucleophilic phosphinidene complexes86 are significantly stabilized to such extent that

many could be isolated, however, they typically react via the P=M double bond, as

exemplified by their most characteristic reactions, i.e. the phospha-Wittig reaction with

carbonyl compounds, the addition to protic reagents and the [2+2] cycloadditions with

alkynes.86 Since all these reactions involve the participation of the metal centre, they will

not be discussed further herein.

Electrophilic phosphinidene complexes85 are strikingly more reactive and are typically

generated as transient species from appropriate precursors. The best known and most

typical reaction of electrophilic phosphinidene complexes is the 1,2-cycloaddition to

carbon-carbon double or triple bonds, which occurs with retention of configuration, to give

3-membered phosphiranes and phosphirenes, respectively.85 In a similar fashion, they

react also with hetero-olefinic C=X double bonds, (X = N, O, Si, S, P), giving access to

CXP 3-membered rings.85 Nevertheless, their reaction with C=X double bonds may also

result into a variety of products. For example, the reaction with carbonyl-containing

compounds leads to different products depending on the carbonyl compound used.92

Numerous phosphinidene C–H and C–C insertion products have been reported, but in most

cases these products result from the initial phosphinidene addition to an unsaturated bond

of the substrate, followed by intramolecular rearrangement into the insertion product.85 A

remarkable example of a direct phosphinidene insertion reaction is the insertion of

phosphinidene complexes R−P–M(CO)5 (R = Ph, Me or CH2CH2Cl; M = Mo, W) into a C–H

bond of one of the Cp ligands of ferrocene (82; Scheme 29), which highlighted its strongly

electrophilic character.93

Scheme 29. Insertion of electrophilic phosphinidene complexes into a C−H bond.

Although a substantial body of elegant work has accumulated on electrophilic

phosphinidenes complexes, utilization of their chemistry for synthetic purposes is

significantly hampered by the limited amount of suitable and easily accessible precursors

and also by the difficult demetallation step required to create metal-free species from the

resulting reaction products.


23

1.4.2. Phosphenium Ions

Phosphenium cations (R2P+) are isoelectronic to carbenes and phosphinidenes and

represent another class of low valent, 6-electron, organophosphorus compounds.81 They

differ from phosphinidenes for the fact that they bear two substituents on the phosphorus

atom and consequently they are positively charged, while free phosphinidenes bear only

one substituent and are therefore neutral. More importantly, in contrast to free

phosphinidenes, they possess a singlet ground state.83 Like phosphinidenes, phosphenium

cations are typically generated in situ, although certain phosphenium cations have been

isolated as stable species, owing to the presence of two amino substituents on

phosphorus, which stabilize the low-coordinate P(III) centre through a push-pull effect,

similar to the N-heterocyclic carbenes.84,95 Additionally, also a few stable N,C-bound

phosphenium cations are known.94 Phosphenium cations81 have been known for more than

40 years and interestingly, the isolation of free N-heterocyclic phosphenium cations (NHPs)

in 197295 preceded the one of their carbon analogues (NHCs),14 although they have

garnered much less attention. Due to the increasing number of publications relative to the

activation of small molecules by low valent main group species, the potential use of

phosphenium cations for this purpose has recently been recognized.

Due to the presence of a strongly electrophilic vacant orbital, phosphenium cations

react with Lewis bases96 such as phosphines,96a-c to give the Lewis base adducts of the

phosphonium ion. Additionally, they are known to insert into C–H bonds. The first example

was reported in 1982 by Cowley and co-workers, who observed the insertion of

[(iPr2N)2P]+ (83; [AlCl4]− as counterion) into the C–H bond of stannocene and plumbocene

(84a,b; Scheme 30).97 Subsequently, other examples of C–H activation by phosphenium

ions have been reported, such as the intramolecular rearrangement of [(Me5C5)(tBu)P]+

(85) into phosphonium salt 86 (Scheme 30).98

Scheme 30. Examples of insertions of phosphenium cations into C–H bonds.

Chapter 1

24

Also cycloaddition reactions to dienes are well documented.99 Typically, the reaction of

phosphenium ions to 1,3-dienes leads to the formation of the 1,4-cycloaddition products,

namely phospholenium salts (87; Scheme 31). It is supposed, based on experimental

observations, that the reaction undergoes via a concerted [1+4] cycloaddition.99c Also the

reaction with 1,4-dienes has been investigated. For example, [iPr2NPCl][AlCl4] (88) reacts

readily with 1,4-pentadiene or 1,4-hexadiene to afford phosphorus bi- and tri-cyclic

compounds 89 and 90 (Scheme 31).99c These cycloaddition reactions, as well as the

insertion reactions mentioned above, firmly established the carbenoid nature of [R2P]+.

Scheme 31. Examples of cycloaddition products for the reaction of phosphenium ions with

1,3- and 1,4- dienes.

In 1994, Burford reported the reaction of the neutral, zwitterionic aluminum derivative

91 and the ionic N,N-stabilized phosphenium salt [(iPr2N)2P][AlCl4] (83) with elemental

sulfur,100 which gave the new bis(spiro)tricyclo-dialumina-tetra-aza-dithia-diphosphetane

92 and the previously reported101 thiophosphoryl chloride-aluminum trichloride complex

(iPr2N)2P(Cl)S·AlCl3 (93), respectively (Scheme 32). The aluminum derivative 91 is also

known to react with P=N bonds,102 as illustrated by the reaction of 91 with

(TMS)2N−P=N(TMS) which gave by-cyclic zwitterion 94, formed upon insertion of the

imminophosphine (P=N) unit of (TMS)2N−P=N(TMS) into the P−N bond of 91, followed by

an unusual ring-opening and cycloaddition involving the formation of three bonds (Scheme

33).


25

Scheme 32. Activation of S8 by phosphenium cations.

Scheme 33. Reaction of phosphenium cation 91 with P=N bonds.

Yet, the most remarkable achievements with regard to the activation of small

molecules by phosphenium cations were obtained with the activation of white phosphorus.

In 2001, Krossing and co-workers reported the insertion of the in situ prepared, highly

electrophilic [X2P]+ cations (95; X = Br, I) into one of the P−P bonds of the P4 tetrahedron,

yielding phosphorus-rich binary cage cations [X2P5]+ (96; Scheme 34). Although highly

reactive, 96 could be successfully isolated using non-oxidizing, weakly coordinating

counteranions of type [Al(OR)4]– (R = C(CF3)3).103 This mode of P4 activation is analogous

to the one observed with silylenes46 (see silylene-P4 adduct 30; par. 1.3.1.) and contrasts

to the mode of activation by carbenes,28-30 which attack the P4 skeleton nucleophilically. In

a similar fashion, such [X2P]+ cations, which are generated by silver-salt metathesis of

Ag[Al(OR)4] and PX3, also insert into X–X and P–X bonds of X2 and PX3 (X = Br, I), forming

[X4P]+ (97) and [X5P2]+ (98) salts of the weakly basic anion [Al(OR)4]–.103b In 2009,

Wiegand extended this chemistry to [Ph2P]+ (99) and reported the preparation of mono-,

Chapter 1

26

di- and tri- cationic clusters [Ph2P5]+ (100), [Ph4P6]2+ (101) and [Ph6P7]3+ (102) (Scheme

34).104 In this work, the source of [Ph2P]+ is a molten medium, readily obtained upon

mixing Ph2PCl and GaCl3 with varied stoichiometries. In a previous study, the molten

medium obtained from a 1:1 mixture of Ph2PCl/GaCl3105 had proved to be able to react

with cyclic polyphosphine Ph5P5 to give P–P insertion product 2,3,4,5-cyclo-

tetraphosphanyl-1,4-diphosphonium dication 103,105 which suggested the use of the melt

for the activation of the P4 cluster.

Scheme 34. Insertion of phosphenium ions into P–P bonds.

Similarly, cyclo-1,3-diphospha-2,4-diazane 104, which acts as phosphenium ion source

in the presence of a Lewis acid, reacts with P4 in a stepwise manner to form the novel

clusters [P4((Dipp)NP)2Cl]+ (105) and [(P4)2((Dipp)NP)2]2+ (106) (Scheme 35).106 In a

similar fashion, also Si-based and Al-based relatives 107 and 91 react with 1 equivalent of

P4 yielding compounds 108 and 109, which feature the [P5]+ unit (Scheme 35). These

results highlight the potential use of phosphenium cations for the transformation of white

phosphorus. The next and most important target is the development of P-transfer

reactions, ideally with the use of only catalytic amounts of activating agent, allowing the

direct synthesis of organophosphorus compound from P4. In view of the growing attention

towards low valent main group species for the metal-free activation of small molecules,

further discoveries in this respect are to be expected in the coming years.


27

Scheme 35. P4 activation by heterocyclic phosphenium ions.

Chapter 1

28

1.1.1.1.5555.... Frustrated Lewis PairsFrustrated Lewis PairsFrustrated Lewis PairsFrustrated Lewis Pairs

1.5.1. Discovery of Frustrated Lewis Pairs (FLPs)

A novel and promising approach to the activation and utilization of small molecules has

emerged very recently after the seminal discovery, by the group of D. W. Stephan, that

certain sterically encumbered Lewis acid and Lewis base combinations, which do not

undergo the ubiquitous neutralization reaction to form “classical” Lewis adducts (later

termed as “frustrated Lewis pairs”), could cleave the robust H–H bond of the dihydrogen

molecule.7

In 2006, D. W. Stephan reported on the reversible metal-free H2 activation by

phosphinoborane 110, which effects heterolytic H–H cleavage under mild conditions to

form phosphonium hydroborate 111 (Scheme 36).6 Similarly to the case of singlet

carbenes and their heavier group XIV analogues, the ability of phosphinoborane 110 to

effect heterolytic H–H cleavage arises from the contemporary presence of unquenched

Lewis acidic and Lewis basic centers.

Scheme 36. Reversible H2 activation by phosphinoborane 110.

This discovery gained enormous attention in the scientific community, since it was the

first example of reversible transition metal free H2 activation under mild conditions. In

fact, at that time, the only other main group compound known to react with molecular

hydrogen under mild conditions was Power’s digermyne (57).5

Consequently to this remarkable discovery, also the reactivity of simple stoichiometric

phosphine/borane mixtures (112―119) towards H2 was explored, using toluene as

solvent.107 Phosphine/borane frustrated Lewis pairs tBu3P/B(C6F5)3 (112) and

Mes3P/B(C6F5)3 (113) reacted smoothly with H2 forming the corresponding phosphonium

hydroborate salts [tBu3PH][HB(C6F5)3] (120) and [Mes3PH][HB(C6F5)3] (121) in

quantitative yield, while frustrated Lewis pair tBu3P/BPh3 (114) effected analogous H–H

cleavage, but required longer reaction times and the product (122: [tBu3PH][HBPh3]) was


29

isolated in low yield (33%), most likely as a consequence of the reduced Lewis acidity of

the borane. Frustrated Lewis pairs Mes3P/BPh3 (115), (C6F5)3P/B(C6F5)3 (116) and

tBu3P/BMes3 (117) gave no reaction upon exposure to an H2 atmosphere at ambient

conditions, while phosphine/borane pairs PPh3/B(C6F5)3 (118) and PMe3/B(C6F5)3 (119)

formed stable Lewis adducts and proved to be unreactive towards H2.

These observations brought across the take-home message that H2 activation by such

phosphine/borane pairs occurs only under favorable steric and electronic conditions. In

particular, steric constrains must be sufficient to preclude the mutual quenching of the

Lewis basic and acidic centers, and the Lewis acidity and basicity of the phosphorus and

boron atoms must be properly matched. Especially, the use of strongly electron

withdrawing pentafluorophenyl substituents on the boron atom seemed to be required to

enhance the Lewis acidity of the borane and consequently, the reactivity of the FLP. Due to

this observation, most FLPs are based on B(C6F5)3 or a –B(C6F5)2 fragment as the Lewis

acidic component. By contrast, a large variety of Lewis bases have been used in FLP

chemistry.108

On the other hand, it was soon envisioned that also weakly bonded Lewis adducts

could manifest FLP reactivity. In 2007, the group of Erker reported that phosphinoborane

Mes2PCH2CH2B(C6F5)2 (123), which forms a classical (intramolecular) Lewis adduct, as

indicated by multinuclear NMR spectroscopy and by calculations, reacts smoothly with H2

at ambient conditions to give phosphonium borate Mes2P(H)CH2CH2(H)B(C6F5)2 (124;

Scheme 37).109 The FLP-like behavior of 123 was attributed to the weakness of the

intramolecular P−B bond which allows its dissociation to give a reactive Lewis pair 123’.

Subsequent investigations revealed that 123 possesses an extraordinary FLP-type

reactivity towards a large variety of molecules110,146 and is an active metal-free catalyst for

the hydrogenation of enamines and imines.121a

Scheme 37. Lewis pair 123/123’ and its reactivity towards H2.

Two years later, the group of D. W. Stephan reported on the peculiar case of the

Lutidine/B(C6F5)3 pair (125), which is “at the boundary of classical and frustrated Lewis

pair reactivity”, since it forms a classical Lewis adduct and also exhibits FLP behavior in the

Chapter 1

30

activation of H2, due to partial dissociation of the Lewis adduct which occurs in solution,

probing that classical and frustrated Lewis pairs are not mutually exclusive.111

More astonishing, was the discovery that the strongly bonded, classical Lewis pair

Ph3P–B(C6F5)3 (118), for which no evidence of dissociation was observed by NMR

spectroscopy, consistent with the calculated P–B bond dissociation energy of 39 kcalmol-

1,112 undergoes P/B addition to the triple bond of phenylacetylene in an E-fashion, to give

phosphonium borate Ph3PC(Ph)=(H)B(C6F5)3 (128), as observed for FLPs (o-tol)3P/B(C6F5)3

(126) and (o-tol)3P/Al(C6F5)3 (127).113 This observation revealed that even classical,

strongly bonded Lewis adducts, previously thought to be unreactive, might manifest FLP-

type reactivity, broadly extending the potential of FLP chemistry for small molecule

activation.113

1.5.2. Proposed Mechanisms for the FLP-mediated H–H Bond Splitting and

Association in Frustrated Complexes

Since the discovery that FLPs could split the H2 molecule heterolytically, computational

effort has been devoted to unravel the mechanism for the H2 uptake by such main group

phosphine-borane species.114,115-120

Guo and Li were the first ones to propose a concerted mechanism for the heterolytic

splitting of H2, resulting from the synergic interaction of the boron and phosphorus atoms

with the dihydrogen molecule.115

The enlightening work of Papai,116 provided insight into the nature of the bimolecular

FLP tBu3P/B(C6F5)3 (112). With the aim to unravel the mechanism of the H2 uptake by

112, initially, they examined the possibility that the H2 activation could be initiated by the

side-on interaction of the H2 molecule with the borane, resulting in the donation of the

H−H σ-bond electrons into the Lewis acidic empty orbital of the boron centre. This

assumption was based on the analogy with the mode of H2 activation by transition metals

and considered previous studies on the existence of weakly bound BH3−H2 adduct.117

Subsequent H+ migration to the phosphorus atom would lead to the final phosphonium

borate species. However, the calculations indicated that this interaction is actually

repulsive, in agreement with the observation that for B(C6F5)3 there is a significant

delocalization of the aryl π-electrons into the p(B) vacant orbital of B(C6F5)3, which

prevents the σ-donation from H2 into the boron vacant p orbital and consequent formation

of a H2−B(C6F5)3 complex. Nonetheless, we note that an example of FLP-mediated H2

activation which occurs through the initial side-on interaction of the H2 molecule with the

boron centre, followed by the

proposed for Lewis pair Ar

tBu3P to H2 was also considered,

isolation work which demonstrated that phosphines can weakly interact with H

presumably via an end-on interaction between the

the empty σ* orbital of the H

unfavorable, owing to Pauli repulsion. Based on these

ease of the activation of H

Importantly, it was envisioned that secondary interactions may lead to weak association

between the molecules of the FLP

identified as a minimum on the PES

characterized as a combination of

interactions. The association energy was predicted to be

surprisingly large, suggesting a certain degree of association even at room temperature.

Another characteristic feature

flexibility, which originates

Figure 1. Structure of the

hydrogen bonds (with d(H

According to this new

the FLP into a loosely bound

the reactive pocket of this flexible FLP

centers. The H−H cleavage was described to occur through simultaneous

phosphorus` lone pair with


31

followed by the deprotonation by the Lewis base, has been recently

proposed for Lewis pair ArF2BH/NEt3.118 As an alternative scenario, the end-on approach of

considered, which is supported by previous low-temperature matrix

isolation work which demonstrated that phosphines can weakly interact with H

on interaction between the lone pair on the phosphorus atom

the empty σ* orbital of the H2 molecule.116 However, also such end-on interaction

owing to Pauli repulsion. Based on these observations and considering the

ease of the activation of H2 by FLPs, an alternative picture had to be sorted out.

envisioned that secondary interactions may lead to weak association

between the molecules of the FLP. Indeed, a weakly bound tBu3PB(C6F5)3 complex

as a minimum on the PES (Figure 1). The bonding in this adduct

characterized as a combination of multiple C−HF hydrogen bonds and dispersion

interactions. The association energy was predicted to be of −11.5 kcal·mol −1

suggesting a certain degree of association even at room temperature.

Another characteristic feature of the described tBu3PB(C6F5)3 complex is its structural

originates from the dominance of weak, non directional long-range forces.

Structure of the tBu3PB(C6F5)3 complex reported by Pápai . C–HF type

hydrogen bonds (with d(H–F) < 2.4 Å) are indicated with dotted lines.

According to this new model proposed by Pápai, which involves the pre-organization of

the FLP into a loosely bound frustrated complex, a small H2 molecule can then insert into

this flexible FLP and interact simultaneously with both

−H cleavage was described to occur through simultaneous interaction

with the σ* orbital of the H2 molecule and of the σ(H−H


has been recently

on approach of

temperature matrix-

isolation work which demonstrated that phosphines can weakly interact with H2,

phosphorus atom and

on interaction resulted

observations and considering the

to be sorted out.

envisioned that secondary interactions may lead to weak association

complex was

. The bonding in this adduct was

HF hydrogen bonds and dispersion 1, which was

suggesting a certain degree of association even at room temperature.

complex is its structural

range forces.

HF type

F) < 2.4 Å) are indicated with dotted lines.

organization of

molecule can then insert into

interact simultaneously with both reactive

interaction of the

−H) bonding

Chapter 1

32

electrons with the vacant orbital of B(C6F5)3 and implies progressive weakening of the H—

H bond along the reaction pathway.

The flexibility of such frustrated complex was further probed by additional

computational work by the group of Rhee,119 aimed to unravel the origin of the stability of

FLP 112. In fact, their calculations further indicated that dispersive interactions between

the phosphorus` lone pair and the delocalized π-system in the borane render the system

highly flexible and provide considerable amount of entropic stabilization in the pair

formation. Additionally, they observed that a substantial stabilization of the frustrated

complex also arises from the non covalent interaction between the boron and phosphorus

atoms.

Once the importance of the secondary non-covalent interactions, which are responsible

for the formation of a “frustrated complex”, was recognized, subsequent computational

studies by Grimme`s group120 shed some doubt on the linear P−HH−B arrangement in

the transition state previously proposed by Pápai116 for the reaction of 112 with H2. In

fact, it was noted that a linear P−HH−B arrangement is not possible for

phosphinoborane 123, although this Lewis pair also had proved to react efficiently with H2

at ambient conditions. The authors suggested that the almost linear P−HH−B

arrangement observed in the transition state computed by Pápai is likely to be an artifact

arising from the insufficient theoretical treatment of intramolecular London dispersion

forces between the large substituents. Furthermore, based on their calculations which

include dispersion forces, they presented a simpler mechanistic picture of the basic

activation step that emphasizes on the polarization of H2 induced by the electric field of the

FLP inside its cavity. In fact, computing a relaxed two-dimensional potential energy

surface (PES) with a fixed linear P−H−H−B unit and with the most important H−H and

P−B distances as variables, revealed that once the H2 molecule is within the FLP cavity,

the H−H dissociation is actually barrierless and that the observed barrier should rather be

attributed to some kind of entrance process of H2 into the FLP`s reactive pocket. A

detailed analysis of the transition state for the reaction of 112 with H2 confirmed that the

entrance of the H2 along with the initial opening of the FLP, is the key step of the reaction

responsible for the observed barrier and that once the H2 inside the FLP cage, the reaction

proceeds without a barrier. This conclusion describes the reaction of 112 with H2 as an

effectively bimolecular process between a prepared Lewis pair and H2. Analogous

conclusions were reported also for the unimolecular Lewis pair 123. Furthermore, the

authors proposed a simpler mechanistic picture where the FLP itself is neglected entirely

and replaced by an electric field. Almost exact (FCI/aug-cc-pVQZ) potential energy curves

for dihydrogen dissociation in an electric field of varying strength applied along the H−H


33

bond axis were computed. Above a critical field strength of about 0.05–0.06 a.u. the

potential energy curves start to exhibit a maximum, which indicates heterolytic

dissociation to the H+/H− ion pair. The corresponding barrier and its position are strongly

field-dependent: with increasing field strength the transition state moves to smaller H−H

distances and the activation barriers is reduced. A rational conclusion is that the

magnitude of the field along the bond axis in the region of the H2 molecule should be as

large as possible for small molecule activation by FLPs.

In conclusion, it is nowadays commonly accepted that FLPs activate H2 (and chemical

bonds in general) by polarization of the chemical bond, which occurs owing to the electric

field created by their donor/acceptor atoms and that the observed reaction barriers are

mainly due to the formation of a prepared FLP.

1.5.3. Reactivity of Frustrated Lewis Pairs

Hydrogenation reactions

Since their discovery, the ability of FLPs to activate H2 has been extensively studied.7

The most interesting aspect, is the fact that the phosphonium hydridoborate salts that

result from the facile H–H splitting by the phosphine/borane pairs, have (in some cases)

the ability to transfer the H+/H– couple to organic substrates, with concurrent regeneration

of the FLP, acting as unique metal-free catalysts for the hydrogenation of organic

molecules.121 This was first demonstrated with FLP 110, which was used for the

hydrogenation of imines and aziridines.121c Such FLP catalyzed imine reductions proceed

via the initial imine protonation, followed by hydride transfer from the hydridoborate. Also

phosphinoborane 123, among others, soon proved to be a very active metal-free catalyst

for the hydrogenation of imines and enamines.121a Similarly, also amine/borane-based FLPs

were used for metal-free catalytic hydrogenations of imines and enamines.121d

Having demonstrated the metal-free catalytic hydrogenation of imines by FLPs, the

groups of Stephan121e and Klankermayer121f probed that the imine itself could be used as

the Lewis base of the FLP, be it that sufficient steric bulk on the imine must be provided to

avoid quenching of the borane catalyst B(C6F5)3 upon Lewis adduct formation with the

imine itself or with the resulting amine.121e In addition, B(C6F5)3 and the additional base

PMes3 were found to catalyze the reduction of electron poor imines and protected nitriles.

The rate acceleration observed with PMes3 as additional base is presumed to be due to the

rapid reaction of PMes3/B(C6F5)3 with H2, giving [Mes3PH][HB(C6F5)3], which reduces the

imine.121e

Chapter 1

34

Analogous borane-catalyzed reductions of imines were subsequently reported by Chen

and Klankermayer, which also reported the first example asymmetric reduction of an

imine, which gave an enantiomeric excess of 13%, using a chiral borane.121f Despite the

poor enantioselectivity, this work served as a proof of principle demonstrating the

possibility to extend FLP chemistry to asymmetric synthesis. Consequently to these initial

findings, more effective chiral boranes were targeted for application in asymmetric

hydrogenations and the first examples of highly enantioselective hydrogenations of

prochiral imines with chiral FLPs were reported (see FLP-H2 adduct 129; Scheme 38).121g

Scheme 38. Asymmetric hydrogenations of prochiral imines using chiral FLP-based

hydrogenation catalysts.

In the few years that followed the discovery of frustrated Lewis pairs, their use for

hydrogenation purposes has been rapidly developing and a substantial amount of work has

been reported since then.7 A comprehensive review7e illustrating the variety of FLP

catalysts that have been studied and the range of substrates where FLP reductions have

been shown to be effective in catalyzing the hydrogenation reaction has been published

very recently, thus we will not discuss FLP hydrogenations more in detail, in this chapter.

Instead, we will conclude this paragraph by pointing out that the group of D. W. Stephan

has recently reported another remarkable achievement, the FLP mediated reduction of

aromatic rings.122 Although the scope studied to date is still limited, it is clear that this

finding should provide synthetic chemists with an unconventional strategy to cyclic amine

derivatives.

Dehydrogenation reactions

Interestingly, FLPs have also been observed to promote de-hydrogenation reactions.

For example, Miller reported that FLP 112 reacts with aminoboranes NH3−BH3 and

Me2NH−BH3 to afford dehydrocoupling products (NH2−BH2)n and (Me2N−BH2)2 and

phosphonium borohydride salt [tBu3PH][HB(C6F5)3] (120).123


35

Additionally, it was observed that the combination of amines iPr2NH and iPr2NEt with

B(C6F5)3 resulted in 1:1 mixtures of the corresponding ammonium salts

[iPr2NH2][HB(C6F5)3] (130) and [iPr2NEt(H)][HB(C6F5)3] (131) with the zwitterionic

products of amine dehydrogenation (iPr)(H)N=C(CH3)CH2B(C6F5)3 (132) and

iPr2N=C(H)CH2B(C6F5)3 (133; Scheme 39),124 while the sterically demanding carbene 1,3-

di-tert-butylimidazolidin-2-ylidene and B(C6F5)3 form a frustrated Lewis pair (134), which

in the absence of reactants, exhibits self-dehydrogenation reactivity to give a mixture of

an imidazolidinium borate (135) and an abnormal carbene-borane adduct (136)(Scheme

40).125

Interestingly, Roesky and co-workers recently employed a NHC/B(C6F5)3 1:1 mixture

for the synthesis of germylenes,126 obtained by the FLP-mediated dehydrogenation of the

appropriate germanium precursors.

Scheme 39. Examples of self-dehydrogenation reactivity of aminoborane Lewis pairs.

Scheme 40. Self-dehydrogenation reactivity of a carbene/borane Lewis pair.

Chapter 1

36

1,2-additions and C−H activations

FLPs have also been shown to undergo a large variety of reactions with organic

molecules, which we do not intend to discuss in this chapter. We will limit our discussion to

saying that typically, FLPs undergo 1,2 addition to double bonds, as first demonstrated by

D. Stephan by exposing a solution of the bimolecular FLP tBu3P/B(C6F5)3 (112) to

ethylene, which resulted in the straightforward formation of the zwitterionic species

[tBu3P(C2H4)B(C6F5)3] (137; Scheme 41). Similarly, the reactions with propylene and 1-

hexene gave products 138 and 139, respectively (Scheme 41). Mechanistically, activation

of the alkene by the Lewis acid, which is bound to the less hindered carbon atom in the

final products, is thought to initiate these reactions.127 Upon reaction with conjugated

dienes, FLPs typically undergo 1,4-additions.128

Scheme 41. Reactivity of intermolecular FLP 112 with olefins.

With terminal alkynes both C–H and C≡C bond activation may occur. This is exemplified

by the phosphine/borane pairs tBu3P/B(C6F5)3 (112) and (o-tol)3P/B(C6F5)3 (126) and

phosphine/alane pairs tBu3P/Al(C6F5)3 (140) and (o-tol)3P/Al(C6F5)3 (127), which gave

phosphonium borate salts [tBu3PH][PhCCB(C6F5)3] (141) and [tBu3PH][PhCCAl(C6F5)3]

(142) and C≡C addition products (o-tol)3P(Ph)CC(H)B(C6F5)3 (143) and (o-

tol)3P(Ph)CC(H)Al(C6F5)3 (144) (Scheme 42).113 In addition, the reactivity of FLPs towards

olefins and alkynes has also been exploited to effect intramolecular cyclizations of

sterically encumbered amines with intramolecular olefin or acetylene fragments in the

presence of a Lewis acid, affording five and six-membered heterocyclic ammonium-borate

species.129

Scheme 42. Reactivity of intermolecular FLPs with phenylacetylene.


37

THF ring-opening

The FLP mediated ring opening of the THF molecule is also well documented.111,130

Interestingly, the first example of THF ring-opening by a Lewis pair was reported back in

1950 when Wittig described the reaction of Ph3CNa with THF(BPh3) which afforded, quite

surprisingly, the anion [Ph3C(CH2)4OBPh3] (145), resulting from the ring-opening of the

THF molecule (Scheme 43).131 Since this early study, the ability of Lewis acidic centers to

promote THF ring-opening reactions in the presence of Lewis bases has been observed for

a consistent number of systems.130

Scheme 43. First example of THF ring-opening by a FLP.

Activation of NH3 and other amines

In an effort to further probe the activation of chemical bonds by FLPs, the activation of

the N–H bond of NH3 and of other amines was also investigated. Carbene-based FLP

NHC/B(C6F5)3 146, which is also capable of cleaving heterolytically the H–H bond of the

dihydrogen molecule forming ionic complex [NHC(H)][HB(C6F5)3] (147),132,133 proved to be

able to split the N–H bond of NH3 to give 148 (Scheme 44) and other amines.133 Such

reactions were performed by pre-coordinating the amine to the Lewis acidic borane

B(C6F5)3 and subsequently reacting the resulting amine-borane Lewis adducts with the

carbene. The high nucleophilicity (basicity) of the carbene is then capable of abstracting a

H+ from the ammonium moiety. It has to be noted that Bertrand observed no reaction

upon reacting NHC-type carbenes alone with either H2 or NH3, whereas both H2 and NH3

reacted with the more reactive AACs.19

Scheme 44. Activation of H2 and NH3 by a carbene-based FLP 146.

Chapter 1

38

Activation of P4

Carbene-based FLP 146 has also been used for the controlled activation of P4, to afford

an adduct (149) in which an abnormal carbene and B(C6F5)3 are bound in a trans,trans

fashion to a butterfly bicyclo[1.1.0]tetraphosphabutane moiety (Scheme 45).134

Interestingly, the selective cleavage of a single P–P bond has rarely been observed since

degradation of P4 typically proceeds further.2,32

Scheme 45. Reaction of NHC/B(C6F5)3 frustrated Lewis pair 146 with P4.

Considering the great interest in the development of new methodologies for the

activation of white phosphorus using main group elements and compounds,32 it is quite

surprisingly that no other examples of P4 activation by FLPs have yet been reported.

Reactions with N2O

Another remarkable finding, was the discovery that FLPs have the ability to form stable

adducts with N2O. N2O is only a minor component of the atmosphere but it was estimated

to be circa 300 times more potent as greenhouse gas than CO2.135 However, N2O is a

potentially strong and environmental friendly oxidant (the side product is N2), but its high

kinetic stability has hampered its use. Main reason for this is that N2O is a very poor

ligand, due to its inability to act as either good σ-donor or π-acceptor and consequently

very few N2O complexes are known.136 In 2009, Stephan reported on the complexation

of N2O by FLPs tBu3P/B(C6F5)3 (112) and tBu3P/B(C6F5)2Ph (150), forming FLP–N2O

complexes 151 and 152 (Scheme 46) which were notably the first N2O complexes ever

characterized by X-ray diffraction.137

Scheme 46. First examples of FLP-N2O complexes.


39

Subsequently to this discovery, the ability of FLP-N2O complexes to undergo Lewis acid

exchange reactions was explored.138 The reaction of 151 with the toluene adduct of

Zn(C6F5)2 resulted in the partial formation of a new compound (156), which showed no

signals in the 11B NMR suggesting that exchange of the borane B(C6F5)3 moiety of N2O

complex 151 with the Zn(C6F5)2 had occurred. To facilitate the Lewis acid exchange, N2O

complex tBu3P—N2O—B(C6H4F)3 (153) was prepared following an analogous protocol, and

its reactivity towards (tol)Zn(C6F5)2 was exploited. Indeed, the Lewis acid exchange

resulted facilitated due to the reduced Lewis acidity of the borane (B(C6H4F)3 instead of

B(C6F5)3) bound to the tBu3P−N2O fragment. Reacting tBu3P−N2O−B(C6H4F)3 (153) with 1,

1.5 and 2 equivalents of (tol)Zn(C6F5)2 resulted in the facile formation of compounds 154,

155 and 156, respectively (Scheme 47). While 154 proved to be the dimer of the

expected Lewis acid exchange product, X-ray analysis of 155 showed a single pseudo-

tetrahedral Zn centre that bridges two [tBu3P−N2O−Zn(C6F5)2] units where the Zn atoms

are coordinated to the O and N atoms of the N2O fragment yielding two chelating four-

membered [ZnN2O] rings. Compound 156, whose formation was previously observed in

the preliminary experiments with tBu3P−N2O−B(C6F5)3 (151), was obtained in high yield

upon reacting tBu3P−N2O−B(C6H4F)3 (153) and (tol)Zn(C6F5)2 in 1:2 ratio. Importantly,

the characterization of 154, 155 and 156 illustrates multiple binding modes for the

interaction of an N2O fragment with a metal.

Scheme 47. Lewis acid exchange reactions. (a) 1 eq. of (tol)Zn(C6F5)2; (b) 1.5 eq. of

(tol)Zn(C6F5)2; (a) 2 eq. of (tol)Zn(C6F5)2

N

N

O

Zn

(tBu)3P

C6F5 C6F5

Zn

C6F5

C6F5

156

(tBu)3P NN O

B(C6H4F)3

153

O

Zn

O

Zn

C6F5

C6F5 C6F5

NN

N P(tBu)3

N(tBu)3P

154

O

Zn

C6F5 C6F5

C6F5

N N

ZnZn

N N

O

P(tBu)3(tBu)3P

C6F5

C6F5

C6F5

155

(a)

(b)

(c)

C6F5

Chapter 1

40

Recently, this chemistry has been extended to other tBu3P/borane combinations, using

boranes B(C6F5)2Mes, B(C6F5)2(OC6F5), B(C6F4-p-H)3, B(C6H4-p-F)3 and diborane 1,4-

(C6F5)2BC6F4B(C6F5)2 (with 2 equivalents of tBu3P) as the Lewis acidic component. Room

temperature reactions yielded mono- and bis-zwitterionic species tBu3P(N2O)B(C6F5)2Mes

(157), tBu3P(N2O)B(C6F5)2OC6F5 (158), tBu3P(N2O)B(C6F4-p-H)3 (159), tBu3P(N2O)B(C6H4-

p-F)3 (160) and tBu3P(N2O)B(C6F5)2C6F4(C6F5)2B(ON2)PtBu3 (161) (Scheme 48).139

N2O capture was similarly achieved using FLP Cy3P/B(C6F4-p-H)3 (162) yielding the

zwitterionic species Cy3P(N2O)B(C6F4-p-H)3 (163) (Scheme 48). Attempts to extend the

chemistry to other phosphorus or nitrogen bases failed, indicating that N2O capture is

strongly dependent on the nature of the base and suggesting that both σ-donation and π-

acceptance stabilize the tBu3P–N2O fragment. In this respect, it is noteworthy to point out

that N2O complexation by NHCs, which are highly Lewis basic compounds, has been

recently reported and the resulting NHC—N2O adducts proved to be remarkably stable.34

Reaction of 160 with [Ph3C][B(C6F5)4] resulted in facile transfer of the robust tBu3P(N2O)

fragment to the stronger Lewis acid Ph3C+ generating [tBu3P(N2O)CPh3][B(C6F5)4] (164)

(Scheme 48). In a similar manner, compounds 152, 157, 158, 159 and 161, can be

obtained from 160 upon the exchange of the weaker Lewis acid B(C6H4-p-F)3 with the

more Lewis acidic boranes.139 Similar Lewis acid exchange reactions of 160 with

titanocene and zirconocene cations generate transition metal and phosphine stabilized

nitrous oxide salts, of the form [tBu3P(N2O)MCp2Me][MeB(C6F5)3], (M = Zr or Ti).140

Scheme 48. FLP-N2O complexes.

(tBu)3P NN O

B(R1)2(R2)

157: R1 = C6F5; R2 = Mes158: R1 = C6F5; R2 = OC6F5159: R1 = R2 = C6F4-p-H160: R1 = R2 = C6H4-p-F

(C6F5)2B

F F

B(C6F5)2

FF

ON

N

O N

N

(tBu)3P

P(tBu)3

161

Cy3P NN O

B(C6F5-p-H)3

163

(tBu)3P NN O

CPh3

164

B(C6F5)4


41

Reactions with NO

Recently, the group of Erker further extended FLP chemistry to another oxide of

nitrogen: nitric oxide (NO). Remarkably, this lead to the discovery of a new strategy that

allows easy access to a novel family of aminoxyl radicals.

Phosphinoborane Mes2P(CH2)2B(C6F5)2 (123) reacts with NO to form a persistent

heterocyclic N-oxyl radical 165 (Scheme 49), related to the well known TEMPO.141 X-ray

analysis of 165 revealed that both the P and B atoms had bound to the N atom of the

incorporated NO. More importantly, while NO itself is a poor H atom abstractor, owing to

the weakness of the resulting H−ON bond (approximately 47 kcal·mol-1),142 FLP-NO

complex 165 proved to be able to easily undergo H atom abstraction reactions. This is

illustrated by the reaction with 1,4-cyclohexadiene (C−H bond dissociation energy of circa

76 kcal·mol−1)142 which gave 166 and benzene as by-product, and the reaction of 165

with cyclohexene or ethylbenzene, which gave 1:1 mixtures of 166 and 167 or 168,

respectively (Scheme 49).141

Interestingly, the reaction of the bimolecular FLP tBu3P/B(C6F5)3 112 with NO did not

give the analogous N-oxyl radical: instead, the already known 112-N2O complex 151,

along with the formation of oxide tBu3P-O-B(C6F5)3, was observed.141 This reaction follows

a course related to the well known disproportionation of NO to give N2O and R3P=O upon

reaction of NO with phosphines PR3,143 which highlights the fact that the ability of the

intramolecular FLP 123 to form a P/B chelate with NO plays a critical role in the isolation

of the NO adduct 165. FLP-NO species 165 represented a novel addition to the important

family of persistent N-oxyl free radicals and expands FLP chemistry to radical species.

In fact, we note that recently this chemistry has been further extended to structurally

related P/B frustrated Lewis pairs (in particular, P and B on adjacent carbon atoms, which

allow N,N-addition of the FLP in a chelate fashion), which showed analogous reactivity

towards NO, and resulted in a unique family of easily accessible aminoxyl radicals.144

Chapter 1

42

Scheme 49. Novel N-oxyl radical 165 and its reactions with C−H bonds.

Reactions with C=O bonds and CO2 capture

Frustrated Lewis pairs also undergo 1,2-addition reactions to carbonyl compounds.

Typical examples are the reaction of FLP Mes2P(CH2)2B(C6F5)2 (123) with benzaldehyde110d

or the FLP addition to the C=O bond of isocyanates.145,147 Remarkably, FLPs were also

discovered to react with CO2 which, owing to its role as a greenhouse gas and potential

use as a C1 chemical feedstock, is gaining increasing attention.1

The first example of CO2 capture by frustrated Lewis pairs dates up to 2009, when

Stephan and Erker reported, in a joint publication, on the reversible complexation of CO2

by tBu3P/B(C6F5)3 (112) and Mes2PCH2CH2B(C6F5)2 (123), to give CO2-adducts 169 and

170, respectively.146 Liberation of CO2 from 169 occurs upon heating at 70 °C, while CO2-

adduct 170, formed under pressurized conditions (2 atmosphere CO2) rapidly loses CO2 in

solution at temperatures above −20 °C, to reform the starting material 123 (Scheme

50).146 In addition, we reported the CO2 capture by P/B-based geminal FLP 171 (see

Chapter 2) which, in contrast to the previously reported FLP-CO2 adducts 169 and 170,

resulted remarkably stable towards loss of CO2.147

Following these initial reports, we further extended this chemistry to P/Al-based FLPs

and reported on the reversible CO2 capture by the geminal P/Al pair 173 (Scheme 51; see

Chapter 3).148 While CO2 adduct 174 is stable at ambient conditions, liberation of CO2

occurred smoothly upon heating at 135 °C under vacuum for 2 minutes, which resulted in

the complete reformation of FLP 173. More recently, our group (see Chapter 4) and the

group of Fontaine reported, independently, on the related CO2 capture by methylene-

Mes2P B(C6F5)2N

O

165

Mes2P B(C6F5)2N

OH

166

+

1/2

1/2

Mes2P B(C6F5)2N

O

167

+

1/2

Mes2P B(C6F5)2N

O

168

+

1/2

166166


43

bridged, P/Al-based dimeric Lewis adducts (175a-c; Scheme 51), probing the concept that

FLP chemistry can be accessible also from classical, strongly bonded Lewis adducts.149

Scheme 50. CO2 capture by P/B-based Lewis pairs.

Scheme 51. CO2 capture by P/Al-based Lewis pairs.

Of particular interest is the fact that the CO2 capture may offer the opportunity to

convert this cheap and renewable C1 source into useful chemicals. Efforts in developing

Mes2P AltBu2

Ph

O

AltBu2Mes2P

Ph

O

+CO2 , RT

-CO2 , 135 °C

+CO2 , RT

173

175a,b,c

174

176a,b,c

P

Al P

Al

Me

Me

MeMe

RR

RR

R2P AlMe2O

Oa: R = tBub: R = Mec: R = Ph

Chapter 1

44

homogeneous as well as heterogeneous processes that utilize CO2 to produce CO, formic

acid or methanol have been undertaken,1 however further improvements are required.

Methanol is considered a very valuable product because it serves as precursor to many

useful organic chemicals, as a substitute for fossil fuels and for the generation of electricity

in fuel cells.150 Additionally, it can be stored and transported safely.

In 2009, Ashley and co-workers demonstrated that the frustrated Lewis pair

TMP/B(C6F5)3 (177) can be used to convert CO2 into MeOH.151 Addition of CO2 to the

already known FLP 177 (4 equivalents) under an H2 atmosphere showed quantitative

conversion (after 6 days, at 160 °C) into (CH3O)B(C6F5)2 via formato complex

[TMPH][H(CO2)B(C6F5)3] (179), formed upon CO2 insertion into the H–B bond of

phosphonium borate salt [TMPH][HB(C6F5)3] (178), which results from the H–H heterolytic

splitting by the TMP/B(C6F5)3 pair. Vacuum distillation at 100 °C lead to the isolation of

MeOH (17–25% yield) as the sole C1 product, alongside C6F5H and TMP by-products.151

The same TMP/B(C6F5)3 FLP 177 was employed later also by the group of Piers to

effect the transformation of CO2 into CH4 using triethylsilane as the reducing agent.152 It

has to be noted that a similar transformation of CO2 using silanes as sacrifical reducing

agents was reported in 2009 by Zhang and Ying: in this work an NHC was used as

organocatalyst for the hydrosilylation of CO2 to give R3SiOCH3 which can be easily

transformed into R3SiOH and methanol upon quenching with H2O.153

In 2010, Stephan`s group reported on the room temperature reduction of CO2 to

MeOH by Al-based FLPs using ammonia-borane as the reducing agent. CO2 is first reacted

with a 2:1 mixture of AlX3 (X = Cl or Br) and PMes3; the resulting species

Mes3P(CO2)(AlX3)2 (180a,b: (a) X = Cl or (b) X = Br; Scheme 52), whic

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Vrije Universiteit Amsterdam 1... · Chapter 1 2 1.111..111.1. ... IIIIntroductionntroductionntroduction The activation of small molecules like H 2, CO 2, NH 3, P 4 and N 2O among