STAGE 2005-2006

M1 de Physique/Chimie

CHERON, Nicolas

École Normale Supérieure de Lyon Université Claude Bernard Lyon1

M1 2005/2006 Option Chimie

HAPTOTROPIC SHIFT OF A CpCo MOIETY ALONG LINEAR [3]PHENYLENE Abstract In an attempt to measure the activation energy of the shift of the CpCo moiety along a phenylene ligand, the synthesis of the complex of the linear [3]phenylene, then its desymmetrization was studied. In this report, we rst present the [N]phenylenes, a novel class of hydrocarbons, then introduce the discovery and the previous work on the haptotropic shift before presenting theoretical calculations.

Finally, we present the work which was done

during this internship.

Keywords [N]Phenylenes, Haptotropic Shift, Cobalt Cyclisation, Aromaticity, Desymmetrization

Latimer Hall (6th oor), Room 635 Department of Chemistry University of California Berkeley, CA 94720, USA http://www.berkeley.edu Prof K. Peter C. Vollhardt

([email protected])

August, 21

th

2006

Contents Introduction

2

1 [N]Phenylenes

3

1.1

Presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

1.2

Synthesis

4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Haptotropic Shifts

7

2.1

The Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

2.2

Previous Work

9

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Theoretical Calculations

11

3.1

The [3]Phenylene Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

3.2

The [5]Phenylene Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

3.3

The Benzodicyclobutadiene Complex . . . . . . . . . . . . . . . . . . . . . .

13

3.4

Binding Energies

13

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 Results and Discussion

14

4.1

The Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14

4.2

Alternative Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

4.3

Experimental Part

16

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conclusion

19

Acknowledgements

19

Bibliography

20

A Theoretical Structures for the Shift along [3]Phenylene

23

B Theoretical Structures for the Shift along [5]Phenylene

24

C Theoretical Structures for the Shift along Benzodicyclobutadiene

28

1

Introduction The concept of aromaticity is one of the most used in organic chemistry [1] (60 000 references to "aromaticity" or "aromatic" can be found in the scientic literature between 1981 and 2000) whereas a precise denition of the aromaticity still remains to be found [2]. The denition of aromaticity has evolved during the years since its introduction by Kekule in 1866 [3]. First identied according to their smell (which, everyone may agree, may no longer be a good denition), the following descriptions was suggested for the definition of aromatic compounds : derivatives of benzene (then extended to cyclic systems with 6

-electrons), compounds with benzene-like reactivity (which was after precised as

electrophilic substitution rather than addition), compounds with a non saturated cyclic structure in which double bonds are conjugated, compounds which follows the Hückel's rule, compounds with special magnetic properties, compounds with particular NMR chemical shifts... [2] Several criteria are nowadays used for aromaticity, such as ASE (Aromatic Stabilisation Energies), RE (Resonance Energies),



(Magnetic Susceptibility Exaltation), HOMA

(Harmonic Oscillator Model of Aromaticity) and NICS (Nucleus Independent Chemical Shift) (this last one is now one of the most used) ; these criteria can be separated in three big families : geometric, energetic and magnetic criteria. The concept of aromaticity was shown to be multidimensional according to the fact that depending on the chosen criteria, a molecule can be more or less aromatic [2]. Aromaticity is clearly linked to benzene and its derivatives. Benzene was discovered by Faraday in 1825 [4] who reported its stability and reactivity.

The rst denition of

aromaticity was given by Kekule in 1866 [3] as compounds which are derivatives of benzene, with a suggestion of a cyclohexatriene structure for benzene in 1865 [5]. The circle in an hexagon as the symbol of benzene was introduced by Armit and Robinson in 1925 [6]. In 1931, Hückel developed a theory of molecular orbitals, which, applied to aromatic compounds, explained the stability of benzene. He suggested what is now known as the

- and -components and he called cyclic systems with (4n +2)  -electrons aromatic, those with (4n )  -electrons antiaromatic

Hückel's rule [7] : molecular orbitals were separated in and the others non-aromatic.

The simplest examples of aromatic and antiaromatic sys-

tems are benzene and cyclobutadiene (rst synthesized in 1965 [8], 140 years after benzene). Thus, aromatic compounds (such as benzene) and antiaromatic ones (such as cyclobutadiene) have really dierent properties. We can therefore wonder what would happen if we juxtapose one of each system (or more), and which of each character would prevailed.

2

Chapter 1

[N]Phenylenes 1.1

Presentation

[N]Phenylenes are the answer to the question "what would happen if we juxtapose aromatic and anti-aromatic cycle ?" ; [N]phenylenes are conjugated systems made by the alternation of benzene rings and cyclobutadiene rings, where N is the number of benzene rings ; unless the structure is cyclic, there are N-1 cyclobutadiene rings.

The smallest

example is the [2]phenylene (Figure 1.1), know as the biphenylene (commercially available [9]), discovered in 1941 by Lothrop [10].

Since this early age, a lot of ways have been

investigated to synthesize biphenylene [11].

Surprisingly, this system which has 12

-

electrons (therefore which shouldn't be stable according to Hückel's rule) is very stable. More, it has an aromatic character : it undergoes electrophilic substitution rather than addition, at a rate which is comparable to the one of naphtalene ([12], [13]).

Figure 1.1: Resonance Forms of Biphenylene The study of the bond length of biphenylene provides some interesting information (Figure 1.2) ([12], [14], [15]). We can there notice that the structure tries to avoid the conjugation between the 2 benzene moieties with for example a long bond of 1.514Å between those two. We can also notice that there is a strong bond alternation, therefore with more localised

-electrons

1

than for benzene (the highest dierence of length is 0.054Å) . The

second resonance form of Figure 1.1 is therefore the one with the highest contribution.

Figure 1.2: Bond Length (Å,

1

italic ) and Bond Angles (°, bold) of some Structures

Typical length are 1.53Å for C-C and 1.34Å for C=C.

3

Several isomers of [N]phenylenes exist, such as linear, angular, zigzag, bent, branched or circular

2 ones (Figure 1.3). For N>5, angular [N]phenylenes get an helical structure

(Figure 1.3(g)). Mixed topologies can occur for high values of N. The number of isomers increases quickly with N : there are 2 [3]phenylenes, 5 [4]phenylenes, 12 [5]phenylenes and 37 [6]phenylenes. The 2 [3]phenylenes, the 5 [4]phenylenes and 7 out of the 12 [5]phenylenes have been synthesized [16]. 3 [7]phenylenes have also been made, including the helical [7], such as the helical [6],[8] and [9]phenylene ([17], [18]) which make a total of 21 synthesized phenylenes (including biphenylene).

(a) Linear

(b) Angular

(e) Branched

(c) Zigzag

(f) Circular

(d) Bent

(g) Helical

Figure 1.3: Several Isomers of [N]Phenylenes Angular phenylenes have been studied a lot, which has lead to the synthesis of the helical phenylenes.

The angular [3]phenylene has a resonance form which can avoid cy-

clobutadienoid circuit, which is not the case of the linear [3]phenylene in which the central ring increases its aromaticity (relatively to the angular structure) : this dierence brings an increase of stability for the angular isomer. In general, angular and zigzag phenylenes are thermodynamically more stable than their linear counterparts and linear phenylenes are more aromatic, therefore they might be more reactive. The central ring of the branched [4]phenylene (Figure 1.3(e)) has very localized double bond, making it a cyclohexatriene unit. Air-sensitivity of phenylenes increases along the series while solubility in common organic solvents decreases and the binding to CpCo moiety increases. It can also be precised

n

that when N is odd, [N]phenylenes have (4 +2)

1.2

-electrons and (4n ) when N is even.

Synthesis

Many ways have been investigated for the synthesis of biphenylene (Figure 1.4; [11], [12]), but only a few were ecient for higher phenylenes.

Just a few were good mainly

because phenylenes have strain and because of their partially anti-aromatic character. The approach used by Lothrop in the rst synthesis of biphenylene is an intramolecular coupling reaction of a 2,2'-dihalobiphenyl [10]. oxide at 350

°C

Compounds were treated with cuprous

producing biphenylene in 5 (X=Br) and 21% (X=I) yield. This method

including copper-mediated Ullman cyclisations of 2,2'-dihalobiphenyl has been improved to give biphenylene in good yield (81%) [19]. The dimerization of benzyne occurs with yields

2

Research for the circular [6]phenylene, called antikekulene, are currently carrying out in the Vollhardt

group.

4

in the range of 4 to 54% [13], but lacks regioselectivity for substituted system. The rst way to phenylenes with N>2 was the ash vacuum pyrolysis of benzocinnolines ; biphenylene was synthesized with 60% yield

via

the intramolecular ring closure of the biphenyldiradical

intermediate [20]. Linear [3]phenylene was made with a 11% yield (800°C, 0.04 torr) [21]. This method is not very ecient because of the diculty to make the precursor and the necessity for them to be volatile and also because the reaction is incomplete and leads to

=

a mixture of products, therefore yields are very low for phenylenes with N6

2 [22].

The most ecient way, which made the research on phenylenes possible, is the cyclotrimerization of alkynes catalyzed by CpCo(CO)2.

This method lead to a 96% yield for the

cyclization of ortho-diethynylbenzene with BTMSA (bis(trimethylsilyl)acetylene) to make 1,2-bis(trimethylsilyl)-biphenylene [23] (right of Figure 1.4).

The cyclotrimerization of

alkynes is very exothermic and was discovered a long time ago : the rst cyclization of acetylene to benzene was reported in 1866 by Berthelot [24], in which high temperatures were required (400

°C).

Figure 1.4: The Early Ages of the Synthesis of Biphenylene and a Recent Ecient Way The mechanism of the cyclization was not solved during a long time ; the remaining question was to know if the insertion of the last alkyne would occur following a Diels-Alder type reaction or an insertion reaction into a Co-C

-bond giving a cobaltacycloheptatriene

(Figure 1.5).

Figure 1.5: Old Proposed Mechanisms

5

The answer was found recently by the mean of DFT calculations [25] (Figure 1.6). The mechanism begins with two carbonyl-alkyne exchanges of dicarbonylcyclopentadienylcobalt(I), followed by an oxidative coupling of the two alkynes to form the cobaltacyclopentadiene. To cleave the Co-CO bonds, a broad visible-projector lamp is used : the excitation of the

-electrons of the CO brings them to an antibonding orbital which destabilize the *

back-bonding resulting from the combination of the cobalt's d-orbitals and of CO's

ones. The cobaltacyclopentadiene then occurs a barrierless addition of acetylene to give the CpCo-complex of benzene (

4 ) in a collapse mechanism (path A).

Figure 1.6: Mechanism of the [2+2+2] Cycloaddition of Alkynes If this reaction doesn't occur fast enough, another way is followed, in which singlet cobaltacyclopentadiene relaxes to the triplet ground state (path B) and then reacts with acetylene to give triplet CpCo(

4 -benzene) (path D) with a barrier of 14.1 kcal/mol.

This

sequences connect the ground states of cobaltacyclopentadiene (singlet to triplet excitation energy) by an energy of energy of


E

S

T


E

S

T

= -16.6 kcal/mol and the ones of CpCo(benzene) by an

= -13.4 kcal/mol. Low-lying minimum energy crossing points (MECPs)

were found between the singlet and the triplet spin-state potential energy surfaces : this lead to the proposition of the possibility of a nonadiabatic mechanism (two-state reactivity). In it, singlet cobaltacyclopentadiene relaxes to the triplet ground state (path B), then

6 -benzene) (path B 6 change then occurs to give 20-electron CpCo( -benzene) (path E).

reacts with acetylene to lead to singlet CpCo(

then C). A last spin

Thus, no cobaltacycloheptatriene was found. Intramolecular [4+2] cycloaddition was not established in the parent series because of the collapsing of all potential intermediates but the presence of substituents on both the alkyne and the cobaltacyclopentadiene generates a barrier which allows the location of several isomeric minima of cobaltacyclopentadiene and their connections with CpCo(

6 -benzene) via intramolecular [4+2] cycloaddition

transition states [25].

6

Chapter 2

Haptotropic Shifts 2.1

The Discovery

Attempts to decomplex the CpCo-complex of linear [5]phenylene by usual ways failed, probably because the fact that the paratropism of the four-membered rings implies a strong bind to CpCo [26]. While trying to decomplex this molecule by UV irradiation, a new set of aromatic peaks was discovered by Peter Dosa (Figure 2.1). The signal of this set increased with irradiation until a steady-state was reached after a few hours ; the new peaks disappeared slowly with time, which is the proof of a thermal reversible reaction. These peaks were assigned to the second complex in Figure 2.1 and a value of 24.5 kcal/mol for


G6=

(at 130°C) was found for this reaction (by measuring rate constants and using

the Eyring equation ;


H 6=

= 27.6 kcal/mol,


S 6=

= 7.7 kcal/mol). The experiment was

repeated with the linear [5]phenylene ligand without hexyl substitutes (R = H in Figure 2.1) and this "CpCo-walk" was observed again. Such a shift, where the connectivity of an organometallic moiety to a ligand changes with multicoordinate site possibilities, is called "haptotropic shift" [27]. This novel migration, photoinduced, thermally and photochemically reversible was found to be of considerable interest.

Figure 2.1: UV Irradiations Lead to the Discovery of Haptotropic Shifts

A clue about such a shift was given during the synthesis of the linear [5]phenylene. As it can be seen in Figure 2.2, there was no reason why the CpCo should be bound to the internal four-membered ring ; it should have been linked to the external one. Going further, the CpCo should be bound to a six-membered ring after the cyclization and not to a four-membered ring.

7

Figure 2.2: First Clue about an Haptotropic Shift

Another shift is possible, but was not observed : the degenerate one which is shown in Figure 2.3. The barrier could be calculated with NMR by measuring the temperature at which two signals which can exchange coalesce (the two TMS groups for example), but the migration needs to be fast enough in order to symmetrize the system. No change was observed at 125°C (the experimental barrier), a minimum barrier (


G6=

) was therefore

1 found to be 21.0 kcal/mol for R=Hex and 20.7 kcal/mol for R=H (at 120°C).

Figure 2.3: Possible Degenerate Shift

Due to the fact that the HOMO-LUMO gap of linear [3]phenylene is larger than for linear [5]phenylene, the metal was thought to be more weakly bond to linear [3]phenylene than to linear [5]phenylene (see Figure 3.8 for theoretical proof ), and the rearrangement was expected to be accelerated and observable by NMR for this ligand [27] (Figure 2.4). No shift was seen, and the minimum barrier was found to be 20.2 kcal/mol (


G6=120 C °

).


G6= at the coalescence temperature
G6=Tc = RTc ln( khkb Tcc ) where the rate constant is calculated with the measurement of
 (frequency between two chosen resonances 
 at slow exchange) : kc = p [27]. 2 1

A derivation from Eyring equation provides

8

Figure 2.4: Haptotropic Shift along [3]Phenylene

The shift of an organometallic moiety along a molecule was not new, even if it was the rst time that the shift from one cyclobutadiene to another was observed. Such thermooptical shifts could nd an interesting application in photo storage system.

2.2

Previous Work

A few previous work has been done. In 1971, Nicholas

et al.

reported the shift of a

Cr(CO)3 moiety along 2 structures [28] (Figure 2.5).

(a)

(b)

Figure 2.5: Chromium-Tricarbonyl Shift along Naphtalene and Fluorene Theoretical calculations were then investigated for these two shifts by Albright

et al

in

1983 [29], with also MLn = FeCp for the shift in Figure 2.5(b). They also studied other thermal organometallic moiety shifts such as those Figure 2.6 and applied molecular orbital analysis to nd the pathways of these shifts. The CpCo shift in Figure 2.6(a) showed no intermediate and an exocyclic coordination transition state and the ones Figure 2.6(b) and Figure 2.6(c) showed

2

transition states. The shifts of the cation of the molecule Figure

2.6(a) and the anion of the one Figure 2.6(c) were also investigated.

(a)

(b)

(c)

Figure 2.6: Theoretical Studies of Organometallic Migration A "CpCo-walk" along 1,3,5-hexatriene was reported in 1989 by King and Vollhardt [30] (Figure 2.7), and the mechanism was suggested to occur via an

2 -intermediate.

Figure 2.7: "CpCo-Walk" along 1,3,5-Hexatriene

9

Other (but only a few) examples can be found in the literature, such as the photoinduced, thermally reversible migration shown Figure 2.8 [31] and the photoinduced, thermally and photochemically reversible rearrangement Figure 2.9 [32] (same type of the one Figure 2.1).

Figure 2.8: Rearrangement of an Arene Chromium Complex

Figure 2.9: Rearrangement of a Molybdene Complex Shift along phenylene frames were also investigated. In 1993, Siegel

et al.

[33] studied

the shift of a Cr(CO)3 moiety along branched [4]phenylene (Figure 2.10). The case of an

6 ,6 -haptotropic shift was studied by Oprunenko et al. in 2002 [34] as shown in Figure 3 2 2.11 : the transition state was found to be an  -coordinated species and an  -complex

was found to be as an intermediate.

Figure 2.10: Cr(CO)3 Shift along Branched [4]Phenylene

Figure 2.11:

6 ,6 -Haptotropic Shift along Biphenylene

10

Chapter 3

Theoretical Calculations Tom Albright the

et al.

from the University of Houston, Texas, lead DFT calculations on

4 ,4 -haptotropic shift of a CpCo moiety along phenylenes to determine the reactional

pathway and to get an energy barrier for it [35]. In every gure, the solid circles ( ) represent the Global Minimum/Ground State (GS), the open circles ( ) are for Local Minimum (LM) and the stars (

) for Transition State

(TS).

3.1

The [3]Phenylene Complex

The pathway for the complex of the linear [3]phenylene was found to have 3 transition states and 2 local minima (Figure 3.1).

Figure 3.1: Projection of the Co Atom during the Haptotropic Shift along [3]Phenylene The relative energy of the local minima was found to be 10.9 kcal/mol (LM and the transition states have relative energies of 26.9 kcal/mol (TS1 kcal/mol (TS2

3 ,  ).

,

2)

,

4)

and of 24.9

The curve of the energy along the reaction path is shown in Figure

3.2 and the potential energy surface diagram is shown in Figure 3.3.

The structures of

each state are shown in Appendix A.

Figure 3.2: Energy along the

Figure 3.3: Potential Energy Surface

Reaction Path

Diagram

11

3.2

The [5]Phenylene Complex

The pathway of the linear [5]phenylene complex was also investigated and ground states, local minima and transitions states are shown in Figure 3.4.

Figure 3.4: Projection of the Co Atom during the Haptotropic Shift along [5]Phenylene For the degenerate walk (between the two internal cyclobutadiene), the pathway is close to the one for the complex of the linear [3]phenylene ; local minima was found to be 19.0

4 ,  ) and the two transition states have relative energies of 35.6 kcal/mol 3 2 ,  ) and of 35.7 kcal/mol (TS2 ,  ).

kcal/mol (LM1 (TS1

For the walk from the internal to the external cyclobutadiene unit, the pathway consists

4 ) and 19.0 kcal/mol 4 4 (LM3 ,  ) to reach a local minima with a relative energy of 9.7 kcal/mol (LM4 ,  ); 3 2 transition states can be observed with relative energies of 37.0 kcal/mol (TS3 ,  ), 36.7 3 2 kcal/mol (TS4, ,  ) and 36.0 kcal/mol (TS5 ,  ). of 2 local minimums with relative energies of 20.3 kcal/mol (LM2

,

The curve of the energy along the reaction path is shown in Figure 3.5, the potential energy surface diagram is shown in Figure 3.6 and the structures of each state are shown in Appendix B.

Figure 3.5: Energy along the Reaction Path

Figure 3.6: Potential Energy Surface Diagram

Experimentally, a barrier of 28 kcal/mol was found for the second walk ; according to the DFT calculations, this barrier is of 27.3 kcal/mol (energy of TS3-energy of LM4). It is interesting to notice that these values are close and that a good accuracy of DFT calculations exists in this series.

12

3.3

The Benzodicyclobutadiene Complex

The pathway of the shift along benzodicyclobutadiene showed the same structure as for linear [3]phenylene (Figure 3.7), except for the values of the relative energies, much

2 ) have a relative energy of 34.5 kcal/mol whereas the 3 transition states have relative energies of 51.4 kcal/mol (TS1, ,  ) and of 57.6 kcal/mol 2 (TS2, ,  ). The structures of each state are shown in Appendix C.

higher. The local minimum (LM

,

Figure 3.7: Projection of the Co Atom during the Haptotropic Shift along Benzodicyclobutadiene

3.4

Binding Energies

The binding energies between the CpCo moiety and the phenylene fragments were also calculated (for the linear [3]phenylene and the two isomers of the linear [5]phenylene). The values are given without correction from basis set superposition error (BSSE) and the cobalt is in a singlet state. The binding energies to the linear [3]phenylene ligand is of 70.3 kcal/mol, the one to the linear [5]phenylene (ground state) is of 81.9 kcal/mol and to linear [5]phenylene in LM4 of 71.9 kcal/mol.

Complex Binding Energy

70.3 kcal/mol

81.9 kcal/mol

71.9 kcal/mol

Thus, the CpCo moiety is more strongly bound to the linear [5] ligand than to the linear [3] ligand (this is due to the fact that the linear [5] ligand has more antiaromatic moieties). This is proved by the energy of the homodesmotic reactions where we can see that the [5]phenylenes are more tightly bound to the CpCo moiety than the [3]phenylene (Figure 3.8).

(a) - 1.5 kcal/mol

(b) - 11.3 kcal/mol

Figure 3.8: Homodesmotic Reactions between Linear [3] and [5]Phenylenes

13

Chapter 4

Results and Discussion 4.1

The Pro ject

The value of the activation energy for the degenerate shift was missing. Theoretical calculations provided a value of 26.9 kcal/mol for the linear [3]phenylene. The aim of the project was to nd the experimental value and to compare it with the theoretical one.



Tom Gro mann developed a powerful way of synthesis for the CpCo complex of the linear [3]phenylene [27] (yield of 65%), method which was used for the synthesis of the complex (Figure 4.1) ; this method is powerful because the solvent is THF (BTMSA is used as a cosolvent with a ratio 4:1) and this allow the temperature to be relatively low (THF boils at 66°C). The shift of the CpCo moiety being degenerate (Figure 4.2), it was necessary to desymmetrize the complex.



Figure 4.1: Gro mann's Way Of Synthesis

Figure 4.2: Degenerate Shift In 1977, Hillard and Vollhardt reported the selective protodesilylation of derivatives of benzocyclobutene [36] (Figure 4.3). It was therefore decided to try the desymmetrization

1

with triuoroacetic acid (CF3COOH) , according to their results. It must be precised that at the beginning of this internship, no ones was sure of which TMS group would rst leave : the one close to the CpCo or the one far from it.

One

possible suggestion is linked with the fact that when the cobalt is bound to a phenylene ligand, it aromatizes the four-membered ring to which it is linked ; a shift of the electron

1

In our case, we were just interested in the desymmetrization : thus, whatever would happen (a rear-

rangement or a desilylation), the result would be good for us.

14

Figure 4.3: Desymmetrization of a Phenylene Model

density of the external six-membered ring close to the cobalt then occurs toward the fourmembered ring to which the cobalt is linked. cobalt are charged

+

can be cleaved more easily.

But another point of view can be adopted : relatively, the

TMS group close to the cobalt are rst the one which is

4.2

Thus, the two TMS moieties close to the

and the bond C-Si bound is fragilized (because of more polar) and



+

and the other ones



; thus, the acid may attack

(far from the cobalt), than the other one.

Alternative Solutions

Other solutions to get the non-symmetric complex are possible, with building the molecule directly non-symmetric. Some of them are shown in Figure 4.4.

(a)

(b)

(c)

Figure 4.4: Alternative Solutions for the Building of Non-Symmetric Structures

15

4.3

Experimental Part

Abreviations DFT : Density Functional Theory Cp : cyclopentadienyl CpCo(CO)2 :

2 -cyclopentadienyldicarbonylcobalt (I)

BTMSA : bis(trimethylsilyl)acetylene THF : tetrahydrofuran TFA : triuoroacetic acid TBAF : tetrabutylammonium uoride TMS : trimethylsilyl TIPS : triisopropylsilyl NMR : Nuclear Magnetic Resonance s : singulet MS : Mass Spectrometry EI : Electron Impact EA : Elemental Analysis calc. : calculated FW : Formula Weight

General procedures All reactions were carried out under nitrogen atmosphere. THF was distilled from sodium/ benzophenone in a nitrogen atmosphere.

Hexanes was distilled from sodium.

All sol:

vents which were used were degassed with N2 at least 20 minutes prior to use. KF 2 H2O and [18]crown-6 were provided by Aldrich and used without further purication. 1,2,4,5Tetrakis[(trimethylsilyl)ethynyl]benzene was used as a product from the lab and its purity was checked by NMR. CpCo(CO)2 (obtained from Strem) had been distilled previously and was vacuum transfer just before using it.

1

NMR spectra were recorded in a Brucker AV300 or AVQ400 in C6D6 (

H

= 7.16 ppm).

Elemental analysis were performed by the U.C. Berkeley Microanalytical Laboratory. Mass spectra were acquired by the U.C. Berkeley Mass Spectroscopy Laboratory. To increase the yield of the reaction, some precautions were taken : all glassware which were used were preliminary washed with water, 10% hydrochloric solution, water then acetone and was dried overnight. The mixture of tetraethynylbenzene and CpCo(CO)2 was introduced by syringe pump by the top of the cooling apparatus (to avoid heating of the end of the needle) in a such manner to make drops falling directly into the reuxing mixture and to avoid drop falling on the wall of the ask. 1,2,4,5-Tetraethynylbenzene is very air-sensitive : it is an unstable compound which can detonate with inammation if heated in air [22]. Thus, it was kept in solution and used immediately without other purication than the plug of silica gel. The linear [3]phenylene-CpCo complex is air-sensitive and it was important to always keep it under a nitrogen atmosphere. It must be also precised that it is very important to purify it by a plug of neutral alumina activity III and not activity I which decomplex the product. The neutral alumina activity III was made by adding the good amount of water (4.4%) to neutral alumina activity I (Aluminum oxide 90, active neutral, 70-230 mesh) provided

2

by EM Science .

Silica gel, 60-200 mesh, for column chromatography (height x Ø) was

obtained from ICBiomedicals.

2

This had been done previously, and the neutral alumina activity III was used directly.

16

Work The synthesis of 2,3,7,8-tetrakis(trimethylsilyl)[3]phenylene cyclopentadienylcobalt(I) was performed several times before getting the desired product. The reasons of these fails are the fact that the purication were carried under air and not under nitrogen, and that then neutral alumina activity I was used several times before realizing it decomplexes the product. After understanding these problems, the complex was made and then tried to be puried. Only a bad XRay structure was available for CpCo complexes of phenylenes (for linear [5]phenylene substitued with hexyls, see Figure 2.1), it was therefore tried to get good crystals for XRay. Many attempts were necessary before nding an ecient way to recrystallize the product. When writing these lines, the XRay structure was submitted. The desymmetrization were then tried with TFA. After stirring 65 hours, the brown/black solution turned green/yellow and resulted in a yellow powder with no more proton peaks in NMR higher than 1.5 ppm.

Time was missing to rationalize this and caracterize the

product. Another way of desymmetrization could be found in the use of TBAF instead of TFA.

Synthesis of 2,3,7,8-Tetrakis(trimethylsilyl)[3]phenylene Cyclopentadienylcobalt(I)

:

A mixture of KF 2 H2O (422 mg, 4.49 mmol), [18]crown-6 (61.3 mg, 0.232 mmol) and 1,2,4,5-tetrakis[(trimethylsilyl)ethynyl]benzene (223.7 mg, 0.484 mmol) in degassed THF (15 ml) was stirred for 110 min at room temperature. The yellow/green solution was ltered through a plug of silica gel (1 x 5 cm) which was washed with degassed THF (15 mL) providing a light-yellow solution.

After adding CpCo(CO)2 (156 mg, 0.864 mmol),

the solution was protected from light and added via syringe pump over a period of 6 h to a reuxing mixture of degassed BTMSA (34 mL) and THF (125 mL) which was irradiated with a projector lamp (500 W) and maintained under nitrogen.

Reux and irradiation

were continued for 15 h after the addition had been complete.

After the solvents had

been removed by vacuum transfer the black residue was dissolved in a degassed mixture of hexanes/THF (50:1) and then ltered through a plug of neutral alumina activity III (1 x 5 cm) eluting with hexanes/THF (50:1).

The resulting black solution was evapo-

rated and the dark brown residue crystallized 4 times from degassed acetone, then from acetone/n-pentane (20:1) yielding 2,3,7,8-tetrakis(trimethylsilyl)[3]phenylene cyclopentadienylcobalt(I) as black crystals. The yield is not reported because it is not signicative (amounts were lost during the tentatives of purication and the product wasn't perfectly pure and dry). The analysis values were consistent with the literature :

1 H NMR (400 MHz, C6D6, 298 K):  = 0.32 (s, 18 H, 1-CH3), 0.36 (s, 18 H, 11-CH3), 4.43 (s, 5 H, 12-CH), 6.92 (s, 2 H, 3-CH), 7.46 (s, 2 H, 6-CH) and 7.96 (s, 2 H, 9-CH). Time was missing to get a clean

13 C NMR spectra. 17

MS (EI, 70eV) : 638 (M+, 100), 639 (M+1, 60), 640 (M+2,32), 514 (34), 515 (20), 516 (13), 73 (22). EA (C35H47CoSi4, FW : 639.02 g/mol) : C calc. : found :

H

65.87

7.41

66.0

7.08

Desymmetrization of 2,3,7,8-Tetrakis(trimethylsilyl)[3]phenylene Cyclopentadienylcobalt(I) :

18.4 mg of the complex (2.5

10 5 10

mol) were dissolved in 10 mL of degassed THF. 0.1 mL

of TFA was dissolved in 10 mL of THF, and then 0.19 mL of this solution was added to the complex (2.89 mg of TFA, 2.5

:

5

mol). The brown solution was stirred 65 hours to get

a green/yellow solution which was ltered through a plug of neutral alumina activity III (eluant : hexanes/THF, 50:1) ; solvents were removed by vacuum transfer to get a yellow powder.

18

Conclusion The story of haptotropic shifts along annulenes, and especially along phenylene frames is at its early ages. This domain, full of interest, from the fundamental aspect to the applied one such as in photo-optical storage, is promissed to an interesting future. During this internship, a method to purify the CpCo complex of linear [3]phenylene was investigated, and an XRay structure was submitted, whereas no clean XRay structures were available previously for phenylene complex. After getting the experimental values for the degenerate shift of linear [3]phenylene by kinetic experiments and nding a way to get the one for the [5]phenylenes, other ways may be explored such as nding an easy method to complex and decomplex the ligand (which would be very interesting for optical storage) or studying the homodesmotic reactions.

Acknowledgements My internship wouldn't have been possible without some people, and I want to sincerely thank them. I thank :

ˆ Prof K. Peter C. Vollhardt

for welcoming me and giving me the opportunity to

work in a such prestigious laboratory.

ˆ All the Vollhardt group

for their nice welcome : almost-Doctor Phil Leonard for

answering my questions and giving me some advices (and for learning me how to stop a re), Rob Padilla for all his help and his motivating interests in my job, Ken Windler for his advices, his help and for giving me the fundamental knowledge of how to make explosives, Dr. Elisa Paredes for giving me some advices and showing me new glassware, Dr. Ingo Janser & Romy Michiels for taking care of the lab during the night and for letting me borrowing them some stu, and also Dr. Alex Lee and Dr. Sabine Amslinger.

ˆ Bonnie Kirk

for everything she did for me before my arrival and for all the admin-

istrative stu.

ˆ Mikael Kepenekian and people from the Tau House

for mentally supporting

me and making me feel comfortable in Berkeley.

ˆ

And at last, but not least

my mother Florence Cheron for nancially supporting

me for this internship.

19

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20

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1,2,3,4-

Template for Cobalt-catalyzed Alkyne Cocyclizations.

lett, Submitted for publication, 2006.

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Total

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Biphenylenes and Tetraphenylenes using Copper-Catalyzed Coupling of

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MLn complexes. 2. Bicyclic polyene-MCp, M(CO)3 sys-

J. Am. Chem. Soc., 105:33963411, 1983.

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5 -cyclopentadienyl)cobalt along 4 -bound 1,3,5-hexatriene. J. Organomet.

Chem., 369:245251, 1989.

21

[31] H. C. Jahr, M. Nieger, and K. H. Dötz. Controlled haptotropic rearrangements  towards a stereospecic molecular switch based on chiral arene chromium complexes.

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Mo(PMe3)6 towards Heterocyclic Nitrogen Compounds: Transformations Relevant Hydrodenitrogenation. J. Am. Chem. Soc., 124:1365813659, 2002.

to

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6 with biphenylene as  -ligand: synthesis, structure, dynamic behaviour in solid state 6 6 and thermal  , -haptotropic rearrangements. experimental (NMR) and theoretical (DFT) studies. J. Organomet. Chem., 656:2742, 2002. University of Houston, Texas, 2006 Computational Details :

[35] T. Albright and O. Oloba.

ˆ

Software Packages :

  

GAUSSIAN03  an

ab initio

package

GaussView 3.0  graphical interface for GAUSSIAN03 ChemCraft 1.5  a Windows-based graphical interface for viewing

ab initio

calculations especially GAUSSIAN and GAMESS (www.chemcraftprog.com)

ˆ

Theory :

  ˆ

Hybrid Density Functional Theory B3LYP

Basis Sets :





Optimization :

* * *

Carbon  321G Hydrogen  321G Cobalt  LANL2DZ with ECP

Energy :

* * *

Carbon  6311G Hydrogen  631G Cobalt  LANL2DZ with ECP, with the last d-orbital released

Benzocyclobutenes, Indans, and Tetralins via Cobalt-Catalyzed Cooligomerization of ,!-Diynes with Substituted Acetylenes. J. Am. Chem. Soc., 99:40584069, 1977.

[36] R. L. Hillard III and K. P. C. Vollhardt. Substituted

22

Appendix A

Theoretical Structures for the Shift along [3]Phenylene Distances are in Angstrom. GS : Relative energy : 0.0 kcal/mol

TS1 : Relative energy : 26.9 kcal/mol

LM : Relative energy : 10.9 kcal/mol

TS2 : Relative energy : 24.9 kcal/mol

23

Appendix B

Theoretical Structures for the Shift along [5]Phenylene Distances are in Angstrom. LM4 : Relative energy : 9.7 kcal/mol

TS5 : Relative energy : 36.0 kcal/mol

24

LM3 : Relative energy : 19.0 kcal/mol

TS4 : Relative energy : 36.7 kcal/mol

LM2 : Relative energy : 20.3 kcal/mol

25

TS3 : Relative energy : 37.0 kcal/mol

GS : Relative energy : 0.0 kcal/mol

TS2 : Relative energy : 35.7 kcal/mol

26

LM1 : Relative energy : 19.0 kcal/mol

TS1 : Relative energy : 35.6 kcal/mol

27

Appendix C

Theoretical Structures for the Shift along Benzodicyclobutadiene Distances are in Angstrom. GS : Relative energy : 0.0 kcal/mol

TS2 : Relative energy : 57.6 kcal/mol

LM : Relative energy : 34.5 kcal/mol

TS1 : Relative energy : 51.4 kcal/mol

28