Co

O Co

This breakthrough has led many research teams to explore the carbonylation pathway and in the late 1960s the Monsanto Company developed a process for carbonylating methanol in the presence of a rhodium catalyst to produce acetic acid in higher yields and lower pressure than the BASF process. Few years latter, Celanese improved the Monsanto process by introducing iodide salts as lithium iodide. This new process need a water content of only 4 wt% whereas 14 wt% are required for the Monsanto process to obtain same carbonylation rate. This low water concentration decreased significantly the energy cost needed for the separation of water from acetic acid. In 1996, BP Chemical announced a low water new carbonylation process using iridium catalyst and ruthenium as co-catalyst named Cativa process. A plant is today operating in Asia using this technology.

Nowadays, the carbonylation processes using rhodium or iridium catalyst are the most used system for the world acetic acid production.

After a bibliographical part recalling the old and the recent processes for the catalyzed methanol carbonylation to acetic acid, we will study in Schlenck tube both, the rhodium and the iridium catalytic cycle. We will pay particular attention on the methods developed to enhance the rate determining step of the iridium catalytic cycle and we will see in Schlenk tube experiments that a rhodium complex can enhance this step.

We will also study the impact of water and lithium iodide on the rhodium catalytic system under catalytic conditions by performing batch run experiments. Thus we will try to explain the role of water and iodide salt in the rhodium-catalyzed methanol carbonylation reaction. oxidative addition CO CO insertion reductive elimination Scheme 2 : Reaction pathway in the methanol carbonylation process. Chapter I 5 I-1. Cobalt-catalyzed methanol carbonylation.

The first catalytic process for the methanol carbonylation to produce acetic acid was commercialized by BASF in 1960 and used cobalt iodide as catalyst 3,4

. This system present weak catalytic activity, therefore, a temperature of 250°C and a p ressure of 700 bars are required to give commercially acceptable reaction rate 5 . Moreover, this cobalt process is very sensitive to hydrogen, which is usually present as impurity in CO gas, giving side products as acetaldehyde and crotonaldehyde 6

. The acetic acid yield is 90% based on methanol and 70% based on carbon monoxide. A catalytic cycle has been proposed 3,7,8 and is depicted in scheme 3.

The first step of this system is the generation of the active nucleophilic complex [Co(CO) 4 ] -by a two-step reduction of halide cobalt (II), usually CoI 2 , by water and CO pressure 4 . Once the active complex is generated, methyl iodide formed by reaction of methanol with HI undergoes a nucleophilic attack by the coordinatively saturated Co(-I) complex [Co(CO) 4 ] -anion to form the neutral Co(I) complex [Co(CH 3 )(CO) 4 ]. The methyl-cobalt complex formed is the stable intermediate of the catalytic cycle, thus, the following methyl migration step affording the acetyl complex [Co(CH 3 CO)(CO) 4 ] is the rate determining step of the catalytic cycle. Since the latter acetyl complex does not hold a iodide ligand, a simple reductive elimination can not occur, thus, acetyl iodide is formed by direct reaction of hydrogen iodide on the acetyl complex regenerating the active complex [Co(CO) 4 ] -. Chapter I 6 Due to the severe conditions required and the low selectivity toward methanol and carbon monoxide, the BASF process is today uncompetitive in comparison with the iridium or rhodium processes. Scheme 3 : Proposed catalytic cycle for the methanol carbonylation with cobalt catalyst. Co CO CO OC OC -Co CH 3 CO OC

GENERAL INTRODUCTION

The catalyzed methanol carbonylation into acetic acid is probably the most successful example of an industrial process homogeneously catalyzed by a metal complex that has yet been realized. For many years, the only source of this acid was the oxidation of ethanol by fermentation and was used as food or as conservative. Today, the production of acetic acid reach 8.10 6 T/year and is used primarily as a raw material for the production of vinyl acetate monomer (VAM), acetic anhydride or as solvent for purified terephthalic acid (PTA) production (Fig 1).

It was not until 1920, in the United States, that catalytic production of acetic acid was begun by the oxidation of acetaldehyde in the presence of manganese acetate. From the beginning both the need for improved production methods and the development of these methods grew rapidly. In 1960, BASF introduced the methanol carbonylation to acetic acid using a cobalt catalyst which was more efficient than all previous system involving oxidation of ethanol, acetaldehyde or hydrocarbons to produce acetic acid. &+$37(5 , %,%/,2*5$3+<

Chapter I

CHAPTER I : BIBLIOGRAPHY

The homogeneous methanol carbonylation reaction (scheme 1) is today the most used process for the production of acetic acid and is now practiced commercially by all major acetic acid manufacturers, including BP-Amoco, Celanese and others. Consequently, more than 60% of the world acetic acid production employs the methanol carbonylation methods 1 (Fig. 1).

This sprightliness for the methanol carbonylation using CO gas is essentially due to the low cost of the raw materials, CO (130 $/T) being synthesized from coal and methanol (160 $/T) being catalytically synthesized from CO and dihydrogene. All others routes to acetic acid are economically obsolete Today, the most used processes are based on iridium as well as rhodium catalyst, cobalt and nickel being only of historical interest. It is worth to note that all these metals have been tested for the catalytic synthesis of acetic anhydride, which is also a methanol carbonylation process, but only rhodium is today industrially used in this process.

In this first chapter, we will study rapidly these processes using different metals underlining similarities and differences in a mechanistic point of view. We will pay particularly attention on the iridium and the rhodium processes which are today in the research stream for the synthesis of acetic acid and acetic anhydride. Finally, we will study the attempts done to enhance the rate determining step and thus, the total catalytic cycle rate. Although the nickel-catalyzed methanol carbonylation process has been deeply studied by Halcon-Rhône Poulenc, leading to numerous patents 9,10,11,12,13 and required milder conditions than the cobalt one (c.a 200 bar and a temperature ranged between 250°C and 300°C), this process has never been commercialized due to the high toxicity and volatility of its active species [Ni(CO) 4 ].

The stable active species [Ni(CO) 4 ] is synthesized from nickel iodide salt NiI 2 as shown in scheme 4.

It was found that Ni species can be charged in the reactor in any form and need not be of very high purity 14 . The accepted catalytic cycle for the nickel-catalyzed methanol carbonylation is depicted in scheme 5. Oxidative addition of methyl iodide on the active species give the methyl nickel complex [Ni(CH 3 )I(CO) 2 ], this step appears to be the slower one of the catalytic cycle 15 . Under CO, a cis-migration reaction occurs leading to the acetyl complex [Ni(COCH 3 )I(CO) 2 ] which gives, after reductive elimination reaction the starting active complex [Ni(CO) 4 ] and acetyl iodide. The latter species is hydrolyzed by water to give acetic acid.

One of the advantage of this system is that nickel metal is very cheap in comparison with rhodium or iridium which are the commonly used metal for carbonylation. This reason has led some research group to develop a process using nickel catalyst and to try to reduce the toxic [Ni(CO) 4 ] in the medium. It is worth to note that nickel can be use as catalyst in the carbonylation of methyl acetate to acetic anhydride. The proposed catalytic cycle for this reaction is similar to this proposed for the nickel-catalyzed methanol carbonylation, the methyl acetate reacting with acetyl iodide to give acetic anhydride. It has been shown that lithium acetate ions enhanced dramatically the reaction rate 18 .

I-3. Palladium-catalyzed methanol carbonylation.

Carbonylation of methanol with palladium catalyst, patented by Shell, used severe operating conditions (182°C, 110 bar of CO) with sulfolane as solvent and nitrogen-containing ligands to avoid catalyst deactivation and thus, to give acceptable carbonylation rate 19 . The proposed active species of this system is [PdI 3 (CO)] -and the proposed catalytic cycle is displayed in scheme 7 20 .

As for the nickel-catalyzed system, palladium catalyzed methanol carbonylation reaction involves the methyl iodide oxidative addition reaction followed by the cis-migration reaction and the insertion of a CO ligand. Concerning the last step of the catalytic cycle which is usually the reductive elimination of CH 3 COI, a direct hydrolysis of the acetyl complex is preferred here, as it is the case for some palladium catalytic reaction as co-polymerization of ethene and carbon monoxide 21 .

Recent advances 22 have shown that a iodo-bridged dimer [Pd 2 I 6 ] 2-was in equilibrium with the active species [PdI 3 (CO)] -but since this system shows lower activity than the rhodium-catalyzed system, that palladium is an expensive noble metal and that 20% water content is required, leading to a high energy cost for the separation of acetic acid and water, this system has not been commercialized and is today less studied. . First mechanistic studies have been reported 24,25 but in a mechanistic investigation performed by Forster in 1979, it became obvious that iridium-catalyzed process was more complex than the first proposed mechanistic pathway 26 .

He proposed that two mains cycle are interconnected, one involving neutral iridium complexes and the other anionique iridium complexes, depending on the iodide ions concentration 8 (sc. 8).

When experiments are performed at low ionic iodide concentration, the observed iridium species is [IrI(CO) 3 ] and the reaction is inhibited by increasing carbon monoxide pressure. This iridium complex present low reactivity toward methyl iodide since it is a poor nucleophile and the complex formed is [Ir(CH 3 )I 2 (CO) 2 ] rather than [Ir(CH 3 )I 2 (CO) 3 ], that explain the inhibition of CO pressure. Without removal of a CO ligand from complex [IrI(CO) 3 ], no oxidative addition occurs. Under these conditions, oxidative addition of methyl iodide is the determining step. Under CO, the complex [Ir(CH 3 )I 2 (CO) 2 ] affords easily the acetyl complex [Ir(CH 3 CO)I 2 (CO) 2 ].

By increasing ionic iodide level, there is a change in the predominant form of iridium to [Ir(CH 3 )I 3 (CO) 2 ] -and the reaction rate is increased by increasing the carbon monoxide pressure. In this system, the active species which prime the catalytic cycle is the anionic complex [IrI 2 (CO) 2 ]

-1

. This species appears more nucleophile than [IrI(CO) 3 ] and thus, the oxidative addition of methyl iodide on [IrI 2 (CO) 2 ] -is rapid. It has been demonstrated that the oxidative addition of methyl iodide on iridium complex (as on rhodium complex) is a nucleophilic attack of the metal center on the electrophile carbon atom of the methyl iodide via a S N2 reaction 27 (scheme 9). Methyl-iridium complex appears to be the resting state of this system since an iodide ligand need to be removed to give the neutral complex [Ir(CH 3 )I 2 (CO) 3 ] which Chapter I

13

allows the formation of the acetyl complex. This reaction occurs at 80°C under 5 bar of CO for several hours. However, the overall catalytic reaction is only slightly faster, because the generation of the iridium acetyl species from the iridium methyl species is more difficult with the anionic system than the neutral system but this anionic cycle is preferred for the methanol carbonylation. Forster has also shown than a too high ionic iodide level in the medium inhibited the acetyl iridium formation 8 . This iodide effect has been well explained by Maitlis et al 28 .

The great advantage of the iridium system is that the maximal carbonylation reaction rate was founded for a 4-6 wt% water content whereas 14 wt% water content are needed for optimal carbonylation rate with rhodium catalyst. It has been shown 8 that increasing water concentration decreased the carbonylation rate. This phenomenon was explain by the formation of the hydride complex [IrHI 3 (CO) 2 ] due to the formation of larger quantities of HI at higher water concentration. Under catalytic conditions, the formation of this hydride complex induces the Water Gas Shift Reaction (WGSR) which is a side reaction since it consumes CO gas without forming acetic acid (see sc. 8).

Although iridium-catalyzed methanol carbonylation process shows slower carbonylation rate than the rhodium-catalyzed one, the low water level used is of industrial interest since a high energetic cost is needed for the separation of water from acetic acid in the distillation column. Thus, many research groups have focus their attention to try to enhance the rate determining step, the methyl migration step, in order to obtain a iridium-catalyzed system suitable for industrial application. As it has been explained above (section I-4-1), the determining step of the iridium-catalyzed methanol carbonylation is the methyl cis-migration from methyl iridium complex [Ir(CH 3 )I 3 (CO) 2 ]

-to produce the acetyl iridium complex [Ir(CH 3 CO)I 3 (CO) 2 ] -via the neutral complex [Ir(CH 3 )I 2 (CO) 3 ]. To obtain this neutral complex, on which the methyl cis-migration reaction occurs rapidly due to the lower electron density on the metal center in comparison with the anionic methyl iridium species, an iodide ligand has to be removed from complex [Ir(CH 3 )I 3 (CO) 2 ] -. This explain why too high iodide concentration in the medium reduces the iridium catalyst activity.

Maitlis et al. noticed that addition of methanol in the medium to performed the methyl cis-migratory reaction accelerated dramatically the reaction rate 29 . Some thermodynamic data were reported for this reaction using chlorobenzene and chlorobenzene/methanol ( . Regarding the lone reaction of methyl iridium complex with ruthenium promoter, a heterobimetallic complex has been observed 36 . Mechanistic details are displayed in section II.

For our part, we have developed in the LCCFP in collaboration with Acetex Chimie and patented a iridium-catalyzed methanol carbonylation system using platinum [PtI(CO) 2 ] as promoter 37 . As for the Cativa process, the platinum promoter acts on the methyl iridium resting state unbolting the slow methyl migratory insertion and a heterobimetallic complex has been detected [START_REF] Gautron | [END_REF][START_REF] Gautron | [END_REF]40,41 . In this thesis we will present in section II the results of a mechanistic studies based on the enhancement of the rate determining step of the iridium catalytic cycle by the use of a rhodium complex as promoter.

It is interesting to note that recently, mechanistic studies on the iridium-catalyzed methanol carbonylation has been carried out using photochemical reaction and time resolved spectroscopic analyses 43 . Thus, the resting state methyl-iridium complex [Ir(CH 3 )I 3 (CO) 2 ] -was placed in dichloromethane under one atmosphere of CO and at room temperature. The solution was subjected to bulk photolysis and the complete formation of the acetyl-iridium complex [Ir(CH 3 CO)I 3 (CO) 2 ] -has been observed within only two hours. This reaction time is short in comparison with the required conditions for this reaction without any promoters meaning that photolysis accelerates the cis-migration reaction rate.

Chapter I

I-5 Rhodium-catalyzed methanol carbonylation

Just few years after BASF commercialized the cobalt-catalyzed process, Monsanto developed a low pressure process with a rhodium iodide promoted catalyst system that demonstrated significantly higher activity and selectivity to produce acetic acid than the cobalt process 43 . Indeed methanol can be converted even at atmospheric pressure with yields of 99% and 90% with respect to methanol and carbon monoxide, respectively 44,45,7 . This system require mild conditions as 30 bar of CO and 190°C and is not as sensitive to hydrogen as the BASF process, and therefore the amounts of reduction products such as methane and propionic acid are comparatively low 5

.

All these advantages have conducted the Monsanto process to be the most used catalytic way to produce acetic acid for 30 years. Its catalytic cycle was well studied by several research group 46,47,48 and a catalytic cycle was proposed (Sc. 10).

Scheme 10 : Proposed rhodium-catalyzed methanol carbonylation catalytic cycle.

[RhI 2 (CO) giving the methyl-rhodium complex [RhI 3 (CH 3 )(CO) 2 ] -followed by a rapid cis-migration affording the acetyl

complex [RhI 3 (COCH 3 )(CO)] 2 2-
. This latter complex was isolated and its structure was determined by X-ray crystallography 49 . Under CO, this complex gives the di-carbonyl acetyl complex [RhI 3 (COCH 3 )(CO) 2 ] which leads after reductive elimination, as for its iridium homologue, to the active complex and acetyl iodide.

The main drawback of the Monsanto process is that a water content of 14% is required to have hight carbonylation rate and to avoid black rhodium precipitation. This high water content needed, in comparison with the iridium-catalyzed methanol carbonylation system, leads to a high energy cost for the separation of water from acetic acid. Furthermore, it has been shown that under these conditions, the Water Gas Shift Reaction (WGSR) occurs, consuming CO without production of acetic acid 50

. The WGSR will be detailed later.

In 1980, Celanese improved the Monsanto process with the Low-Water Acid Optimisation (AO)

Technology, adding large quantities of iodide salt as lithium iodide, potassium iodide or sodium iodide in the medium

51

. Thus, methanol carbonylation reaction proceed at only 4% water content at hight carbonylation rate without rhodium loss. The Monsanto catalytic cycle is still operating in the low-water process. Industrial application of the Low-Water AO Technology led Celanese Company to be the first acetic acid producer in the world.

Chapter I

19

I-5-1 Mechanistic approach

In this section, we will study step by step the rhodium catalytic cycle and describe advance in its comprehension and its improvement.

The oxidative addition

As for the nickel-, palladium-and iridium-catalyzed methanol carbonylation, the first step is the oxidative addition of methyl iodide to the active complex [RhI Second, they have studied the effect of additives on the oxidative addition. They observed that the presence of halogen salts in the medium enhanced the rate. Results are summarized in Figure 2. They proposed as explanation that this phenomenon appears to arise from a general salt effect. ) in dichloromethane in the presence of various salts.

21

The general mechanism of the oxidative addition is a S N 2 reaction as it has been theoretically proved 27 . As explained in scheme 9, the nucleophilic metal center attack the electrophilic carbon of the methyl iodide forming at first a five coordinated methyl complex which evolves rapidly to the six coordinated methyl complex containing three iodide ligands. The experimental evidence of this pathway was held by Salzer et al. in 2004 starting from a neutral rhodium complex (sc. 11).

The cationic methyl complex has been isolated, crystallized and its structure determined by X-ray analysis 55 . This reaction can continue leading to the expected methyl-iodide complex.

Since it was proved that oxidative addition reaction was a nucleophilic attack of the metal center, it was proposed that in the case of the Celanese low-water AO process that the high lithium iodide content involved two competitive pathway between four-coordinated and five coordinated nucleophilic intermediates (sc. 12). The formation of a dianionic complex [RhI 2 L(CO) 2 ] 2-, where L could be an iodide ligand or an acetate ligand according to the equilibrium displayed in scheme 13, is more nucleophilic than the classical [RhI 2 (CO) 2 ] -and thus can react more rapidly with methyl iodide

22

It has also been proposed that the enhancement of the rate for the low water process could be due to the formation of additional methyl iodide by the reaction of LiI with methyl acetate (Sc. 13)

57

.

However, the increase of CH 3 I concentration in the medium appears to be not sufficient 2 On the basis that the rate-determining step for rhodium-catalyzed methanol carbonylation is the oxidative addition of CH 3 I to Rh(I), numerous attempt have been made to improve catalytic activity by introduction of strongly donating ligands that increase the nucleophilicity of the rhodium centre.

A class of rhodium complexes containing mono-phosphine ligands as P(Et) This system shows high carbonylation rate at high water content but present a loss of activity with the time. This phenomenon is explain by the catalyst degradation via the loss of phosphine ligand (to give Et 3 PI + ) under the harsh conditions used. Kinetic studies performed with this phosphine-rhodium system show that the oxidative addition remains the rate determining step of the catalytic cycle. It is worth noting that no appreciable benefit is obtained with this system at low water content in comparison with the "classical" system using the [RhI 2 (CO) 2 ] -catalyst.

Chapter I

24

In order to avoid catalyst degradation, several studies using ligand chelate effect have been made in attempts to stabilize the ligand-metal interaction. with methyl iodide was found to lead rapidly to the corresponding acetyl complex. Moreover, no trace of [RhI 2 (CO) 2 ] -has been detected. Chapter I

Rh

25

BP researchers have found that the use of P-S ligands as Ph 2 PCH 2 P(S)Ph 2 (dppms) as a promoter for rhodium catalyzed methanol carbonylation gave a substantial acceleration of the rate in comparison with the "classical" rhodium system and even with the use of dppeo 63 -system for methanol carbonylation, the aim being to increase the electron density on the metal center in order to accelerate the methyl iodide oxidative addition determining step.

Rh

29

Most of these new rhodium-catalysts containing chelating ligands mentioned above are presented in the literature as new systems having higher carbonylation rates than the "classical" system using [RhI 2 (CO) 2 ] -catalyst. Some of these new catalysts were been patented for an industrial application 64,71 .

One of the first patent concerning methanol carbonylation and using additional ligands as amines or phophines was published in 1981 by the Monsanto Company 72 but to our knowledge, after around 25 years of research, no company uses today these new catalysts for acetic acid production.

One reason that can explain the preference for the Monsanto, Celanese or Cativa process for industrial acetic acid production is that the published catalytic tests using new rhodium catalysts were performed in batch run under conditions far from the real industrial conditions. Indeed, the reported batch runs using these new catalysts were performed at low water content (less than 8%) or even with no water or no acetic acid as solvent and no iodide salts as lithium iodide are introduced in the reactor. For example, one of the best reported Turn Over Frequency (TOF) for the use of trans-chelating ligands is around 2400 h -1 and only h -1

.mol -1 for the "classical" system under the same conditions but reach 2200 h -1

under Monsanto conditions (test performed in our laboratory). Thus, the use of phosphine, amine or mixed chelating ligands in the rhodium catalyzed methanol carbonylation presents only a low improvement of the TOF values in comparison with the Monsanto process. Moreover, there are no information concerning the behavior of these new catalyst in continue experiments since all the reported runs were performed in batch conditions.

Thus, the advantage of these σ-donor ligands is that they increase the electron density on the rhodium metal center, accelerating by this way the oxidative addition of methyl iodide. But by giving a metal center more electron rich, the rate of next step, the migratory insertion of a CO ligand into the Rh-CH 3 bond, is decreased so that the overall rate is not substantially enhanced. In comparison with the iridium system which is more electron rich than rhodium, the oxidative addition is fast whereas the cismigration is the rate determining step of the catalytic cycle.

Chapter I

30

The migratory insertion, CO insertion and the reductive elimination

The next step of the rhodium-catalyzed methanol carbonylation is the migratory insertion 75 of a CO ligand from the 18e -complex [RhI 3 (CH 3 )(CO) 2 ] -to the 16e -acetyl complex [RhI 3 (COCH 3 )(CO)] -(Sc.

20).

This reaction is fast 45 so that the methyl rhodium complex was only detected recently 56,74 . Indeed, an unknown infrared band at 2104 cm -1 was detected using a computer subtraction technique. By comparison with the infrared spectra of the stable complex [IrI 3 (CH 3 )(CO) 2 ] -(2098 cm -1 2045 cm -1

), the authors assigned of the band at 2104 cm -1 to the high frequency ν CO mode of the cis-[RhI 3 (CH 3 )(CO) 2 ] complex. 13 C NMR observations of a new doublet at δ = 175.9 (d, 1 J Rh-C = 59.9 Hz) when reacting

[RhI 2 ( 13 CO) 2 ]
-in neat CH 3 I at low temperature confirm the latter infrared observation. Density functional calculations 75 indicates that the methyl rhodium complex with the CO in trans position is energetically favored to the proposed methyl rhodium complex with the CO in cis position but this trans complex can only arise from the oxidative addition of methyl iodide to the complex trans-[RhI 2 (CO) 2 ] -which is largely energetically disfavoured to the complex cis-[RhI 2 (CO) 2 ] -and furthermore, was not detected in the catalytic medium.

The CO migratory insertion reaction appears to be exothermic with a calculated energy of -32 kJ/mol and involves a transition state having a trigonal bipyramidal structure with a partially formed

31

The resulting acetyl complex is five coordinated. A X-ray crystal structure of this intermediates has been obtained 49

33

The first way is the direct reductive elimination of acetyl iodide mentioned above and the second involves the intervention of methanol or water in a solvolytic mechanism giving a hydride rhodium complex which eliminates HI to complete the catalytic cycle. Forster indicated that the acetyl iodide elimination is preferred since evidence of this pathway has been reported with rhodium phosphine complexes 79 . Kilner et al. proposed also that water can react directly with the acetyl complex via a hydrolysis reaction to give acetic acid and the starting complex and noted that addition of acetic acid increases the rate of the elimination reaction 80 .

This solvolysis reaction can be linked on the reaction of methanol with cationic acetyl palladium

35

The Water Gas Shift Reaction (WGSR) in the rhodium catalyzed methanol carbonylation.

The catalysis of the WGSR by a rhodium iodide system has been noted to occur as a side reaction in the rhodium catalyzed methanol carbonylation at elevated temperatures and pressures

38

Kinetic measurements based on the production of CO 2 in a medium containing rhodium catalyst, acetic acid, water and HI under CO for a total pressure of 28 bar at 185°C have been performed in orde r to evaluate the effect of water and HI on the WGSR rate [START_REF] Singleton | Preprints-ACS[END_REF] . Results are displayed in figure 3.

Figure 3 shows that water, HI and rhodium concentration play an important role in the WGSR rate.

The three curves present the same Gaussian type shape. The WGSR rate increases with the increase in water concentration to reach a maximum and then, decreases for higher water concentration. Higher concentrations of HI lead to a shift of the maximum of the curve towards high water concentrations. As, it can be expected, the increase of rhodium concentration increases the WGSR rate. These results have been connected to the total acidity level of the medium, depending on water and HI concentration, indicating that the WGSR rate increases to a maximum when the acidity level increases and then

decreases whereas the acidity level continues to increase -and the catalytic reaction shows first-order dependence on the partial pressure of CO.

However, below 65°C the reaction is independent of CO pressure, and exhibits a second-order dependence on HI concentration. The major species observed is now the Rh(I) complex [RhI 2 (CO) 2 ] -.

These observations mean that at high temperature, the determining step of the WGSR reaction is the reaction of CO with the tetra-iodo complex [RhI 4 (CO) 2 ] -or with [RhI 3 (CO)] Another main draw-back of the homogeneous catalytic process is the loss of the catalyst. Thus, for the rhodium-catalyzed methanol carbonylation reaction, some expensive rhodium catalyst can be lost either 1) by precipitation of black rhodium, 2) by the formation of volatile rhodium species when separating catalytic mixture from acetic acid or 3) by degradation of phosphine-rhodium ligands in drastic conditions used as mentioned above. Thus, immobilisation of rhodium catalyst on resins or zeolytes is an economical challenge for industrial applications 62 .

An answer has been held in the Acetica process by the use of polyvinypyridine resin tolerant of elevated temperature [88][89][90] . The catalytically active complex [RhI 2 (CO) 2 ] -was supported on a ion exchange resin in which the pendant pyridyl groups had been quaternized by reaction with CH 3 I (Sc. 33). This process shows increased carbonylation rates for batch experiments at low water content (3-7 wt%) in comparison with the catalytic process using [RhI 2 (CO) 2 ]

-and shows an extremely effective ionic attachment with a very low rhodium concentration in the effluent from the reactor.

Rhodium/iodide catalyzed methanol carbonylation.

In spite of all recent research made to find higher efficiency rhodium process, the two most largely used process in the methanol carbonylation are the Eastman process for the production of acetic anhydride and the Celanese process for the production of acetic acid. In both processes, large amounts of lithium iodide are introduced.

Concerning the production of acetic anhydride from methyl acetate, Monsanto has discovered first the powerful iodide promoted rhodium carbonylation system Water effect:

By increasing the water concentration in the reaction medium containing rhodium catalyst, methyl acetate (4-6 wt%), LiI (18-20 wt%), CH 3 I (14 wt%) and acetic acid, the carbonylation rate increases rapidly between a water concentration of 0-4 wt% and stabilizes whereas water concentration continues to increase. High water contents (20 wt%) under these conditions do not show significant enhancement of the carbonylation rate.

Lithium iodide effect:

The increase of LiI concentration in the medium increases linearly the carbonylation rate. This enhancement is very effective for low water contents. Furthermore, gas analysis shows that the CO 2 production, due to the WGSR, is significantly reduced for lithium iodide concentration ranging between 10-20 wt%. Rhodium loss is also reduced when using significant quantities of LiI.

Methyl acetate effect:

Methyl acetate concentration effect exhibits almost the same behavior than the water concentration effect with a dramatic enhancement of the carbonylation rate when methyl acetate content increases from 0 to 8 wt% and a plateau is observed with no significant carbonylation rate enhancement for high methyl acetate concentrations. The CO 2 production is also reduced when methyl acetate content increases.

Chapter I

44

The reduction of the WGSR has been explained by the reduction of the HI concentration in the medium, this latter reacting with the acetate ions formed by reaction of methyl acetate with lithium iodide (Sc. 13).

Thus, the Celanese process present numerous advantages with the reduction of water content, the reduction of rhodium loss and reduction of the WGS reaction. Altought several new rhodium complexes were developed and showed high activity for methanol carbonylation, the most part of acetic acid is produced by the Celanese process. Indeed, these new catalysts may be show high carbonylation activity in batch experiments but their behaviors and especially their stabilities in a continue unit could be very different, batch experiments being limited to the reactor part.

Chapter I

45

I-6 Context of the present work

Our PhD work was essentially devoted to study the mechanism of the catalytic system in the carbonylation reaction of methanol at low water content. During this work, we performed numerous batch experiments in order to study the catalyst activity for the methanol carbonylation in conditions close to industrial ones. But the behavior of the catalytic system in batch experiments present several differences in comparison with the behavior of the same catalytic system under industrial conditions in a continue plant. In this part, we will describe the conditions used in an industrial plant and the major differences that exist in comparison with batch experiments. We will explain also how to mimic the catalytic conditions used in an industrial plant by changing the catalytic medium for batch experiments.

Description of an industrial plant:

A simplified flow sheet describing the commercial unit used for industrial production of acetic acid for the Celanese process or the Cativa process is displayed in scheme 35.

Flash tank Reactor

Once the plant is started, the reactor, where the carbonylation reaction occurs, is only fed with methanol and CO gas. The CO partial pressure in the reactor is around 10 bar whereas the total pressure is around 30 bar due to the volatile organic compounds as methyl iodide and methyl acetate present in the reactor and the temperature is set to 190°C. Even i f the reactor is fed with methanol, analysis of its composition does not show its presence since it is directly transformed to methyl iodide by reaction with HI produced from the hydrolysis of acetyl iodide, coming from the reductive elimination reaction of the acetyl rhodium complex, and water.

The catalytic mixture is then transferred to the flash tank, its main role being to separate the catalyst from the catalytic solution. Thus, the flash tank cool down the catalytic mixture to 150°C an d depressors it, the total pressure being around 2 bar, essentially due to the volatile compounds. This difference of pressure and temperature separate the catalytic mixture in two parts:

-The residue containing the catalyst and a large part of acetic acid and water is fed back to the reactor (B).

-The over head containing water, acetic acid, methyl acetate, methyl iodide and a small quantity of catalyst is sent to the light ends column (A).

It is worth noting that the loss of catalyst due to its precipitation is observed in the flash tank and not in the reactor. Thus, stabilization of the catalyst in the flash tank is an important feature since rhodium is very expensive.

The light end column: the over head of the light ends column containing methyl iodide, methyl acetate and water is fed back to the reactor (C). The side stream containing the acetic acid and water is sent to a second column (E). The residue containing remaining catalyst and acetic acid is sent back to the reactor.

The over head of the heavy ends column containing essentially water is sent back to the side stream of the flash tank (F). The residue containing heavy propionic acid by-product is exited (G) and the pure acetic acid is collected by the side stream (H).

We can note that for the high water content Monsanto process, a third distillation column is needed to remove the water in order to obtain pure acetic acid, generating higher energy cost.

By feeding back the water, methyl acetate, methyl iodide and some acetic acid in different parts of the unit, their concentrations are kept constant in the reactor. Table 4 summarizes the optimized concentrations in the reactor used for the different processes, acetic acid content being the balance.

We can notice that for each process, the methyl iodide concentration, the temperature and the CO partial pressure are the same, the differences arising from the methyl acetate concentration, the water content and the nature of the catalyst. For the Monsanto process, a water content of 14 wt% is required to have a good stability in the flash tank whereas the Cativa process using a [Ir/Ru] catalyst shows good stability as well as high activity at 6 wt% and concerning the Celanese process, a good stability of the rhodium catalyst at 4 wt% of water is observed when lithium iodide is introduced in the medium. The catalyst concentration as well the methyl acetate content are optimized for a good compromise between both activity and stability.

48

Thus, in a industrial plant, two main factors are important, the activity of the catalyst in the reactor but also its stability in the flash tank.

Batch experiments

Batch experiments are usually performed in order to evaluate the activity of a catalyst system before any eventual industrial application. In our case, this technique presents major differences in comparison with a continue unit:

First, batch experiments are performed in a closed reactor meaning that the behavior of the catalytic system in the flasher tank, particularly the stability of the catalyst, cannot be evaluated.

Second, the batch reactor is only fed by gas during the experiment, thus, the concentration of the introduced compounds evolve along the duration of the experiment that it is not the case in a continue plant where all the concentration are kept constant in the reactor.

Taking into account these two comments, we performed batch experiments following the general procedure described below:

First, the active catalyst species is preformed in the reactor by introducing the desired metal salt (RhI 3 or IrI 3,4 ) with an acetic acid/water mixture of 10/1 wt%. The reactor is sealed and flushed three times with CO. Then, the CO pressure is adjusted at 5 bar and the temperature at 190°C. We checked by HP-IR experiments that, under these conditions, a preformation time of 25 min. is required to observe a maximum intensity of the active species CO bands.

Chapter I

49

Second, we introduced the rest of the catalytic medium via an additional over-pressurized tank and the CO pressure is adjusted to 30 bar:

-Methyl iodide: 10 wt%. Same concentration than that used in an industrial plant.

-Methyl acetate: 27 wt%. We introduced higher concentration of methyl acetate in comparison with that present in the reactor of a continue plant in order to keep constant the concentration of methyl iodide in the reactor. Indeed, we consider that for one molecule of methyl iodide consumed in the oxidative addition reaction, one molecule of HI is released by the reaction of acetyl iodide with water, which can react with one molecule of methyl acetate giving back a molecule of methyl iodide and acetic acid (Sc. 36). Thus, the high concentration role of methyl acetate is to mimic the continue feed of methanol in a continue unit and its concentration decreases during the batch experiment.

-Water to the desired starting concentration. In order to study the water effect on the activity of a catalytic system, we introduced various water concentrations via the additional tank, taking into account the water introduced for the preformation reaction and considering that its concentration does not changed during the preformation of the active catalyst. According to the equations displayed in scheme 36, we note that water is consumed during the carbonylation reaction and no water is added during the experiment. Thus the water concentration decrease during the experiment.

-Acetic acid as the balance. Acetic acid is introduced as the balance taking into account the different product introduced for the experiment. According to scheme 36, we note that two molecules of acetic acid are produced, one arising from the real carbonylation reaction and one arising from the reaction of methyl acetate with HI. Thus, the acetic acid concentration increase during the experiment.

Chapter I

50

-CO pressure is kept constant at 30 bar during the batch experiment via an over-pressurized ballast where the CO consumption is measured. According to scheme 36, one molecule of CO is consumed whereas two molecules of acetic acid are produced.

Determination of the carbonylation rate:

The carbonylation rate is given in moles of acetic acid produced per liter of the catalytic medium per hour (mol/L/h). According to the equations depicted in scheme 36, we consider that one molecule of acetic acid is produced from the carbonylation reaction (Eq.: 1, 2, 3) and one molecule of acetic is produced from organic reaction (Eq.: 4).

[RhI 2 (CO

) 2 ] -+ CH 3 I CO [Rh(COCH 3 )I 3 (CO) 2 ] - [Rh(COCH 3 )I 3 (CO) 2 ] - reductive elimination [RhI 2 (CO) 2 ] -+ CH 3 COI CH 3 COI + H 2 O CH 3 COOH + HI HI + CH 3 COOCH 3 CH 3 COOH + CH 3 I CH 3 COOCH 3 + H 2 O CO 2 CH 3 COOH
Scheme 36 : Consumption and production of the different products during a batch experiment Eq. 1

Eq. 2

Eq. 3

Eq. 4

Eq. 5

Chapter I

51

Thus, regarding equation 5, the carbonylation rate can be calculated from the consumption of methyl acetate, the consumption of water, the consumption of CO but half the production of acetic acid.

For our part, carbonylation rates were determined at 20 wt% and 15 wt% of remaining methyl acetate, starting from 27 wt% initial concentration of methyl acetate as explained above. We chose these values to evaluate the catalytic system activity which is not to its maximum when the carbonylation reaction starts and to have a remaining water concentration in the medium close to its initial concentration since water is consumed during the experiment.

To evaluate the carbonylation rate at 20 wt% and 15 wt% of remaining methyl acetate, we consider, regarding the equation 5 (Sc. 36), that one molecule of CO is consumed for one molecule of methyl acetate consumed. Thus, taking into account the temperature and the volume of our gas ballast, we calculate the consumption of CO required, considering CO as a perfect gas.

Once all the catalytic medium is introduced in the reactor and the CO pressure adjusted to 30 bar, a chronometer is started when the ballast pressure, measured with a digital equipment, starts to decrease.

At the calculated CO consumption for 20 wt% and 15 wt% of remaining methyl acetate, the time is noted and a sample is taking out from the reactor for chromatography analysis. Thus, concentration of acetic acid, methyl acetate, water and methyl iodide are calculated.

Batch experiments were performed starting from the same weight of catalytic solution (60g), the density of its solution being measured. For the determination of the carbonylation rate, this weight was corrected taking into account the CO introduction in the solution for the formation of acetic acid and considering that the density of the solution at 20 wt% and 15 wt% of remaining methyl acetate is the same than the initial solution.

Chapter I

52

Thus, regarding the consumption of methyl acetate, the comsuption of CO, the consumption of water and the half production of acetic acid, we calculated for each product, with the reaction time and the calculated volume of the solution, the carbonylation rate and we obtained in each case similar values.

For the batch run experiments performed with lithium iodide, we not take into account its wt% in the balance of acetic acid introduced in order to have similar acetic acid concentration in the batch runs performed without lithium iodide. It is worth noting that by introducing LiI in the reactor, we change slightly the catalyst concentration.

Thus, batch run experiments can be a good technique to evaluate the activity of a catalytic system considering the carbonylation rate and the behavior of the catalyst in reactor

conditions. But we have to keep in mind that the reactor is one part of a continue unit and that both, behavior and stability of a catalyst in the flash tank can't be evaluated using this technique.

CHAPTER II : FROM THE ACTIVE COMPLEX [MI 2 (CO) 2 ] -TO THE ACETYL COMPLEX [MI 3 (COMe)(CO) 2 ] -

Nowadays the most largely used processes for methanol carbonylation to acetic acid are the low-water-content systems using a rhodium catalyst and an iodide salt (LiI, NaI...) as co-catalyst, patented by Celanese 1 in 1991, and the Cativa process 2 , which employs an iridium catalyst and ruthenium as co-catalyst, patented by BP Chemical in 1996. Although the two iridium and rhodium catalytic cycles show some differences related to the nature of the intermediates 3,4 , several complexes present the same structures, especially the stable active and acetyl species (scheme 1). The active species results from the preformation step that we will describe later. In both cases, it is a square planar complex containing two carbonyl ligands in a cis-arrangement and two iodide ligands linked to the metal center. It starts the catalytic cycle leading, after further steps, to the acetyl species. The acetyl complex has two CO ligands, three iodide ligands and the acetyl group coordinated to the metal center, showing a pseudo-octahedral geometry. It is worth noting that two iridium acetyl isomers have been observed at ambient temperature, the cis-[IrI -was observed under the same conditions. The cis-rhodium acetyl isomer was detected by 13 C NMR at -30°C 6

. These acetyl compounds are key intermediates of the two catalytic cycles since they provide, after reductive elimination, acetyl iodide that gives acetic acid by immediate reaction with water giving back the active species [MI 2 (CO) 2 ] -.

In this chapter, at first we studied the rhodium catalytic cycle from the active species

[RhI 2 (CO) 2 ] -to the acetyl species [Rh(COCH 3 )I 3 (CO) 2 ]
-by synthesizing the different intermediates in the cycle stabilized with the bulk counter ion PPN + (Ph 3 P=N=PPh 3 + ), using Schlenk techniques. These complexes were characterized by various techniques in order to collect their spectral data and to have the possibility of identifying them in other experiments. Hence, we did not study these species under catalytic conditions.

Secondly, we also synthesized the intermediates involved in the iridium catalytic cycle with the same aim as for the rhodium catalytic cycle. These iridium complexes show different reactivities than their homologue rhodium complexes. Then, we compared their behavior and their reactivity.

Thirdly, we tried to promote the rate determining step in the iridium catalytic cycle by adding a metal co-catalyst. We know that ruthenium is a good co-catalyst in the Cativa process, and studies performed in our laboratory 7,8,9 by introducing a platinum co-catalyst have also shown also good carbonylation rates. Thus, we tried to determine if any rhodium and iridium complexes are able to exent a positive synergetic effect and, considering that at low-water-content rhodium alone is unstable and shows low carbonylation rates, if rhodium can behave as a co-catalyst in the iridium-catalyzed methanol carbonylation.

II-1. Study of the rhodium catalytic cycle.

The rhodium catalytic process for the carbonylation of methanol to acetic acid was described in detail since it was patented by Monsanto Company in 1973 The first step in this catalytic cycle is the oxidative addition of methyl iodide to [RhI 2 (CO) 2 ] -1

giving [RhI 3 (CH 3 )(CO) 2 ] -2. This is the rate determining step of the Monsanto catalytic system at high water content, since it was demonstrated by Paulik et al. 12 that this reaction corresponds to a nucleophilic attack of the rhodium metal center at the methyl iodide carbon atom. A kinetic study performed by Roth et al. 13 has shown that this reaction is first order with respect to methyl iodide and rhodium catalyst and zero order with respect to CO pressure. This reaction is very slow if compared to the same reaction with the iridium system. This is due to the higher electron density on the iridium atom, giving to iridium a higher nucleophilic nature. As the oxidative addition is very slow for the rhodium system, the next step, the CO cis-migration is very fast as shown by the fact that complex 2

was not isolated up to now. Thus, reaction of [RhI 2 (CO) 2 ] -1 with methyl iodide gives directly complex [RhI 3 (COCH 3 )(CO)] 2 2-3. This complex was isolated and crystallized by Forster We can observe from these spectra the slow formation of complex 3. The ν CO stretching bands (red spectrum) at 1986 cm -1 and 2058 cm -1 correspond to the starting complex 1. After 30 min., we note that the intensity of the band at 1986 cm -1 has decreased whereas the band at 2058 cm -1 is shifted to higher frequencies. We also observe the appearance of ν CO stretching bands in the acetyl area at 1737 cm -1 and 1717 cm -1

. After a 2 h reaction time (black spectrum), we only observe a shouldered wide band at 2063 cm -1 and two acetyls bands at 1737 cm -1 and 1717 cm -1

. This could mean that we have two isomers of complex 3 in solution. By bubbling CO through the solution for 5 sec., we observe (spectrum in yellow) the total disappearance of the former bands assigned to leads us to believe that the CO ligands are in trans position since we only observe the antisymmetrical infrared vibration. We have isolated complex 3 and labeled it (Fig. 5). We can see that some unlabelled complex 3 (bands at 2056 cm -1 and 1734 cm -1

) is still present. The presence of these features, terminal CO ligands/acetyl CO ligands, shows that we have two isomers in solution, according to the infrared spectrum (Fig. 5, black spectrum) that showed two acetyl vibration bands.

1 H NMR of unlabeled complex 3 (Fig. 7) shows two signals at δ = 3.15 ppm and at δ = 3.13 ppm, which confirms the presence of two isomers. We might assume that a molecule of solvent could be coordinated to complex 3 in two different sites in solution. dinitrogen to see by IR if the reductive elimination occurrs under these conditions. Surprisingly, we didn't observe the expected reductive elimination but the formation of complex 3 after 10 min., meaning that a CO ligand of complex 4 is very labile and can dissociate under ambient conditions giving back complex 3. By bubbling again CO through the solution, we observed in the formation of complex 4 (Sc. 3). At 0°C, this phenomenon is not observed a nd complex 4 lives in solution for more than one hour.

Therefore, we synthesized complex 4 in dichloromethane at 0°C, then added hexane in or der to form a bilayered mixture, and freezed the Schlenk tube to -18°C. After 3 days, we obtained crystals of [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4. A crystal was selected and its X-ray structure determined (Fig. 10).

The crystal system is monoclinic, the space group is P 2 1/c and the geometry is pseudo-octahedral. This componud is the mer-trans isomeric form of complex 4. The trans CO position of the ligands inferred from the IR spectrum (Fig. 4) of complex 4, is thus confirmed by its X-ray structure. The 13 C NMR spectrum of compound 6 (Fig. 13) shows a singlet at δ = 156 ppm corresponding to the two carbon atoms of the CO ligands and other peaks around δ= 130 ppm corresponding to the carbon atoms of the [PPN] counter-ion.

Figure 13 : 13 In a glass reactor under 5 bar of CO, compound 6 dissolved in dichloromethane gives slowly, after 2 h, the iridium acetyl complex 8. Maitlis et al. 20 showed that addition of a protic solvent such as methanol to the reaction mixture, increases dramatically the CO cis-migration reaction rate, and the authors proposed the mechanistic pathway depicted in scheme 5.

This accelerating effect could be explained by the ability of methanol to coordinate an iodide ligand linked to the iridium, center giving under CO the neutral tricarbonyl complex 7. Thus, the electron density on the metal center is reduced and the CO migratory insertion can occur more rapidly.

Since methanol is not present in batch experiments or even in industrial plants (see preliminary information), this accelerating effect was only observed in Schlenk tube experiments but provides an insight into the reaction mechanism. Thus, action of methanol could be mimicked by the use of others --co-catalysts able to abstract an iodide ligand, which could also be introduced in a reactor industrial plant.

On our part, we tried to synthesize compound 8 using a glass reactor (see Experimental Section). We dissolved [PPN][Ir(CH 3 )I 3 (CO) 2 ] 6 in a mixture of CH 2 Cl 2 /MeOH 20mL/10mL. After 1 hour under 5 bar of CO and at ambient temperature, the initial orange solution turns yellow. The solvent is removed under reduced pressure affording an orange-yellow powder. The infrared spectrum of the resulting solid (Fig. 14) shows three CO bands at ν CO = 2110 cm -1 and at 2069 cm -1 with a shoulder, corresponding to the terminal CO ligands linked to the metal center, and a band at 1657 cm -1 characteristic of the CO acetyl stretching mode. This system of two CO terminal bands could correspond to a cis isomer, but the intensities of the two bands are expected to be the same, that is not the case, thus we think that a mixture of at least two isomers is present in the medium. Since the effect of methanol on this cis-migration reaction was previously established, we tried to determine the influence of methanol concentration on this reaction by performing further reactions under the same conditions (1 hour, 5 bar of CO, 500 mg of [PPN][Ir(CH 3 )I 3 (CO) 2 ], ambient temperature) except for methanol concentration. By this way, we studied the effect of the polarity and the protic nature of the solvent on the cis-migration reaction. Figure 15 shows the infra-red spectra of the product obtained witth three of these reaction mixtures. We could observe that methanol concentration exerts a strong influence on the cismigration reaction, and not only on the rate of this reaction as it was published 20 but also on the nature of the species formed.

At low methanol concentration (spectrum in red), we can see two terminal CO bands at ν CO = 2110 cm -1 and 2061 cm -1 with a shoulder. These two vibrations are due to the cis-isomer complex : cis-

[PPN][IrI 3 (COMe)(CO) 2 ] cis-8.
Examination of the acetyl region reveals two bands at 1679 cm -1 and 1658 cm -1

. These two acetyl bands and the shoulder on the low frequency terminal CO band make us think that two different isomers are present. Maitlis et al observed the same phenomenon 5 .

At a higher methanol concentration (spectrum in blue), we observe the decrease of the terminal CO band at 2110 cm -1 and the increase of the terminal CO band at 2061 cm -1 which is shifted to 2070 cm -1

. The acetyl region bands show a large band at 1657 cm -1 that might hide two acetyl bands. It is worth noting that the acetyl band at 1680 cm -1 has disappeared or is very weak. We believe that, under these conditions, two isomers are formed, cis- , and an acetyl band at 1653 cm -1

, which is sharper than that observed in the second experiment. A single terminal CO band is characteristic of a trans complex, that is to say trans-

[PPN][IrI 3 (COMe)(CO) 2 ], trans-8.
We monitored the same reactions under the same conditions by 1 H NMR. The different experiments were conducted according to the synthetic method depicted above. The solvent was removed under reduced atmosphere, and the resulting powder analyzed by 1 H NMR in CD 2 Cl 2 at ambient temperature (Fig. 16). We can observe in spectrum a) at low methanol concentration, a major peak at δ = 3.12 ppm that can be assigned to compound cis-8. The signal at δ = 2.18 ppm corresponds to unreacted complex 6, which is observed (Fig. 15, red spectrum) as a shoulder on the main bands. At higher methanol concentration, spectrum b) shows three mains peaks. The first at δ = 3.12 ppm corresponds to compound cis-8. The second at δ = 2.77 ppm is attributed, according to the literature 5 , to trans-8 and the third one at δ = 3.60 ppm that has not been identified yet. It could correspond to a third acetyl compound that may be the neutral iodo bridged dimer [Ir 2 I 4 (COMe) 2 (CO) 4 ]

or trace of methanol in the coordinating sphere of trans-8. At higher methanol concentration, spectrum c), we observe two major signals at δ = 2.77 ppm and δ = 3.60 ppm corresponding to trans-8, and to the hypothetical neutral acetyl dimer or methanol and a weak signal at δ = 3.12 ppm due to cis-8. It comes out that the infrared spectra in the figure 15 and the 1 H NMR spectra in the figure 16 are in good agreement with each other.

Thus, from these observations we can conclude that by increasing the concentration of methanol, i. e. the protic character and the polarity of the medium as well, the ratio trans-

[PPN][IrI 3 (COMe)(CO) 2 ]/cis-[PPN][IrI 3 (COMe)(CO) 2 ]
does increases. We can also observe that in the two last 1 H NMR spectra b) and c), the signal corresponding to

[PPN][Ir(CH 3 )I 3 (CO) 2 ] 6 is weak, which leads us to propose that the reaction route from compound 6 to compound trans-8 may involve compound cis-8, followed by a cis/trans isomerisation. Theoretical studies on these two isomers have shown that the trans-isomer has a lower energy than the cis one 21 , meaning that cis-8 could be the kinetic product and trans-8 the thermodynamic one.

We have encountered some difficulties in labeling these two isomers with 13 CO under the above reaction conditions due to the high cost of this gas and the required conditions for the synthesis (5 bar of CO). In order to collect 13 C NMR data we synthesized the two isomers, cis-8 and trans-8, according to the aformentioned procedure using a glass reactor and a solvent mixture of CH 2 Cl 2 /MeOH = 25mL/5mL to obtain the cis isomer (sample 1, Fig. 17) and of CH 2 Cl 2 /MeOH = 10mL/20mL to obtain the trans isomer (sample 2, Fig. 18). We analyzed these two samples by 13 C NMR spectroscopy in CD 2 Cl 2 at ambient temperature over night.

Figure 17 : 13 Although the infrared spectra of these two samples present the expected CO bands relevant to cis-8 for sample 1 and to trans-8 for sample 2, the 13 C NMR present many peaks (Fig. 17 and18). In the case of the first sample (Fig. 17 These 13 C NMR experiments confirm the results observed previously by infrared and 1 H NMR spectroscopy concerning the protic solvent effect and the cis/trans isomerisation reaction.

In order to study the cis/trans isomerisation of compound 8, we monitored the cis-migration reaction using our high-pressure infrared (HP-IR) reactor (see Experimental Section). We introduced is fromed by the reductive elimination from complex 8. Indeed, under these conditions, we don't observe the cis/trans isomerisation of complex 8.

Figure 19 : HP-IR spectra of the cis-migration reaction using CH 2 Cl 2 /MeOH = 15 mL/10 mL solvent mixture Figure 21 : 13 C HP-NMR spectra of the reaction of complex 6 in CD 2 Cl 2 /CD 3 OD = 1mL/2mL at 40°C under 5 bar of 13 CO for 10 min. reaction.

Figure 22 : 13 C HP-NMR spectra of the reaction of complex 6 in CD 2 Cl 2 /CD 3 OD = 1mL/2mL at 40°C under 5 bar of 13 CO for 40 min. reaction. ppm for the hypothetical neutral dimer and at δ = 165.02 ppm for unidentified species. We also note the appearance of a signal at δ = 172.33 ppm, which could correspond to complex 5 but also of From these 13 C NMR experiments, we can confirm that an isomerisation occurs from

complex cis-[PPN][IrI 3 (COMe)(CO) 2 ] cis-8 to complex trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-8.
This cis/trans isomerisation could occur via unidentified intermediates which could correspond to the unidentified signals. On one hand, methanol accelerates the cis-migration reaction from complex 6 to complex 8, and on the other hand the increase in methanol concentration favor the cis/trans isomerisation. We can note that the signal at δ = 185 ppm corresponding to free 13 CO dissolved in the reaction mixture decreases significantly after some time since it passes from the liquid phase to the gas phase. That could explain why some complex 6 is found at the end of the reaction. Moreover, under these conditions, the reductive elimination step is likely to be very slow since after 90 min., we only observed a small signal corresponding to the starting complex 5 whereas large signals corresponding to the acetyl species cis-8 and trans-8 are still observed.

In order to increase the rate of the CO migratory insertion step, we increased the polarity and the protic character of the medium, by adding a small quantity of water. Since water is not soluble in dichloromethane, we were obliged to add a small amount of methanol to obtain a homogeneous solution. By reacting the methyl iridium complex 6 in CH 2 Cl 2 /MeOH/H 2 O = 20 mL/5 mL/5 mL under 5 bar of CO and at ambient temperature, we only observed, after 30 min., the CO stretching bands corresponding to trans-8, meaning that water is much more effective than methanol for the cismigration reaction and the cis/trans isomerisation. Moreover, water is used in the industrial plant reactor. Therefore, we can believe, regarding the former result, that the increase in water content in batch or continuous experiments could increase the carbonylation rate as it was observed for the Monsanto process using a rhodium catalyst. We will see that it is not the case for iridium-catalyzed batch experiments, where a maximum activity is observed at about 4 wt% water concentration and a decrease in activity is observed for higher water concentrations (Fig. 25). We can correlate these pieces of information to the observations made by Forster 22 for the rhodium-catalyzed water gas shift reaction (WGSR), where the rate of the WGSR increases to a maximum when the water content increase, to about 10 wt% and then decreases, whereas the water concentration continues to increase. Moreover, a too high water content involves high energy costs to separate acetic acid from water.

The X-ray structure of cis-[PPN][IrI 3 (COMe)(CO) 2 ] cis-8 was reported in 2004 by Maitlis et al. -by photolysis of [IrI 3 (CH 3 )(CO) 2 ] -under CO. They obtained crystals of this complex and published its structure with AsPh 4 + as counter-ion. The bond distances are in good agrement with those we obtained for compound trans-8. A density functional study concerning this complex was published in 2001 19 , and calculated data for trans-

[IrI 3 (COMe)(CO) 2 ]
-fit well with those we have observed. It's worth noting that this trans effect is not observed in the rhodium homologue complex shown in figure 10. In this context, some research groups introduced into the iridium catalytic system another metal co-catalyst in order to promote the slowest step and accelerate the carbonylation rate of the process. That is the case of the BP-chemical group who found that addition of a ruthenium complex to the iridium catalytic system enhances dramatically the carbonylation rate, leading to a new process suitable for industrial use named Cativa 2 .

On our part a few years ago, we found that addition of a platinum co-catalyst also could enhance the carbonylation rate of the iridium catalytic system. Mechanistic studies were previously performed to understand the role of the platinum in this system 7,8,9 , and continuous catalytic experiments showed good activity for this bi-metallic system at low water content in comparison with the iridium system alone. In addition we tried to establish via Schlenk tube experiments if a rhodium complex could promote the cis-migration reaction slow step in the iridium catalytic cycle.

In this section, we will describe the [Ir/Ru] and [Ir/Pt] systems and the action of the [RhI(CO) 2 ] 2 complex on the rate determining step in the iridium-catalyzed methanol carbonylation cycle.

II-3-1 The Cativa process

In the first part, we will briefly describe the basics of the Cativa process. The addition of a ruthenium promoter, such as [RuI 2 (CO) 4 ], to the iridium catalytic system, accelerates the carbonylation rate. This effect is explained by the ability of [RuI 2 (CO) 4 ] to loose a CO ligand and to abstract an iodide II-3-2 The iridium-platinum system.

In our laboratory, we developed and patented 25 for the carbonylation reaction, and that iridium alone has a moderate activity (16 mol/L/h).

We performed a second set of experiments in order to determine the desired water percentage to obtain the maximum activity. These batch runs were based on the same conditions as the former, except for the platinum/iridium molar ratio maintained at Pt/Ir = 3/7 and the variation of water percentage between 2% and 8%. Results are shown in figure 25. The curve representing the activity of the iridium-platinum system as a function of water content presents the shape of a gaussian curve with a maximum for a water content of 5-6 wt%. We chose to express the activity in terms of turnover frequency (TOF), based on metal concentration, to compare our results with those published for the Cativa process 26 . One can see that the TOF reached with the Cativa system are slightly higher than those obtained with the iridium-platinum system. Both of them have their maximum for a water content around 6%, the [Ir-Ru] system being a little bit more active than the [Ir-Pt] system.

RUN

In order to get mechanistic information on the [Ir-Pt] system, Schlenk tube experiments were performed under nitrogen using the stabilizing counter ion PPN + .

[PPN][IrI 3 (CH 3 )(CO) 2 ] 6 was prepared as described in section II-2. [PtI 2 (CO)] 2 9 was synthesized 27 introducing the PtI 2 salt to nhexane in a reactor. The reactor was pressurized to 12 bar, stirred and heated at 75° C for 1h. The reactor was then cooled down to ambient temperature and a red solid corresponding to the [PtI 2 (CO)] 2 dimer was obtained. Complex 9 was labeled with 13 CO and its 13 -for m/z = 1161. Hence, we can envisage the mechanistic route described in scheme 7.

-5490 -5490 -5480 -5480 -5470 -5470 -5460 -5460 -5450 -5450 -5440 -5440 -5440 -5460 -5480 ppm

New platinum complex

: -5462 ppm, J 13 Rhodium is known to have high activity for methanol carbonylation at high water content but shows low activity at low water concentration, where iridium catalytic system shows the best activity.

The two catalytic cycles show similarities with respect to their intermediates, but their rate determining steps are different and the rhodium system shows high activity at low water content with high iodide salt concentration, whereas, in the iridium system, a iodide ligand must be removed from complex The infrared spectrum of complex 10 (Fig. 30), shows CO streching bands at 2095 cm -1

[IrI 3 (CH 3 )(CO) 2 ] -to
, 2080 cm -1 and 2028 cm -1

. The presence of these three bands shows that the atoms of the molecule are not in the same plan. The 13 .17 Hz) is not observed as well. Thus, the same reaction was analyzed by mass FAB spectroscopy in negative mode (Fig. 33). In a second set of experiments, we monitored this reaction by 13 C NMR varying the relative concentrations of the neutral rhodium dimer 10 and the methyl iridium complex 6. Figures 34, 32 and 35 show three representative 13 C NMR spectra for Rh/Ir = 1/2, Rh/Ir = 1 and Rh/Ir = 2, respectively.

For a molar ratio of Rh/Ir = 1/2 (figure 34), we can note the presence of a doublet at δ = 182.95 ppm, (d, Figure 34 : 13 C NMR spectra of the mixture coming from reaction between 2 and 6, Rh/Ir = 1/2, in CD 2 Cl 2 , r.t.

For the last case (Rh/Ir = 2, Fig. 35), we observe a wide signal not well defined between δ = 180 ppm and δ = 183 ppm, pointing to a mixture of rhodium complexes, such as the heterobimetallic dimer and complex 10 still in solution since it is in excess, and also a wide signal not well at δ = 155.53 ppm that could hide the two signals that correspond to the carbon atoms of the CO ligands of the iridium moiety in the heterobimetallic compound and the carbon atoms of the CO ligands of the homobimetallic iridium dimer.

In order to investigate the above system and iys reactivity pattern, CO gas bubbled for 5 sec.

through a solution of 25 mg of [PPN][IrI 3 (CH 3 )( 13 CO) 2 ] 6 (2.11.10 -2 mmol), with 0.5 equivalent of complex [RhI( 13 CO) 2 ] 2 10 (1.05.10 -2 mmol, 6 mg) in 2 mL of CD 2 Cl 2 and analyzed the reaction mixture by 13 C NMR (Fig. 36). -are still present in solution. By bubbling CO through the same reaction solution for 5 min., we can observe the disappearance of the signal seen by 13C NMR because of the exchange between 13 CO and 12 CO. After CO bubbling, we extracted this solution with hexane and analyzed the Catalytic batch experiments starting from IrI 3,4 and RhI 3 were performed varying water concentration and the Rh/Ir molar ratio. Unfortunately, the observed carbonylation rates don't present significant enhancement in comparison with the iridium system alone. We know that the dimeric species [RhI(CO) 2 ] 2 10 can be easily formed from complex H[RhI 2 (CO) 2 ] 1, even by evaporating a solution of 1, but we don't know if it can be formed under catalytic conditions. We can believe that this dimer might be produced in the flasher zone of a continuous plant. Thus, continuous catalytic experiments need to be made in order to know if the [Rh/Ir] system presents higher activities than those we have observed in batch experiments.

II-4. Conclusion

In this chapter, we describe the synthesis and characterization of the different intermediates in the rhodium and iridium catalytic cycle for methanol carbonylation into acetic acid and studied their reactivity, except for the methyl rhodium complex 2, which is a too short-lived species to be isolated.

At first, we investigated the rhodium catalytic cycle. In this case, the oxidative addition of methyl iodide to the starting complex 1 is the rate determining step. In comparison with the iridium cycle, only the trans-acetyl rhodium complex trans-4 has been observed under ambient conditions. Brunel et al. 6 have observed complex cis-4 by 13 C NMR at low temperature after oxidative addition of CH 3 COI to the starting complex 1, but its presence under catalytic conditions is quite unlikely to occur.

The rhodium catalytic cycle includes two acetyl intermediates, a mono-carbonyl species 3 and the dicarbonyl complex trans-4. We showed that insertion of CO into complex 3 giving complex 4 is reversible under ambient conditions, meaning that a CO ligand of complex 4 is very labile. The X-ray structure of mer-trans-4 has been solved.

As for the iridium system, in which the rate determining step is the cis-migration reaction, two acetyl isomers cis-8 and trans-8 were synthesized and isolated. The X-ray structure of trans-8 has been solved. The protic nature and the polarity of the solvent used for the cis-migration reaction play a prominent role on this reaction. Firstly, the increase in the concentration of a protic solvent, as methanol or water, increases the rate of the reaction. Secondly, the increase in the concentration of the protic solvent exerts an effect on the nature of the iridium acetyl isomer formed. At low protic solvent contents, the cis-8 isomer is preferentially formed, whereas at high protic solvent contents, the trans-8 isomer is formed. We checked by HP-IR and HP-NMR that a cis/trans isomerisation occurs.

Thus, we believe that the cis-8 isomer is at first formed in the reaction mixture and then isomerises to the trans-8 isomer, the protic solvent allowing this isomerisation process.

In the second part, we observed that addition of co-catalyst as [RhI(CO) 2 ] 2 complex accelerates the CO migratory insertion reaction from complex 6 to complex 8. We showed that this . This process shows a selectivity of 99% with respect to methanol and 85% with respect to CO for high activity, but its major drawback is that it needs at least 14 %wt of water to achieve high carbonylation rates and good rhodium stability. This high water content leads to a high energy consumption and to high costs due to the distillation process needed to separate water from acetic acid. In 1991, Celanese patented an improved process 2 using rhodium catalyst and iodide salts such as LiI, NaI or KI as promoters. This new process shows a similar range of selectivities at only 4 %wt of water, and improved stability for similar activity. However, the role of iodide salt promoters in the catalytic cycle of rhodium remains not well understood.

In this chapter we will study the role of iodide salt promoters in the low water content process for methanol carbonylation to acetic acid using a rhodium catalyst. We will show the importance of the reductive elimination step at low water content, and the important role of water, as well of the iodide salt promoters on the reaction.

III-1. Effect of water and of the iodide salt promoter on the low water content process

Since Celanese patented its rhodium-based low water content process, research groups 3,4

have carried out studies in order to understand the role played by the iodide salt promoter, mainly LiI.

They proposed that lithium iodide reacts with the starting complex [RhI 2 (CO) 2 ] -1 to form a dianionic species more nucleophilic than 1 that could react more rapidly with methyl iodide to give the methyl rhodium complex 2. Then, the acetyl complex 3 is formed by the rapid migratory CO insertion reaction.

They also proposed that LiI may react with methyl acetate or acetic acid to give an acetate ion which might coordinate to the metal center giving a dianionic complex (Sc. 1).

% water Carbonylation rate (mol/L/h)

17% of LiI without LiI

These two figures show indeed the same trend.

-For the curves showing the carbonylation rate measured in the absence of lithium iodide, we observe a break point at around 6 % of water for 20 % of methyl acetate remaining and at 8 % of water for 15% of methyl acetate remaining. The shift in the position of the break point observed in the two figures is due to water consumption between the two samplings, since no water is added during the batch experiment and the abscissa in the plot represents the water % introduced at the beginning of the experiment. We note that beyond these break point, the carbonylation rate is nearly constant while the water content increases. Before the break points, we can see that the carbonylation rates decrease dramatically and linearly. When we opened the reactor at the end of the experiment, we observed, for the run at 2% of water, a small quantity (not measured) of dark deposit on the walls of the reactor indicating that under these conditions some rhodium black is formed.

Carbonylation rate at 15% d'AcOMe

% water Carbonylation rate (mol/L/h)

17% of LiI without LiI

Figure 2 : Carbonylation rate at 15% methylacetate remaining -As for the curves related to the carbonylation rate measured in the presence of lithium iodide, we don't observe a sharp breaking point. Indeed, the carbonylation rates decrease slowly and regularly while the water content is decreasing. For the curve at 20 % methyl acetate remaining, the carbonylation rates decrease equals only 0.5 mol/L/h when we reduce the water content from 14% to 2%. In each case, we never observed any dark deposit at the end of the experiments. It is also worth noting that above 6 % water content for the curves representing the calculated carbonylation rate at 20% of methyl acetate remaining, or above 8% water content for the curves at 15% of methyl acetate remaining, we observe only a slight difference between the two curves, with or without LiI, meaning that the carbonylation rates are quite similar. Some questions arise from the analysis of these results:

-Firstly, why do we observe a sharp and dramatic decrease of the activity while the water content decreases below 6% (or 8%) without LiI? We don't believe that precipitation of rhodium black can fully explain this phenomenon since we observed a deposit only at 2% water, and in very small quantities. Thus, does water exert a role on the catalytic cycle and what is this role? -Secondly, why the carbonylation rates remain quite constant whatever the water content when LiI is introduced in the reactor? The hypothesis of the formation of a dianionic complex increasing the methyl iodide oxidative addition rate is not convincing since the carbonylation rates above a water content of 6% (or 8%) are similar with or without lithium iodide. So, what is the role of lithium iodide in the rhodium catalytic cycle? -Thirdly, if the carbonylation rates are quite the same beyond a water content of 6% (or 8%) with and without LiI and if these carbonylation rates are maintained below a water content of 6% (or 8%) with LiI and decrease dramatically without LiI, do lithium iodide and water play a similar role? Is LiI taking the place of water when the concentration of the latter is too low? It is largely accepted that the main role of water in the catalytic cycle is to react with acetyl iodide produced from reductive elimination of [RhI 3 (COCH 3 )(CO) 2 ] -4 to give acetic acid and HI (Sc. 2), but in fact this reactivity cannot explain the marked decrease in the carbonylation rates at low water content since, even at low water content, water is in large excess with respect rhodium concentration.

Lithium iodide can react with acetic acid to form lithium acetate and HI, the latter being able to react with methyl acetate producing acetic acid and methyl iodide (or LiI reacts directly with methyl acetate to give lithium acetate ion and methyl iodide) 5 (Sc. 3). These reactions could increase the methyl iodide concentration in the medium and, since the oxidative addition is first order in CH 3 I, accelerate the rate determining step in the rhodium catalytic cycle. As for the higher concentration of methyl iodide when LiI is present in the reaction mixture, the carbonylation rate of the LiI promoted process should be faster than that of the high water content process, but this is not the case, the increase of methyl iodide concentration being to low to really promote the oxidative addition step .

In order to gain information about the role of water and lithium iodide in the reaction mixture, we performed a series of 13 Then, 13 C HP-NMR experiments at different water contents were performed. The most representative spectra are shown in figure 5, 6, 7, 8 and 9. We have introduced to the sapphire tube 25 mg of [PPN][RhI 2 ( 13 CO) 2 ] 1 (1000 ppm), 27% of methyl acetate, 14% of methyl iodide, water (5%, 10% or 20%) and deuterated acetic acid (balance). The sapphire tube is then sealed and filled with 15 bar of 13 CO. Before placing the tube into the NMR probe heated at 90°C, we have shaked it vigorously in order to dissolve the catalyst and 13 CO into the solvent. We have observed a significant decrease in the 13 CO signal (δ = 185 ppm) during the experiment due to its consumption or its going back to the gas phase at 90°C without stirring. Thus, we analyz ed the reaction mixture during 10 min. in order to have the most representative medium comparing to batch experiments performed in an autoclave. We have also checked by gas phase chromatography, before and after the NMR experiments, that we have consumption of water and methyl acetate and formation of acetic acid but it was not possible to provide carbonylation rates since we have no information on the CO consumption and the real reaction time. Finally, we have repeated the same experiments by introducing 17% of LiI. In the above HP-NMR spectra we observe intense peaks: the signal at around δ = 185 ppm corresponds to free dissolved 13 CO, the signal at δ = 175 ppm corresponds to CH 3 COOH and the signal at arbout δ = 173.5 ppm to CH 3 COOCH 3 . We cannot explain the presence of a second signal in figure 6 and why this signal is shifted down in figure 9. Moreover, we observe carbon atoms in rhodium complexes. Since we have checked for every each experiment by gas chromatography that the carbonylation reaction occurs, the rhodium complexes observed are likely to be those with the longer life-times in the medium, that is to say those preceding the rate determining step(s). Results are summarized in table 1.

In the spectrum shown in figure 5 1. Such signal could also correspond to the dimeric acetyl complex 3 but by running the reaction for 30 min. we observed the loss of the CO signal together with the appearance of a doublet at 179.52 ppm (d, 1 J Rh-C = 68.04 Hz) and a signal at 209 ppm strongly suggested that the dimeric acetyl complex 3 is formed wia the loss of CO in the liquid phase

Therefore we can conclude that at high water content, the rate determining step is the oxidative addition reaction as expected.

Table 1 : Summarized HP-NMR experiments.

We also observe a signal at δ = 213.06 ppm that has not been identified. Its position in the acetyl region could point to an acetyl species but no CO terminal corresponding to rhodium complexes of this kind are detected. It could correspond to acetate ions. Another small signal is present at 186 ppm, which we coul not identify.

When decreasing the water content to 10% (Fig. 6), we note some changes in the spectrum.

Indeed, we observe the appearance of two novel signals at δ = 177.73 ppm (d, These pieces of observation mean that under these operating conditions two steps presents similar rates: oxidative addition as expected but also reductive elimination.

By decreasing the water content to 5% (Fig. 7), we only observe a doublet at δ = 177.86 ppm (d, 1 J C-Rh = 55.92 Hz) assigned to complex 4, the signal corresponding to the acetyl group being presumably hidden by the spectrum noise.

At low water content, the reductive elimination reaction is slower than oxidative addition and becomes the rate determining step in the rhodium catalytic cycle.

It is worth noting that, at low water content, when we removed the sapphire tube from the NMR probe, we noted dark metallic deposit on the tube wall. This can explain the low definition of the spectrum.

We ran the same experiments again by introducing 17% of LiI. In figure 8, at 20% of water, we observe two doublets at δ = 182.68 ppm (d, As far as these results are concerned, at low water content without LiI and at 10 wt% of methyl iodide the rate determining step in the rhodium catalytic cycle is reductive elimination. Addition of higher amounts of water changes the rate determining step to oxidative addition. Hence we can propose that one role of water in this process is to accelerate the reductive elimination rate.

Concerning the role of lithium iodide, we didn't observe acetyl rhodium species in the medium at low water content for the experiments using 17% of LiI. Hence, in the same way as water, LiI may accelerate the reductive elimination step. In these experiments we also observed the presence of experiments, the rhodium active species can also catalyze the water gas shift reaction, and that lithium iodide may reduce this side reaction since two iodide ligands must be removed from 11 to produce complex 1. The increase of iodide ions concentration in the medium should then limit this reaction, the resting state becoming complex [PPN][RhI 4 (CO) 2 ] 11.

Jones proposed 6 that the reductive elimination reaction could be the rate determining step in the rhodium-catalyzed methanol carbonylation cycle under 8% water content, but no proofs were given for this hypothesis. Regarding the present 13 C HP-NMR experiments, we can conclude that water plays a major role, not only on the organic reaction with acetyl iodide to form acetic acid, but also on the final rate of the rhodium catalytic reaction, affecting the reductive elimination step, as well as lithium iodide. We have summarized these conclusions in the catalytic cycle depicted in scheme 4. After careful study of the results obtained in section III-1, we have chosen to focus our attention on the reductive elimination step in the rhodium catalytic cycle. To the best of our knowledge, this reaction has not been examined in detail since it has been admitted thirty years ago it was to be a reductive elimination reaction of CH 3 OI from complex 4 8 (Sc 5).

Such reductive elimination pathway doesn't indeed explain the observed effect of water and LiI of on the previously HP-NMR experiments described in section III-1.

We monitored by IR the elimination reaction from [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4 and water (Fig. From these spectra, we can note at first sight that the IR active CO stretching band corresponding to the terminal CO ligands of the rhodium complexes in THF is shifted to lower frequencies if compared to the bands observed in dichloromethane. Indeed, the CO band for complex 4 is at 2084 cm -1 in dichloromethane and 2074.6 cm -1 in THF.

In fact, the IR monitored reaction shows that the reductive elimination reaction occurs under these very conditions. The band at 2074.61 cm -1 (spectrum in red) corresponds to complex 4. The two bands at 2049 cm -1 and 1975 cm -1 correspond to the starting complex 1. During the reaction course, the intensity of the bands corresponding to complex 1 increaes, meaning that the concentration of 1 is increasing in the reaction mixture. After 195 min. reaction, we observed in solution (spectrum in brown) that two complexes are present: complex 1 and an acetyl complex (band at 2070.7 cm -1 ). It is worth noting that in the spectra at 60 min. (spectrum in green) and at 195 min (spectrum in blue), we observe no major differences in the intensities of the bands corresponding to complexes 1 and 4.

Presumably, under these conditions, water induces reductive elimination which seems to be very slow so that an equilibrium is reached after a long reaction time.

Since water induces elimination, we thought that methanol could be able to induce this reaction too. Hence, we reacted 25 mg of complex 3 in 1mL of dichloromethane and 5 mL of freshly distilled methanol (see Experimental Section) and bubbled CO through the solution in order to obtain complex 4. This reaction was monitored by IR (Fig. 11). Surprisingly, we could not observe any elimination reaction but only the reformation of complex 3, meaning that methanol doesn't induce the expected reductive elimination reaction.

Thus, on one hand, water induces reductive elimination by reacting with acetyl iodide to give acetic acid, and in on the other, methanol does not induce any reductive elimination although it seems possible that methanol may react with acetyl iodide. To confirm this hypothesis, we independently synthesized acetyl iodide 9 starting from acetyl chloride and trimethylsilyliodide and studied its reactivity. The IR spectrum of acetyl iodide is shown in figure 12, and is in agreement with the calculated spectrum. After this synthesis, we introduced acetyl iodide (2 g, 11.76 mmol) using a dry glass syringe into a dry Schlenk tube containing 5 mL of distilled dichloromethane and dry methanol (0.376 g, 11.76 mmol). Immediate IR analysis of the solution shows the CO stretching band of methyl acetate at 1738 cm -1 (Fig. 13).

As acetyl iodide reacts readily with methanol to form methyl acetate, we can believe that in the IR monitored experiment shown in figure 11 there is no formation of acetyliodide by elimination from complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4. On the contrary, we observe a reductive elimination reaction in the IR monitored experiment using water ( Fig 10). Thus, we have no proof of the formation of acetyl iodide from acetyl rhodium complex 4 under these conditions.

We believe that in the case of the reaction with water there is a direct interaction of water with the acetyl complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4 whilst that is not the case with methanol, this latter being less reactive than water. In the same way as in the methanolysis An overview of all the experimental results, and observations from the literature, led us to the hypothesis that a relationship could exist between the reactivity of water, acetic acid, and lithium iodide. We can suppose that water with acetic acid and LiI with acetic acid are in equilibrium with acetate ions as shown in the two equations of scheme 7

Then, we reacted the rhodium acetyl complex These spectra show that sodium acetate induces elimination reaction from complex 4. Indeed, we observe the appearance of the CO stretching bands at 2058 cm -1 and 1986 cm -1 corresponding to the starting complex [PPN][RhI 2 (CO) 2 ] 1 while the bands at 2084 cm -1 and 1704 cm -1 corresponding to complex 4 decrease. We can note the appearance of a ν CO band at 1826 cm -1 that corresponds to acetic anhydride according to a calculated spectrum.

For this reaction the ratio Rh/AcO -= 2.10 solubility in dichloromethane. Indeed, addition of 1 mL of methanol to the reaction mixture induces a total and instantaneous elimination giving complex 1, acetic anhydride (1826 cm -1 ), methyl acetate (1737 cm -1 ) and acetic acid (1711 cm -1 ) ( Fig 15).

In addition, we performed the same reaction starting from 10 mg of [PPN][RhI 3 ( 13 COCH 3 )( 13 CO) 2 ] 4, obtained after vigorous agitation under of 1 atm. of 13 CO of a solution of complex 3, in deuterated dichloromethane with more than 100 excess of sodium acetate. We analyzed the solution by 13 Figure 16 : 13 C NMR of the reaction between complex 4 and sodium acetate after 1 min. The 1 H NMR spectrum at the end of the reaction is shown in figure 18. We note the presence of three signals that in fact consist in a singlet at 2.21 ppm and in a doublet (d, 2 J H-C = 6.7 Hz). This is due to the asymmetric labeling of acetic anhydride: the acetyl group coming from the acetyl rhodium complex 4 is labeled whereas that is not the case for the acetyl group coming from the acetate ion. We can also note the presence of a singlet at 2.17 ppm corresponding to the methyl iodide that remains after the methylation reaction. Figure 17 : 13 C NMR of the reaction between complex 4 and sodium acetate after 5 min. was added via an additional over-pressurized reservoir. The total pressure was then adjusted to 30 bar. During the first 30 min., we did not observe any CO consumption, but after this induction period a low CO consumption was noted (carbonylation rate < 1 mol/L/h). After a reaction time of 3 h, the reactor was cooled down to room temperature and depressurized. The analysis of the solution by gas chromatography showed the presence of large amounts of methyl acetate (about 80 wt%) but of also methyl iodide (about 5 wt%), acetic anhydride (about 13 wt%) and small amounts of acetic acid (less than 2 wt%) due to the presence of water in traces.

Thus, the presence of acetic anhydride and methyl iodide at the end of this experiment shows that the carbonylation reaction occurs (but at a very slow carbonylation rate ,since the catalytic medium employed is far from being the best and since the presence of PPN as counter ion may also reduce the activity of the catalyst) without any introduction of methyl iodide into the reactor. This result confirms that methyl iodide is formed from methyl acetate and lithium iodide and that acetate ions can indeed be formed under catalytic conditions (190°C, 30 bar) .

After having confirmed that acetate ions react with the acetyl rhodium complex [RhI 3 (COCH 3 )(CO) 2 ] -4 and that acetate ions are formed in the catalytic medium in the presence of LiI, we investigated other nucleophilic compounds that could react with complex 4. In a first set of experiments we tested a non-ionic nucleophilic family of molecules such as para-methoxyphenol, triphenylmethanol or urea (Sc. 10). Using different solvent mixtures to dissolve complex 4 and the different nucleophilic compounds, we never observed any reaction, even by heating slightly the reaction mixture (50°C).

In a second set of experiments, we tested some strong ionic nucleophilic/basic species as methanolate, obtained by the reaction of sodium with methanol, and sodium hydroxyde (Sc. 11).

Introduction of methanolate or sodium hydroxyde into a solution of complex 4 leads instantaneously to the formation of rhodium black. No CO stretching band was observed by IR spectroscopy. In a third set of experiment, we checked other carboxylate anions than the acetate ions, such as sodium butyrate or sodium benzoate (Sc. 12)

In each case we observed reaction between the carboxylate ion and the rhodium acetyl complex 4 giving the active complex 1 and mixed anhydride compounds. We monitored by IR (Fig. 19) Figure 19 : IR spectrum of the reaction between sodium butyrate and complex 4.

the
Scheme 13 : Proposed side reaction between sodium butyrate and mixed anhydride Figure 20 : 13 C NMR spectrum of the reaction between sodium butyrate and labelled complex 4.

H 3 C H 2 C C H 2 C O O C O H 3 C H 3 C H 2 C C H 2 C O O - H 3 C H 2 C C H 2 C O O C O H 2 C C H 2 H 3 C + H 3 C C O O - H 3 C H 2 C C H 2 C O O C O H 3 C or 4 H 3 C C O O C H 3 C O + H 3 C H 2 C C H 2
anhydride signal

Thus, almost all of the carboxylate salts we tested react with the rhodium acetyl complex 4

giving complex 1 and mixed anhydrides, their reactivity depending upon their solubility in the medium.

As a further step, we were also interested in investigating the reactivity of the monocarbonyl rhodium acetyl complex was added to the solution. The reaction was monitored by IR (Fig. 22).

In the first spectrum (black one) we observe at the very beginning of the reaction the bands at 2063 cm -1 , 1736 cm -1 , and 1713 cm -1 corresponding to complex 3. After 5 min. (spectrum in blue), we see clearly the presence of the bands at 2058 cm -1 and 1985 cm -1 corresponding to the active complex

[PPN][RhI 2 (CO) 2 ] 1, a band at 1825 cm -1 attributed to acetic anhydride and bands corresponding to As for this result, we were really surprised that, starting from the monocarbonyl rhodium acetyl complex 3, we could obtain after reaction with acetate ions the dicarbonyl complex 1 and this, without no CO pressure and that, under the same reaction conditions employed in the experiment shown in figure 14 (same temperature, same materials, same solvent and same concentrations), the reaction involving complex 3 appears to be faster than that involving complex 4. We run the same experiments several times, starting from 4 and 3, and always obtained the same results.

With regard to CO exchange, we run the same experiment in 5 mL of dichloromethane while increasing by 5 times both the concentration of complex 3 and of sodium acetate. We observed the rapid formation of complex 1 together with acetic anhydride but the solution becomes green whereas it was red-yellow for the other experiments, with precipitation of rhodium black. By bubbling CO through the solution for 20 min., the latter became yellow and the precipitation disappeared. We have no clear explanation for this phenomenon but we can suppose that an intermolecular CO exchange occurs, which provides a black precipitation of RhI 3 or of colloidal Rh(0).

With regard to the rapidity of this reaction, we performed the same experiment as that described in figure 14, starting from the dicarbonyl rhodium acetyl complex 4 and bubbling CO through the solution. The reaction was monitored by IR (Fig. 23). We can observe in the two first spectra (spectra in blue and black, respectively) that the reaction between complex 4 and acetate ions starts quickly but spectra after a reaction time of 10 min (blue one) and 30 min (black one) are almost the same meaning that the reaction stops rapidly. After 30 min, bubbling of CO was stopped. The last spectrum (red one) was recorded 10 min after CO bubbling was stopped. We can see that the reaction has occurred rapidly and almost totally without any CO bubbling, affording the active species 1.

That means that first, CO decreases the rate of the reaction that leads to complex 1 and second, that very likely a CO ligand must be removed from complex 4 to provide the reaction. Indeed, with CO bubbling through the reaction mixture, we have observed a nearly complete reaction after more than 3 hours.

As for these results, we cannot be sure that a nucleophilic attack of the acetate ions at the acetyl moiety in the rhodium acetyl complexes may occur since that does not explain why CO gas inhibits the reaction. As we believe that a CO ligand must be removed from complex 4 to afford the reaction, we could propose two reaction routes:

-Either a nucleophilic attack effectively occurs, but the hexacoordinated complex 4 is too crowded to provide this reaction, moreover, the presence of a second CO ligand increases the electron density on rhodium and thus reduces the electro-deficient character of the acetyl carbon atom. -Or, an acetate anion must coordinate the metal center in the rhodium acetyl complex, taking the place of a CO ligand before a classical reductive elimination may occur (Sc. 14).

Despite all our efforts, our attempts at separating or observing spectroscopically the acetate rhodium complex were not successful. Indeed, we were not capable of proving the acetate coordination hypothesis but, as far as all our results are concerned, we can confirm that acetate ions react rapidly with the mono-carbonyl rhodium acetyl complex [PPN] 2 [RhI 3 (COCH 3 )(CO)] 2 3 giving the starting complex 1 and acetic anhydride, but react more slowly with the di-carbonyl acetyl rhodium complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4.

In order to determine if reductive elimination can occur from complex 4, we dissolved in 10 mL of anhydrous dichoromethane 100 mg of complex 3 in a glass reactor. The reactor was sealed and flushed three times with CO. The pressure was then adjusted to 5 bar and the reactor was heated at 50°C and the solution stirred at 1000 rpm. After a reaction time of 2 hours, the stirring and the heating were stopped. After the solution was cooled down to room temperature, the reactor was depressurized and the solution analyzed by IR ( Fig 24). By observing the spectrum of the solution coming from the reactor, large amounts of unreacted [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4, characterized by the bands at 2084 cm -1 and 1703 cm -1 are detected. We also note the presence of the bands at 2058 cm -1 and 1986 cm -1 corresponding to complex 1, which results from reductive elimination from the complex 4. Another band at 1733 cm -1 is also observed. It could correspond to the acetyl band of complex 3, other bands being hidden by the bands of complex 4 and 1. Or since acetyl iodide bands are not observed, it could even correspond to an organic compound formed by condensation of acetyl iodide. This experiment shows that classical reductive elimination can occur from complex 4 even if no water or acetate ions are present in the medium, but at a very slow rate. Thus, we propose that under catalytic conditions, two reductive elimination pathways are possible:

-A slow "classical reductive elimination" from complex 4 at low water content in the absence of lithium iodide.

-Or a rapid elimination reaction by nucleophilic attack or coordination of acetate ions onto complex 3 at high water content or in presence of lithium iodide. 2 eq/Rh, 1 eq/Rh, 0,5 eq/Rh). The IR spectrophotometer was initialized before each reaction and the period between two recorded spectra was as short as possible (usually 2 or 3 min.).

Spectra were recorded in transmittance mode. We chose to follow the appearance of the band at 1987 cm -1 of complex [PPN][RhI 2 (CO) 2 ] 1 owing to the fact that it is far from the other bands. Thus, the reaction was monitored in transmittance mode by following the increase in intensity (calculated by the computer) of the aforementioned band. Then, we calculated the logarithm of each measured intensity, to transform the values into absorbance, directly related to the concentration of

[PPN][RhI 2 (CO) 2 ] 1 in the medium according to the Beer-Lambert law. An example of the recorded spectra is shown in figure 25.

If we measure the concentration of complex [PPN][RhI 2 (CO) 2 ] 1 formed very close to the initial time, we can make the approximation that the reactants concentration has not changed. Thus, we can write the following rate law regarding the reactants concentration at the initial time: Second, we can assume that for the same absorbance value for each experiment, the concentration of complex [PPN][RhI 2 (CO) 2 ] 1 formed is the same, so, if we look at the time (t) for the same absorbance value close to the initial time for each experiment, (1/t) can be approximated to be plot the neperien logarithm of (1/t) as function of the neperien logarithm of the initial concentration of acetate ions in the medium we obtain a straight line in which the slope indeed is the reaction order toward acetate ions (Fig. 27). In this case, we have calculated the pseudo-initial rate for the same absorbance values equal to 0,003.

-(d[[PPN][RhI 3 (COCH 3 )(CO) 2 ]/dt) = k.[PPN][RhI 3 (COCH 3 )(CO) 2 ] α .[CH 3 COO -] β or d[[PPN][RhI 2 (CO) 2 ]]/dt = k.[[PPN][RhI 3 (COCH 3 )(CO) 2 ] α . [CH
The coefficient slope of the straight line is close to 0,45. This value indicates that the reaction between complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4 and acetate ions does not consist of a single step, but may pass through a few slow steps, may be via the loss of a CO ligand. and by varying the sodium acetate concentration (5 eq/Rh, 2 eq/Rh, 1 eq/Rh, 0,5 eq/Rh). The procedure is identical to that described above. The curves representing the absorbance as function of time are plotted in figure 28.

By comparing figure 26 and figure 28 we can confirm that the reaction between acetate ions and complex 3 is more rapid than the same reaction with complex 4.

Then, we plotted the neperien logarithm of the pseudo-initial rate (calculated for the same absorbance equal to 0,015) as function of the neperien logarithm of the sodium acetate concentration and we obtained the straight line shown in figure 29.

eq

The slope of the straight line is found to be close to 1, meaning that we have a direct reaction between the acetyl complex [PPN] 2 [RhI 3 (COCH 3 )(CO)] 2 3 and the acetate ions but on the basis of this result this result, it is not possible to determine whether the nucleophilic acetate ions attack the acetyl in complex 3 or acetate ions slowly coordinate onto complex 3 and then a rapid elimination occurs.

We also tried to determine the reaction order toward water for the elimination from complex 4.

Hence we run the same reaction using the same procedure described above, but adding to the dichloromethane solution of 4; 5mL of a acetic acid/water mixture with water content of 12 wt%, 10 wt%, 8 wt% and 6 wt%. The plots of the calculated absorbance as function of time are shown in figure 30. This reaction seems to be rather fast for high water concentrations but becomes very slow for low water contents. By plotting the neperien logarithm of the pseudo-initial rate, calculated for an absorbance value equal to 0,004, as function of the neperien logarithm of water concentration, we obtained a straight line as shown in figure 31. The calculated slope is 2,8. This value is difficult to explain but we could imagine that further reactions may take place during the this elimination process and that water exerts a strong influence for this reaction.

We also tried to study the elimination reaction for higher water concentrations in the medium but we encountered some solubility problems by using water and dichloromethane. Then, we run these experiments starting from complex 3 and using the same acetic acid/water mixture but surprisingly, we observed only a very slow rate of formation of complex [PPN][RhI 2 (CO) 2 ] 1 even at 12 wt% water content and the data obtained cannot be use to determine the reaction order.

We could propose that a molecule of water may be coordinated onto the unsaturated complex 3 inhibit the elimination reaction and that the concentration of acetate ions in the medium, formed by dissociation of acetic acid, is not high enough to induce the elimination process.

More kinetic measurements should be made, first to determine the reaction order toward the concentration of acetyl rhodium complexes and second, to study the effect of the temperature on the elimination reaction. It is worth noting that we performed these reaction with the acetyl iridium complex

III-4 Conclusion

In this chapter, we tried to explain the role of lithium iodide and water on the rhodiumcatalyzed methanol carbonylation.

Firstly, batch experiments showed that the presence of lithium iodide in the catalytic medium was able to maintain high carbonylation activity even at low water content, whilst the carbonylation rate decreased rapidly below a water content about 6-8 wt% without lithium iodide in the medium.

Thus, we focused our attention on the role played by lithium iodide and water on the catalytic process.

Secondly, we performed batch experiments in a sapphire NMR tube in order to get information on the rate determining step in the catalytic process under different water and lithium iodide concentrations. We observed that:

At low water content without lithium iodide the rate determining step in the catalytic cycle is the reductive elimination reaction whereas, at higher water content, oxidative addition of methyl iodide becomes the rate determining step, meaning that high water concentration accelerate the reductive elimination process.

By introducing lithium iodide into the medium, we observed that at low water content the only complex present was the tetra-iodo carbonyl complex [PPN][RhI 4 (CO) 2 ] 11, but we didn't observe any acetyl species, meaning that, in the same way as water lithium iodide accelerates reductive elimination, the rate determining step becoming the formation of the active species 1 from complex 11.

Then, we tried to explain the accelerating effect of water and lithium iodide, and we found that acetate ions were able to dramatically increase the reductive elimination reaction rate. Hence, we proposed that acetates could be formed by reaction of lithium iodide with methyl acetate or acetic acid or by dissociation of acetic acid at high water content.

Thirdly, we determined order of the reductive elimination reaction toward acetate ions and water. As for acetate ions, we found a reaction order of 0.5 when starting from the dicarbonyl acetyl complex 4, meaning that the elimination process could include other slow reactions. But starting from the acetyl complex 3 we found a reaction order close to 1 indicating that the reaction between acetate ions and complex 3 involves no intermediates, or could include other rapid reaction(s). The reaction of acetyl complexes in a reaction mixture containing acetic acid and water seems to be more complicated. The reductive elimination process would pass through other routes maybe including the dissociation of acetic acid in water. More investigations ought to be made to explain this kind of reactivity.

Experimental section

Materials:

All reactions and manipulations of Ir and Rh complexes were performed under argon or dinitrogen atmosphere using Schlenk tube technique. Dichloromethane was distilled using CaH toluene-d 8 (Eurisotop), argon, dinitrogen, carbon monoxide, 13 C-labeled carbon monoxide (Air Liquide), hydriodic acid (Merck). Labeling experiments were performed by stirring the relevant dissolved complex under a 13 CO atmosphere for some hours.

Instrumentation:

IR spectra were recorded on a Perkin-Elmer 1710 Fourier Transform spectrophotometer using either a solution cell with CaF 2 windows. C NMR spectra were recorded on a Bruker AC250, on a AMX400 or on a DPX300 spectrometer. Mass spectra were recorded by negative FAB method on a NERMAG R10-10 mass spectrometer.

X-ray crystallographic data were collected on a Stoe IPDS diffractometer for complex 1 and on an Oxford-Diffraction Xcalibur diffractometer for complexes 4, 10 and 8.

Batch experiments:

High pressure batch carbonylation experiments were performed in a Hastelloy ® B2 100 mL autoclave (Top Industrie) equipped with a stirring axis. The reactor can be pressurised up to 90 bar from a gas reservoir and can be heated up to 200°C. A cons tant pressure can be maintained in the reactor during the batch experiment. We measured carbon monoxide consumption in the reservoir and then calculated the carbonylation rate at 20 wt% and 15 wt% of unreacted methyl acetate calculated from CO consumption and corrected by gas-chromatography analyses. The reactor is equipped with a feed-line to introduce liquids under pressure. All batch experiments were started with the RhI 3 salt in a mixture of acetic acid and water (10 : 1). The reactor was closed and flushed several times with carbon monoxide, and then pressurized at 5 bar, heated at 190°C, and the mixture stirred at 1000 rpm to preform the iridium active species. After 30 minutes, the rest of the catalytic mixture is added through the liquid feed-line overpressurized with carbon monoxide. Pressure was then adjusted to 30 bar. It can be maintained constant by supplying gas from the reservoir during the experiments. A glassware reactor (Top Industrie) was used for some experiments. Technical features and working conditions were the same as those used for the Hastelloy ® autoclave, except for the maximum operating pressure, which was 8 bar. With such a reactor it was possible to observe the reaction mixture during the experiments, and perform crystallizations afterwards.

High-pressure/high-temperature IR analyses:

Batch experiments were monitored in situ by high-pressure IR spectroscopy in an autoclave equipped with a Cylindrical Internal Reflectance (CIR) silicon rod. A "Cassegrain" lens system was used to focalize the IR beam onto the rod placed across a Hastelloy® B2 (Top Industrie) batch 100mL autoclave and to refocalize the IR beam onto the detector (Scheme 1). Spectra were recorded on a Perkin-Elmer GXII spectrophotometer. The reactor can be pressurized up to 50 bar with CO and heated up to 200°C. A solution can be added into the reactor under pressure using an additional tank. With this technique, it is possible to reproduce catalytic conditions and to observe the species present in the catalytic mixture by infrared analyses.

High-pressure NMR analyses:

Batch carbonylation experiments were monitored in situ by high-pressure NMR spectroscopy using sapphire tubes (Universiteit van Amsterdam) (10 mm e.d.), pressure rated to 100 bar, fitted with a titanium valve. Reaction mixtures can be heated up to 100°C. .

Figure 1 :

 1 Figure 1 : Use of acetic acid.

4

 4 Scheme 1 : Methanol carbonylation into acetic acid

2 .

 2 Nickel-catalyzed methanol carbonylation.

Scheme 4 : 8 Scheme 5 : 9 17 .Scheme 6 :

 4859176 Scheme 4 : Synthesis of the active species [Ni(CO) 4 ].

Chapter I 11 Scheme 7 : 12 I- 4

 117124 Scheme 7 : Proposed catalytic cycle for the palladium-catalyzed methanol carbonylation.

Scheme 8 :

 8 Scheme 8 : Iridium-catalyzed methanol carbonylation

15 I- 4 - 2

 1542 Scheme 9 : Oxidative addition mechanism

Figure 2 :

 2 Figure 2 : Variation of rates of oxidative addition of CH 3 I to [AsPh 4 ][RhI 2 (CO) 2 ] (5.10-3 mol. L -1

12 :Scheme 13 :

 1213 Scheme 12 : Different pathways for the oxidative reaction in the low-water AO process. L = I -or AcO -

14 :

 14 Scheme 14 : Catalytic cycle of the methanol carbonylation catalyzed by the neutral complex [RhI(PEt 3 ) 2 (CO)]

2 Scheme 16 :

 216 Scheme 16 : [RhI(CO)(dppms)]-catalyzed carbonylation cycle

19 :

 19 Scheme 19 : Example of trans-chelating ligands rhodium complex for methanol carbonylation

CH 3 75 .Scheme 20 :

 37520 Scheme 20 : Cis-migration reaction.

24 :

 24 Scheme 24 : Different pathways for the reductive elimination step.

44 .

 44 Scheme 27 : Proposed catalytic cycle for the rhodium catalyze WGSR.

Figure 3 : 39 Otherbeen performed 86 .

 33986 Figure 3 : Variation of the WGSR rate with water and HI concentration.

33 :

 33 Scheme 33 : Supported rhodium-catalyzed methanol carbonylation.

Scheme 35 :

 35 Scheme 35 : Simplified flow sheet for an industrial acetic acid plant

1 :

 1 Scheme 1 : Active and acetyl species involved in the iridium and rhodium catalytic cycles

5 ,

 5 and the trans-[IrI 3 (COMe)(CO) 2 ] -; by contrast only the trans-[RhI 3 (COMe)(CO) 2 ]

10 . 11 Scheme 2 :

 10112 Scheme 2 : Proposed catalytic cycle for methanol carbonylation to acetic acid catalyzed by rhodium.

Figure 2 : 13 (

 213 Figure 2 : Infrared spectra of [PPN][RhI 2 (CO) 2 ] 1 (top) and [PPN][RhI 2 13 (CO) 2 ] 1 (bottom).

Figure 4 :

 4 Figure 4 : Reaction of [RhI 2 (CO) 2 ]-1 with methyl iodide monitored by IR.

The 13 C

 13 NMR spectrum of labeled complex 3 (Fig 6) shows two doublets at δ = 182.03 ppm (d, 1 J C-Rh = 66.72 Hz) and δ = 182.37 ppm (d, 1 J C-Rh = 70.95 Hz) in the region of terminal CO ligands, and two others doublets at δ = 204.14 ppm (d, 1 J C-Rh = 18.85 Hz) and δ = 211.35 ppm (d, 1 J C-Rh = 19.82 Hz) corresponding to two acetyl ligands.

Figure 5 :

 5 Figure 5 : IR spectrum of [RhI 3 ( 13 COCH 3 )( 13 CO)] 2 2-3 in CD 2 Cl 2 , r.t.

Scheme 3 :

 3 Scheme 3 : Reversibility of the CO coordination on complex 3.

Figure 10 : 4 II- 2 :

 1042 Figure 10 : X-ray structure of [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4

[Figure 12 :

 12 Figure 12 : Infra-red spectra of [PPN][Ir(CH 3 )I 3 (CO) 2 ] and [PPN][Ir(CH 3 )I 3 ( 13 CO) 2 ] in dichloromethane.

Scheme 5 :

 5 Scheme 5 : Proposed mechanistic pathway for the formation of [PPN][Ir(CH 3 CO)I 3 (CO) 2 ] 8 from [PPN][Ir(CH 3 )I 3 (CO) 2 ] 6.

Figure 14 :

 14 Figure 14 : Infra-red spectrum in CH 2 Cl 2 of the product of the reaction of [PPN][Ir(CH 3 )I 3 (CO) 2 ] 6 in CH 2 Cl 2 /MeOH 20mL/10mL under 5 bar of CO.

  [PPN][IrI 3 (COMe)(CO) 2 ] cis-8 and trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-8 which possesser only one terminal CO band. The last spectrum, at high methanol concentration, shows mainly one terminal CO band at 2070 cm -1

Figure 16 : 1 H

 161 Figure 16 : 1 H NMR spectra of the CO migratory insertion reaction at different methanol concentration : a) CH 2 Cl 2 /MeOH = 25mL/5mL, b) CH 2 Cl 2 /MeOH = 20mL/10mL; c) CH 2 Cl 2 /MeOH = 5mL/25mL, CD2Cl2, r.t.

  ), we observe signals at δ = 151.58 ppm, δ = 160.72 ppm, δ = 170.25 ppm, δ = 196.35 ppm and at δ = 199.02 ppm, all not well defined. According to the literature 5 , we can attribute the signal at δ = 151.58 ppm to the carbon atoms of the terminal CO ligands and that at δ = 196.35 ppm to the carbon atom of the acetyl group in the complex cis-8. The signal at δ = 160.72 ppm could correspond to the carbon atoms of terminal CO ligands in complex trans-8 and signal at δ = 199.02 ppm to the carbon atom of the acetyl group; the signal at δ = 170.25 ppm corresponds to compound 5 resulting from the reductive elimination from the acetyl complexes. The presence of complex trans-8 might be due to a cis/trans isomerisation occurred overnight. In the case of the second sample (Fig. 18), we observe signals at δ = 160.75 ppm and at δ = 197.9, corresponding to the carbon atoms of complex trans-8, the signal at δ = 167.97 ppm attributed to complex 5, and a small signal at δ = 151.86 ppm, which corresponds to complex cis-8. We also observe two new signals at δ = 163.23 ppm and δ = 201.4 ppm which could be attributed to the hypothetical neutral acetyl dimer observed previously in 1 H NMR. In these two spectra (Fig. 17 and18), we also observe signals at δ = 50.04 ppm and at δ = 51.05 ppm in figure 17 corresponding to the carbon atoms of the methyl group of cis-8 and trans-8, respectively. In figure 18, these two later signals are still present and we observe a new signal at δ = 51.47 ppm which could correspond to the neutral dimer. We have not shown these signals for the clarity purporse.

[ 1 ,

 1 PPN][Ir(CH 3 )I 3 (CO) 2 ] 6 and a CH 2 Cl 2 /MeOH solvent mixture into the HP-IR reactor. To have a good signal/noise ratio, we have to introduce at least 0.700 g of compound 6 into the reactor for a maximum volume of 25 mL. To dissolve all of the solid, 10 mL of CH 2 Cl 2 are needed, since compound 6 is not soluble in methanol. In a first run, we chose to use a CH 2 Cl 2 /MeOH = 15 mL/10 mL solvent mixture, a pressure 5 bar of CO and a temperature of 40°C. The spectra obtained by this experiment are shown in figure 19. After 5 min (black spectrum) we can observe the growth of the two CO vibration bands at ν CO = 2110 cm -1 and 2063 cm -1 characteristic of the cis-[PPN][IrI 3 (COMe)(CO) 2 ] cis-8 complex and of the two bands at 2098 cm -1 and 2046 cm corresponding to [PPN][Ir(CH 3 )I 3 (CO) 2 ] 6. After 30 min, we can neathly observe the CO vibration bands of the two complexes 6 and cis-8. 50 min are needed to observe the CO bands of complex cis-8. After 2h30 no major change was noted, except for the loss of the signal which is due to the loss of the adjustement between the reactor and the IR beam. The reactor is then depressurized and the red solution placed in a Shlenck tube, the solvent being removed in vacuo. A red-orange solid was obtained. The IR spectrum of this solid (Fig. 20) shows the CO bands of compound cis-6 and two additional CO bands corresponding to [PPN][IrI 2 (CO) 2 ] 5 which

  After 10 min., we observe essentially a signal at δ = 152.06 ppm attributed, together with the signal at δ = 197.3 ppm, to the complex cis-[PPN][IrI 3 (COMe)(CO) 2 ] cis-8. We also note the presence of trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-8 characterized by the signal at δ = 160.87 ppm (the signal corresponding to the acetyl group may be hidden by the noise), of [PPN][Ir(CH 3 )I 3 (CO) 2 ] 6, characterized by the signal at δ = 156.23 ppm and three other signals at δ = 163.22 ppm , δ = 204.44 ppm, which could be assigned to the neutral dimeric acetyl species, and at δ = 165.30 ppm which was not identified. After 40 min., we observe in figure 22 we observe a change in the relative intensities of the signals corresponding to cis-[PPN][IrI 3 (COMe)(CO) 2 ] cis-8, δ = 151.97 ppm, δ = 197.54 ppm, and trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-8, δ = 161.00 ppm, δ = 200,38 ppm. We note the presence of the signals at δ = 156.21 ppm corresponding to unreacted complex 6, at δ = 162.95 ppm and δ = 203.54

Figure 23 : 13 C

 2313 Figure23:13 C HP-NMR spectra of the reaction of complex 6 in CD 2 Cl 2 /CD 3 OD = 1mL/2mL at 40°C under 5 bar of13 CO for 90 min. reaction.

5

  On our part, following the synthetic procedure using water and after many crystallization attempts, we obtained, from a hexane/dichloromethane bilayered mixture in a Schlenk tube at -18°C, crystals of compound trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-8. A crystal was selected and analyzed by X-ray crystallography. The octahedral structure of trans-8 is shown in figure24. The crystal lattice is monoclinic and the space group is P 2 1/C . By comparing the Ir-I bond length, we can note that the Ir -I(2) bond is longer than the two other Ir-I bonds, due to the trans-effect exerted by the acetyl ligand. Volpe et al23 have recently synthesized compound trans-[IrI 3 (COMe)(CO) 2 ]

  Bond length: Ir(1) -I(1) = 2.6849(8), Ir(1) -I(2) = 2.8147(8), Ir(1) -I(3) = 2.6984(9), Ir(1) -C(1) = 1.929(11), Ir(1) -C(2) = 1.915(13), Ir(1) -C(3) = 2.058(12), C(1) -O(1) = 1.116(13), C(2) -O(2) = 1.104(14), C(3) -C(4) = 1.487(18), C(3) -O(3) = 1.183(14). Angles: I(1) -Ir(1) -I(2) = 91.85(3), I(1) -Ir(1) -I(3) = 174.65(3), I(2) -Ir(1) -I(3) = 93.07(3), I(1) -Ir(1) -C(1) = 89.8(3), I(2) -Ir(1) -C(1) = 89.8(3), I(3) -Ir(1) -C(1) = 88.1(3), I(1) -Ir(1) -C(2) = 91.6(4), I(2) -Ir(1) -C(2) = 86.1(3), I(3) -Ir(1) -C(2) = 90.8(4), C(1) -Ir(1) -C(2) = 175.7(5), I(1) -Ir(1) -C(3) = 87.4(3), I(2) -Ir(1) -C(3) = 178.3(3), I(3) -Ir(1) -C(3) = 87.7(3), C(1) -Ir(1) -C(3) = 88.7(5), C(2) -Ir(1) -C(3) = 95.5(5), Ir(1) -C(1) -O(1) = 177.8(11), Ir(1) -C(2) -O(2) = 175.4(12) Ir(1) -C(3) -C(4) = 117.9(9), Ir(1) -C(3) -O(3)= 119.9(9), C(4) -C(3) -O(3) = 122.0(12).

Figure 24 :

 24 Figure 24 : X-ray crystal structure of trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-4 (counterion omitted for clarity purposes

from [IrI 3 ( 6 :

 36 scheme 6 24

Figure 25 :

 25 Figure 25 : Influence of water content on the turnover frequency.

  C NMR spectra show a signal at δ = 154.5 ppm with two satellites ( 1 J 13C-195Pt = 1843 Hz) due to the coupling with 195 Pt (I=1/2, abundance = 33.7%) and the 195 Pt NMR spectrum shows a signal at δ = -5328 ppm with two satellites ( 1 J 13C-195Pt = 1843 Hz). Then, 0.3 equivalent of complex [PtI 2 ( 13 CO)] 2 9 were added into a solution of [PPN][IrI 3 (CH 3 )( 13 CO) 2 ] 6 in dichloromethane. 195 Pt NMR spectra performed at -30° C (Fig. 26) show the disappearance of the signals corresponding to complex 9 and the appearance of two news signals. The one at δ = -5460 ppm ( 1 J 13C-195Pt = 1759 Hz), corresponds to [PPN][PtI 3 (CO)], which was previously fully characterized 8 and the other at δ = -5462 ppm ( 1 J 13C-195Pt = 1650 Hz), corresponds to a novel platinum species. Also the corresponding 1 H NMR spectra at -30° C on this reaction (Fig. 27) also show the presence of new proton signals. Those at δ = 1.9 ppm and 1.85 ppm correspond to the neutral dimer [IrI 2 (CH 3 )(CO) 2 ] 2 , which was previously fully characterized 16 , a weak one at δ = 2.13 ppm corresponds to unreacted compound 6, and two new signals at δ = 2.37 ppm and at δ = 2.28 ppm are observed. The FAB mass spectrum (in negative mode) on this solution (Fig 28) shows two peaks at m/z = 1121 and m/z = 1161 corresponding to two negatives species, and others fragmentation peaks that correspond to the loss of CO ligand. We could propose the following formula: [IrI 2 (CH 3 )(CO) 2 (µ-I)PtI 2 (CO)] -for m/z = 1121 and [IrI 2 (CH 3 )(CO) 2 (µ-I)IrI 2 (CH 3 )(CO) 2 ]

Figure 28 : 7 :Scheme 8 : 3

 28783 Figure 28 : Mass FAB spectrum on the product from the reaction between [PPN][IrI 3 (CH 3 )( 13 CO) 2 ] 6 and 0.3 eq of [PtI 2 (CO)] 2 .

We synthesized complex 10

 10 by reaction of RhI 3 in a methyl acetate/ methanol solvent mixture with CO to obtain first H[RhI 2 (CO) 2 ] 1. By evaporation of the solvent under CO bubbling, complex 1 dimerizes to form [Rh(µ-I)(CO) 2 ] 2 10. Details on the synthetic procedure are given in the Experimental Section. Crystals of [RhI(CO) 2 ] 2 were obtained and analyzed by X-ray diffraction (Fig. 29).

Figure 29 :

 29 Figure 29 : X-ray crystal structure of [RhI(CO) 2 ] 2 10

Scheme 9 :

 9 Scheme 9 : Proposed reaction between [RhI(CO) 2 ] 2 10 and [PPN][IrI 3 (CH 3 )(CO) 2 ] 6.

Figure 32 : 13 CFigure 33 :

 321333 Figure 32 : 13 C NMR spectrum of the products from the reaction between [RhI( 13 CO) 2 ] 2 10 and [PPN][IrI 3 (CH 3 )( 13 CO) 2 ] 6 Rh/Ir = 1/1 in CD 2 Cl 2 , r.t.

  Figure35:13 C NMR spectra of themixture coming from reaction between 2 and 6, Rh/Ir = 2, in CD 2 Cl 2 , r.t.

Scheme 10 :

 10 Scheme 10 : Proposed mechanistic pathway for the reaction between [RhI(CO) 2 ] 2 10 and [IrI 2 (CH 3 )(CO) 2 ] -6

Scheme 1 :

 1 Scheme 1 : Proposed effect of LiI or acetate ions on [RhI 2 (CO) 2 ] -1

Figure 1 :

 1 Figure 1 : Carbonylation rate at 20% methylacetate remaining

6

 6

1 , 13 C, 1 J

 1131 C HP-NMR experiments. Firstly, we have synthesized [PPN][RhI 4 (CO) 2 ] 11, which is an intermediate in the water gas shift reaction, in order to collect all IR and NMR data about the long-lived complexes that may be present in the catalytic reaction mixture. [PPN][RhI 2 (CO) 2 ] 1 is dissolved in neat dichloromethane and 10 equivalents of HI are added. After 2 h, we only observed by IR (Fig 3) the presence of one CO stretching band at 2090 cm assigned to [PPN][RhI 4 (CO) 2 ] 11. Complex 11 was labeled (Fig 3) and its NMR spectrum (Fig 4) shows a doublet at δ = 173.09 ppm (dRh-C = 48.10 Hz).

Scheme 2 :Figure 3 :

 23 Scheme 2 : Reaction of water with acetyliodide

Figure 6 : 13 C

 613 Figure 6 : 13 C HP-NMR spectrum of the carbonylation reaction mixture at 10% water.

Figure 7 :

 7 Figure7:13 C HP-NMR spectrum of the carbonylation reaction mixture at 5% water.

Figure 9 :

 9 Figure9:13 C HP-NMR spectrum of the carbonylation reaction mixture at 10% water with 17% of LiI.

  corresponding to the reaction mixture containing 20% of water, we note the presence of a doublet at δ = 182.90 ppm (d, 1 J C-Rh = 70.04 Hz). It's position and the relevant coupling constant are in good agreement with those observed for complex [PPN][RhI 2 (CO) 2 ]

1 J C -

 1C Rh = 54.63 Hz) and at δ = 216.90 ppm, not well defined. The observed chemical shifts are good agreement with those obtained for complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4. We observe also a signal at δ = 182.82 ppm (d, 1 J C- Rh = 69.51 Hz) assigned to complex 1, and another unidentified signal at 173 ppm.

  1 J C-Rh = 68.87 Hz) corresponding to [PPN][RhI 2 (CO) 2 ] 1, and at δ = 172.97 ppm (d, 1 J C-Rh = 48.77 Hz) corresponding to [PPN][RhI 4 (CO) 2 ] 11. Thus, addition of lithium iodide in the reaction mixture does affect the equilibrium between [PPN][RhI 2 (CO) 2 ] 1 and [PPN][RhI 4 (CO) 2 ] 11 without changing the carbonylation rate (Fig 1 and 2). When we decrease the water content to 10% (Fig 7), we only observe a doublet at δ = 173.17 ppm (d, 1 J C-Rh = 48.35 Hz) assigned to complex [PPN][RhI 4 (CO) 2 ] 11. It is worth noting that no signal corresponding to the acetyl complex 4 is observed.

[

  PPN][RhI 4 (CO) 2 ] 11 and [PPN][RhI 2 (CO) 2 ] 1 at high water content, but only complex 11 at low water content. This could mean, that high lithium iodide concentrations in the medium might favor the formation of HI by reaction with acetic acid or might accelerate the reductive elimination reaction that also produces HI thus favoring the formation of complex 11, and that water might cause formation of [PPN][RhI 2 (CO) 2 ] 1 from [PPN][RhI 4 (CO) 2 ] 11. We think that under the conditions used for HP-NMR

Scheme 4 :III- 2

 42 Scheme 4 : Proposed catalytic cycle depending on water and lithium iodide concentration.

10 ).

 10 Scheme 5 : Classical reductive elimination reaction

Figure 10 :

 10 Figure 10 : IR-monitored elimination reaction in the presence of water.

Figure 11 :

 11 Figure 11 : IR monitored elimination reaction using methanol.

Figure 12 :Figure 13 :Kilner

 1213 Figure 12 : IR spectrum of acetyliodide.

Figure 14 :

 14 Figure 14 : IR monitored spectra of the mixture from reaction between complex 4 and sodium acetate.

Figure 15 :

 15 Figure 15 : IR spectrum of the reaction between complex 4 and sodium acetate ions in methanol/CH 2 Cl 2 mixture

Scheme 8 :

 8 Scheme 8 : Proposed mechanistic pathway for the reaction between complex 4 and the acetate anions.

10 :

 10 Scheme 10 : Example of non-ionic nucleophilic molecules tested in the reaction with complex 4.

11 :

 11 Scheme 11 : Example of strong ionic nucleophilic/basic species tested in the reaction with complex 4

Figure 22 :

 22 Figure 22 : IR spectra of the reaction between sodium acetate and complex 3 in dichloromethane.

Scheme 14 :

 14 Scheme 14 : Proposed mechanistic pathway for the reaction of acetate with rhodium acetyl complex.

Figure 24 :

 24 Figure 24 : IR spectrum of the reaction of complex 3 at 50°C under 5 bar of CO for 2 h.

Figure 25 :

 25 Figure 25 : IR spectra of the elimination reaction with 1 equivalent of sodium acetate.

Figure 26 :

 26 Figure 26 : Calculated absorbance of the band at 1987 cm -1 as function of time for the reaction between complex 4 and sodium acetate ions.

Figure 27 :

 27 Figure 27 : Plot of ln(1/t) as function of ln([CH 3 COO -]).

Figure 28 :

 28 Figure 28 : Calculated absorbance of the band at 1987 cm -1 as function of time for the reaction between complex 3 and sodium acetate ions

Figure 29 :

 29 Figure 29 : Plot of ln(1/t) as function of ln([CH 3 COO -]).

Figure 30 :

 30 Figure 30 : Calculated absorbance of the band at 1987 cm -1 as function of time for the reaction between complex 4 and a solution of acetic acid/water.

  cis-[PPN][IrI3 (COMe)(CO) 2 ] cis-8 and with trans-[PPN][IrI 3 (COMe)(CO) 2 ] trans-8 but no elimination reaction was observed.

Figure 29 :

 29 Figure 29 : Plot of ln(1/t) as function of ln([H 2 O]).

  2 . Methanol was distilled after reaction with Mg/I 2 and used immediatly. Other reagents were used as supplied: the salt IrI 3 /IrI 4 , named IrI 3,4 (Johnson-Matthey), RhI 3 salt from the Celanese plant at Pardies, bistriphenylphosphoranylidene chloride [PPN]Cl, triphenylphosphine (Aldrich), acetyl chloride, methyl iodide, (Acros), acetone, acetic acid, chlorobenzene, chloroform, dichloromethane, dimethylformamide, n-hexane, n-heptane, methanol, pentane, toluene (Scharlau), chloroform-d 3 , dichloromethane-d 2 , methanol-d 4 ,

13 CO) 2 ] 2 13 CO) 2 ] 7 1 . 13 C

 13221327113 Scheme 1 : High-pressure infrared cell

Fac 8 . 1 H

 81 The salt[PPN][IrI 3 (CH 3 )(CO) 2 ] 6 (0.5 g, 0.42 mmol) was dissolved in 40 mL of dichloromethane containing 5 mL of methanol. The solution was placed in a glass reactor and stirred for 90 minutes under 5 bar of CO at room temperature (Sc. 6). Then 20 mL of n-hexane were added by a liquid feed-line and the reactor was cooled to 0°C. After 2 hours we observe d the formation of yellow crystals (380 mg, 75% yield), corresponding to fac,cis-[PPN][Ir(COCH 3 )I 3 (CO) 2 ]. IR (CH 2 Cl 2 ) ν(CO)/cm -1 : 2111, 2061, 1679, 1660. NMR (CD 2 Cl 2 ): δ 3.13 (s, CH 3 ).

13 C { 1 H}

 131 Scheme 6 : Preparation of fac,cis-[PPN][Ir(COCH 3 )I 3 (CO) 2 ] 8.

Table 1 :

 1 Thermodynamic data for the methyl cis-migration reactionThis accelerating effect was explained by the ability of methanol to remove an iodide ligand from complex [Ir(CH 3 )I 3 (CO) 2 ]-via H-bonds and then, facilitate the CO insertion affording the neutral complex

	[Ir(CH 3 )I 3 (CO) 2 ] -+ CO		[Ir(COCH 3 )I 3 (CO) 2 ] -
	Solvent	∆H # /kJ mol -1	∆S # /kJ mol -1
	Ph-Cl	155	91
	Ph-Cl/MeOH	33	-197

3/1) solvent mixture (table. 1)

  from rhodium salt RhI 3 , RhCl 3 or Rh(OAc) 3 in an acetic acid/water mixture with a small quantity of HI under CO pressure. The different steps of this catalytic cycle are the same than those of the anionic iridium catalytic cycle. The first reaction is the oxidative addition of methyl iodide on [RhI 2 (CO) 2 ]

	The active species which primes the catalytic cycle is the 16e	-complex [RhI 2 (CO) 2 ]
		2 ] -
				Slow
	MeCOI	H 2 O	MeI
	[(MeCO)RhI 3 CO 2 ] -			[MeRhI 3 (CO) 2 ] -
	CH 3 COOH + HI
				MeOH
	Fast			Fast
	[(MeCO)RhI 3 (CO)] 2	2-
	CO

-

. This complex can be generated

Table 2 :

 2

Activation parameters for the oxidative addition of CH 3 I to [Ph 4 As][RhI 2 (CO) 2 ] in different solvents.

Table 3

 3 Entries 1 and 5 in table 3 indicate that the higher carbonylation rates are obtained for a ligand/Rh molar ratio of 1. High pressure infrared spectroscopy studies identified [RhI(CO)(dppms)] as the only

	: Carbonylation rates using different bidentate ligands for the
	rhodium-catalyzed methanol carbonylation

27

  On the basis of the same kind of ligands,Dilworth et al. have developed P-S molecules giving a rate enhancement in the rhodium-catalyzed methanol carbonylation (Sc. 17) Süss-Fink et al. have recently developed a class of trans-chelating ligands containing phosphorus or nitrogen atom (Sc. 19) 67,68,69 . These catalysts show higher activities than the [RhI 2 (CO) 2 ]

	Chapter I							28
						65 . They observed a maximum
	carbonylation rate four times higher than when starting from [RhI 2 (CO) 2 ]
	Ph 2 P	S	CO	Ph 2 P	S
		Rh	Rh		Rh	Rh
	OC	S	PPh 2	OC	S	PPh 2
	Ph 2 P	PAr 2	[RhCl(CO) 2 ] 2	2	Ph 2 HP OC	Rh	PHAr 2 Cl	+ 2 CO
	Ar = 4-C 6 H 4 OCH 3 or 3,5-C 6 H 3 F 2 or 3,4,5-C 6 H 2 F 3
	Scheme 18 : Unsymmetrical rhodium catalysts for methanol carbonylation reaction

-

. The catalytic cycle appears to be the same than the one displayed in scheme 16, the methyl intermediate hasn't being observed. Some unsymmetrical ethylene diphosphines (Sc. 18) were reported as more efficient ligands than symmetrical diphosphine for rhodium catalyzed methanol carbonylation and are longer-lived under industrial conditions. In situ infrared spectroscopy only shows the diphosphine rhodium complex without traces of [RhI 2 (CO) 2 ] -. But all these diphosphine-rhodium catalysts are less active than the [RhI 2 (CO) 2 ] system

66

. CO Scheme 17 : P-S-rhodium catalyst used for methanol carbonylation -system. Some other trans-chelating diphosphine ligands were reported very recently by van Leeuwen et al.

70

. As the latter trans-chelate displayed in scheme 19, these ligands show higher activities than the [RhI 2 (CO) 2 ]

Supported rhodium-catalyzed methanol carbonylation.

	Chapter I			40
				2-affording complex [RhI 2 (CO) 2 ]	-
	and at lower temperature, the determining step becomes the oxidative addition of 2 moles of HI on the
	Rh(I) complex.
	Moreover, it has been shown that the WGSR is also dependent on the dihydrogene partial
	pressure. In situ infrared investigations indicated that complex [RhI 4 (CO) 2 ]	-can be converted into complex
	[RhI 2 (CO) 2 ]	-by addition of dihydrogen in the catalytic medium meaning that the addition of 2 moles of HI
	on complex [RhI 2 (CO) 2 ]	-and the resulting liberation of dihydrogen can be reversible (Sc. 32)	87 .
			2 HI	H 2
			[RhI 2 (CO) 2 ] -	[RhI 4 (CO) 2 ] -
			2 HI	H 2
		Scheme 32 : reversible reaction between [RhI 2 (CO) 2 ]	-and [RhI 4 (CO) 2 ]	-.

Table 4 :

 4 Product concentrations in the reactor for the different processes.

		catalyst (ppm) water wt%	methyl acetate wt%	methyl iodide wt%	temperature °C	CO partial pressure (bar)
	Monsanto	600	14	2	10	190	10
	Cativa	1200 (Ir) 800 (Ru)	6	10	10	190	10
	Celanese	700 (Rh) 10 wt% (LiI)	4	2	10	190	10

  RhI 3 (COCH 3 )(CO) 2 ] 4. The single band corresponding to the terminal CO ligands

		complex 3 and the appearance of two new ν CO stretching bands at 2083 cm	-1 and 1702 cm	-1 assigned
		to complex [PPN][
		t = 0 min
				2058,13
							1985,79
	t = 30 min.
											1717,80
						2059,43	1985,14
	%T									1737,82
	t = 120 min.
				2063,30					1734,59		1717,15
	CO bubbling
				2082,68								1702,30
	2500,0	2400	2300	2200	2100	2000	1950 cm-1	1900	1850	1800	1750	1700	1650,0

  Hz), attributed to the carbon atoms of the terminal CO ligands of complex 4, and at δ = 215.93 ppm (d, 1 J C-Rh = 18.17 Hz), attributed to the carbon atom of the acetyl group in complex 4. We can note that a small quantity of complex 3, signals at δ = 182.05 ppm and δ = 182.44 ppm is still present. Since the

		99,1			The	13 C NMR spectrum in figure 9 shows two doublets at δ = 177.81 ppm (d,	1 J C-Rh = 54.06
		95	211.507	211.192				204.292	203.993			3.151	3.126	182.936	182.559	181.808	181.498	6000
		90
																	5000
		85 methyl group of the acetyl ligand was not labeled (in 3 and 4), we didn't observe	13 C signals due to
	%T	80 methyl carbon during	[PPN] 2 [RhI 3 ( C NMR experiments. 13 COCH 3 )( 13	13 CO)] 2	1697,49	4000
		75															66.716
					70.951 After getting [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4 in dichloromethane, we left the solution under	3000
		70												2015,10
																	2000
		19.825 65				18.850				1674,91
		61,4 2500,0	2400			2200		2000	1900 cm-1	1800	1700	1600	1550,0	1000
						Figure 8 : Infrared spectrum of [PPN][RhI 3 (	13 COCH 3 )(	0
		ppm (f1)	3.40 216.079	215.790	3.30 211.560 211.263					3.20			3.10	3.00	2.90	183.002	182.585	181.874	181.511	2.80 178.243	177.383	2.70	-1000
				210.0			205.0					200.0			195.0	190.0	185.0	180.0	175.0
		ppm (f1)					Figure 7 :	1 H NMR of [PPN] 2 [RhI 3 (COCH 3 )(CO)] 2 3 in CD2Cl2, r.t.
													[PPN][RhI 3 (	13 COCH 3 )(	13 CO) 2 ] 4	54.062
								Figure 6 :	13 C NMR of [PPN] 2 [RhI 3 (	13 COCH 3 )(
														[PPN] 2 [RhI 3 (	13 COCH 3 )(	13 CO)] 2 3
		which means that under reduced pressure complex 4 loses a CO ligand giving back complex 3. Thus, 18.171
		we dissolved [PPN] 2 [RhI 3 (	13 COCH 3 )(	13 CO)] 2 3 in CD 2 Cl 2 and stirred it under one atmosphere of	13 CO.
		After 10 min., the infrared spectrum (Fig 8) shows ν CO stretching bands at 2015 cm	-1 and 1675 cm	-1
		corresponding to [PPN][RhI 3 (	13 COCH 3 )(	13 CO) 2 ] 4. We could attribute the band at 1697 cm	-1 , some
		unaltered [RhI 3 (	13 COCH 3 )(	13 CO)] -3 still present in the solution.
									210								200	190	180
		ppm (f1)
							Figure 9 :

13

CO)] 2 3 in CD2Cl2, r.t.

Then, we tried to isolate the complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4. After CO bubbling through the solution of complex 3, we removed dichloromethane under reduced pressure, and obtained a red powder. When analyzing this powder by IR, we observed the very bands corresponding to complex 3, 13 CO) 2 ] 4 in CH 2 Cl 2 , r.t.

13

C NMR spectrum of [PPN][RhI 3 ( 13 COCH 3 )( 13 CO) 2 ] 4 in CD 2 Cl 2 , r.t.

  The IR spectrum of complex [PPN][IrI 2 (CO) 2 ] 5 in dichloromethane shows two stretching bands at ν co = 2046 cm -1 for the symmetric vibration and ν co = 1968 cm -1 for the antisymmetric vibration(Fig 11). These two ν co bands are characteristic of a cis-isomer, in a with its X-ray structure. Figure 11 : IR spectrum of [PPN][IrI 2 (CO) 2 ] 5 in dichloromethane. In order to synthesize compound 6, [PPN][IrI 2 (CO) 2 ] 5 is dissolved in CH 3 I. After 10 min. under ambiant conditions, the solution turns to orange. Two IR stretching bands are observed at ν sym = 2098 cm -1 and ν asym = 2046 cm -1 (Fig 12). The shift to higher frequencies of the CO bands corresponding to compound 6 with respect to those corresponding to [PPN][IrI 2 (CO) 2 ] 5 is due to the reduced electrondensity on the metal center. Indeed, the lower electron density on the metal center decreases the back-bonding effect on the anti bonding π* orbitals of the carbon CO ligand and thus stabilizes the C=O bond. The presence of these two bands is characteristic of a cis-complex, which was confirmed by a X-ray crystal structure16 .

			The iridium catalytic cycle was first described by Forster in 1979 15 and is depicted in scheme
		4. In order to start this cycle, one must form the active species [IrI 2 (CO) 2 ] -. A synthetic procedure was
	105,0	previously described by our laboratory staff	16 , starting from the commercial salt IrI 3,4 which is a mixture
		of IrI 3 and IrI 4 . The procedure to obtain [PPN][IrI 2 (CO) 2 ] 5 is the same that we used for the preparation
		of [PPN][RhI 2 (CO) 2 ] 1 (see Experimental Section). A single crystal of this compound was obtained and
		its x-ray structure solved 16 .
							IrI 3,4
	%T
							[IrI 2 (CO) 2 ] -
					2098,18		5
							2045,25
	%T				CH 3 COI			Fast CH 3 I
				[(CH 3 CO)IrI 3 (CO) 2 ] -8 AcOH 2046,82	H 2 O HI		1968,39 CH 3 OH	[CH 3 IrI 3 (CO) 2 ] -6
	42,6
	2500,0	2400	2300	2200	2100	2000	c m-1	1950	1900	1850	Slow	1800	1750	1700,0
						[CH 3 IrI 2 (CO) 3 ]	1998,38			I -
				I -		2048,49	7
												CO
	2500,0	2400	2300	2200	2100	2000		1950	1900	1850	1800	1750,0
							cm-1
			The next step in the iridium catalytic cycle is the oxidative addition of methyl iodide, which is
		Scheme 4 : Iridium catalytic cycle for methanol carbonylation produced in the catalytic medium by reaction of HI with methanol, to the active species [IrI 2 (CO) 2 ] -5
		giving the Ir(III) complex [Ir(CH 3 )I 3 (CO) 2 ] -6. This reaction is fast 17 , and appears to be a S N 2 type
		oxidative addition which proceed via a nucleophilic attack of the metal center at the carbon atom of
		CH 3 I 18,19

. 6 is known to be the resting state in the catalytic cycle 17 .

  C NMR of [PPN][Ir(CH 3 )I 3 ( 13 CO) 2 ] 6 in CD 2 Cl 2 , r.t.

		156.147
				[Ir(CH 3 )I 3 (	13 CO) 2 ][PPN] 6
	170	160	150	140	130	120
	ppm (t1)

T rans 8 PPN T rans 8 PPN T rans 8 PPN C is 8 PPN C is 8 PPN Cis 8 PPN

	(a) (a)	3.597 3.60 3.597 3.60	3.120 3.12 3.120 3.12	2.767 2.77 2.767 2.77	2.175 2.18 2.175 2.18
			C is 8 PPN cis-8
				T rans 8 PPN trans-8
	4 4 4		3 3 3		2 2 2
	(b) (b)	3.602 3.60 3.602 3.60	3.125 3.12 3.125 3.12	2.772 2.77 2.772 2.77	2.180 2.18 2.180 2.18
			C is 8 PPN cis-8	T rans 8 PPN trans-8
	4 4 4		3 3 3		2 2 2
	(c) (c)	3.609 3.61 3.609 3.61	3.132 3.13 3.132 3.13	2.780 2.78 2.780 2.78
				T rans 8 PPN trans-8
			Cis 8 PPN cis-8
	4 4 4		3 3 3		2 2 2

  C NMR spectrum in CD 2 Cl 2 , r.t. of the products coming from reaction of 2 in CH 2 Cl 2 /MeOH : 25mL/5mL under 5 bar of CO.

		199.017	196.352		170.250		160.717	151.579
								300
								200
								100
								cis-8
								0
					5		trans-8
		trans-8	cis-8
								-100
								-200
	210	200		190	180	170		160	150
	ppm (f1)
	201.396	197.893				167.968	163.228	160.752	151.865
								500 0
							trans-8
	Und.	trans-8		5	Und.	cis-8
								0
	210	200		190	180	170		160	150
	ppm (f1)
	Figure 18 :

13

C NMR spectrum of the products coming from reaction of 2 in CH 2 Cl 2 /MeOH : 10ml/20ml under 5 bar of CO.

  Additionally, we monitored by HP-IR another experiment using a CH 2 Cl 2 /MeOH = 10 mL/15 mL solvent mixture, but we observed the same series of spectra than those shown in figure19with a signal loss due to the weak solubility of complexes in this medium. Hence, we chose to follow this type of experiments by High Pressure NMR (HP-NMR), using a sapphire tube (see Experimental Section). Figure 20. IR spectrum of the product remaining after HP-IR experiment.

		98,5				204.437		197.307							165.304	163.224	160.867	156.235	152.061
																250
		95
		90														200
		85
									1967,64						150
		80
																100
		75
	EGY	70														50
		65														1656,85
																0
		60
		55								2048,33						-50
		50	ppm (f1)	210		2110,27 200	2061,93 190	180		170			160	150
		45	t = 0 min
		42,9 2500,0	2400		2300	2098,18 2200	2100	2000	2046,82 1950	1900 cm-1	1850	1800	1750		1700	1650	1600	1550,0
			t = 5 min 205.583	203.536	200.386	197.541					172.328	165.019	162.955	160.779	159.727	158.477	156.215	151.974	150.292
			t = 30 min		2110,27		2045,31
									2064,95
	%T							2099,69
			t = 50 min
			t = 90 min
		We introduced 50 mg of [PPN][Ir(CH 3 )I 3 (CO) 2 ] 6, 1 mL of CD 2 Cl 2 and 2 mL of CD 3 OD into the tube
		which is then filled with 5 bar of	13 CO and placed in the NMR probe at 40°C. We collecte d spectra
		2110,27 every 10 min. for a total time of 90 min. The most representative spectra are shown in figure 21, 22 2063,44 t = 150 min
		2500,0 and 23. 2400		2300		2200	2100	2000	1950	1900 cm-1	1850	1800	1750	1700	1650	1600	1550,0
						200		190	180		170			160	150
			ppm (f1)

Table 1 :

 1 Carbonylation rate by varying the Pt/Ir ratio

		Pt/Ir (mol/mol)	Carb. Rate * at 20% [AcOMe] (mol. L -1 . h -1 )
	1	100/0 %	0
	2	0/100 %	16
	3	19/81%	31
	4	33/67 %	35
	5	47/53 %	33
	6	67/33 %	24

* Carbonylation rate expressed in mol of AcOH formed per liter of solution per hour.

TOF (h -1 ) [Ir] [Ir/Ru] [Ir/Pt]

  accelerate the rate determining step. Hence, we tried to see if a synergistic effect could exist between iridium and rhodium. After Schenlk tube experiments on the relevant reaction and on the different intermediates in the two catalytic cycles, we have not observed any reaction. As [PtI 2 (CO)] 2 can react with [IrI 3 (CH 3 )(CO) 2 ]

-by abstracting an iodide ligand, we tried to see if the iododeficient complex [Rh(µ-I)(CO) 2 ] 2 10 is able to do the same.

  accelerating effect is apparently due to the intermediate formation of a heterobimetallic species, not yet isolated, in which the electron density on the iridium metal center is reduced thus facilitating the cis-migration reaction. But batch experiments using the [Ir/Rh] system do not show any significant acceleration of the carbonylation rate, and we cannot know if the dimer [RhI(CO) 2 ] 2 10 forms under the

	CHAPTER III : MECHANISTIC FEATURES OF THE LOW
	WATER CONTENT PROCESS USING RHODIUM CATALYST.
	catalytic conditions used. Further mechanistic experiments (H-P NMR) and continuous experiments STUDY OF THE REDUCTIVE ELIMINATION PROCESS.
	are needed to know whether this system is based on bimetallic catalysis or the two metals do catalyze
	the reaction independently.
		Methanol carbonylation to acetic acid using a rhodium catalyst was patented by Monsanto in
	1973	1

  C NMR after 1 min. and 5 min of reaction, respectively (Fig 16 and 17). In figure 16, we can note after 1 min. a doublet at 177.73 ppm (d, 1 J C-Rh = 54.27 Hz) together with a doublet at 215.69 ppm corresponding to the dicarbonyl acetyl rhodium complex 4. We can also

	101,2
	100
	99
	98
	97								1826,93
	96
											1737,17
	95
											1711,34
	94
	%T
	93
	92
	91
	90
	89
	88						1991,60
	87
					2063,30
	85,9
	2500,0	2400	2300	2200	2100	2000	1950	1900	1850	1800	1750	1700	1650,0
							cm-1

  Summary of the differents processes depending on water content and the presence of LiI.

								5 0
			2.219	2.207	2.193
								0 0
		O						5 0
		C	CH 3
		O						0 0
		C *	CH 3	1
		O						5 0
						Acetic anhydride
	Coupling of the protons					0 0
	with the labelled carbon
	atom		6 .7 1 6
								0
		In summary, in one hand, at high water concentration (Monsanto process) acetate
	ions are formed in the medium by acetic acid dissociation. In another hand, at low water
	concentration (Celanese process), acetate ions are formed by the reaction of lithium iodide	-0
	2 . 5 0	2 . 4 0	2 . 3 0	2 . 2 0	2 . 1 0	2 . 0 0	1 . 9 0
	p p m ( f 1 ) with acetic acid or methyl acetate. If no water is introduced in the medium, we should obtain
	only the formation of acetic anhydride. This corresponds to the process used today by
	Figure 18 : Eastman compagny 11 . These conclusions are summarized in scheme 9. 1 H NMR of the reaction between complex 4 and sodium acetate salt at
			a reaction time of 5 min.
	Water content Water content	Process name Process name	Acetate ions formation Acetate ions formation	Product obtained Product obtained
	14 % 14 %	All of these observations permit the confirmation that acetate ions react with the Monsanto Acetic acid dissociation by water Acetic acid Monsanto Acetic acid dissociation by water Acetic acid
	acetyl rhodium complex [PPN][RhI 3 (COCH 3 )(CO) 2 ] 4 affording the active complex
	[PPN][RhI 2 (CO) 2 ] 1 and acetic anhydride, which in the reaction medium, reacts with water to 4 % Celanese. Presence of LiI Reaction of LiI with acetic acid or methyl acetate Acetic acid 4 % Celanese. Presence of LiI Reaction of LiI with acetic acid or methyl acetate Acetic acid
	form acetic acid. We can reasonably believe that this reaction consist in a nucleophilic attack
	of the oxygen atom of the acetate ion at the electrophilic acetyl carbon atom in complex 4 0 % Eastman. Presence of LiI Reaction of LiI with acetic acid or methyl acetate Acetic anhydride 0 % Eastman. Presence of LiI Reaction of LiI with acetic acid or methyl acetate Acetic anhydride
	(Sc. 8). Under these conditions, the reductive elimination reaction occurs without formation of
	acetyl iodide. Scheme 9 :

  NMR spectrum of this reaction mixture is shown in figure21. We observe several proton signals. The triplet at 0.98 ppm corresponds to the terminal CH 3 of the butyrate chain, a sextet at 1.68 ppm corresponds to the β-CH 2 of the butyrate group and a triplet at 2.44 ppm is related to the α-CH 2 group. For every each signal, we can observe a further smaller signal, respectively a triplet, a sextet and a triplet, meaning that another anhydride molecule containing a butyrate part is present. We

			184.557	183.437		166.903	166.717
			70.406
	200	190		180	170	160
	ppm (f1)
	* 1 H observe also two doublets at 2.22 ppm (d, The	* 2 J H-C = 7.35 Hz) and 2.20 ppm (d,	* 2 J H-C = 7.30 Hz),
	attributed to two CH 3 signals of the acetate in two different anhydride molecules. We note that each
	O coupling constant is different from those observed in figure 18 for acetic anhydride, whereas the
				*
					C	or 1
				*		O -

chemical shifts are in good agreement. We have no explanation for this phenomenon at this time, but these results obviously confirm that two different anhydride molecules are present. We should stress the presence of methyl iodide (2.17 ppm) in the medium.

  On the basis of the observations in sections III-1 and III-2, the relevant catalytic cyclewas modified and propose a new one is proposed(Sc. 15). order to get a deeper insight into the effect of acetate ions on the elimination process, we performed a series of reactions with the dicarbonyl acetyl rhodium complex[PPN][RhI 3 (COCH 3 )(CO) 2 ]4 at different concentrations of acetate ions and monitored them by infrared spectroscopy.For each reaction, we introduced in a dry Schlenk tube containing stirring bar complex[PPN] 2 [RhI 3 (COCH 3 )(CO)] 2 3 (100 mg, 4.57.10 -2 mmol) dissolved in 5 mL of dry dichloromethane. The Schlenk tube was placed in a water bath at 25°C. CO was bubbled through the solution for five seconds and 5 mL of a solution of sodium acetate in acetic acid were added with a glass syringe and a chronometer was started. The same reaction (same temperature, same Schlenk tube, same stirring magnet and same agitation) was run three times at different sodium acetate concentrations (5 eq/Rh,

	III-3 Determination of reaction order.
								Resting state at low water content with LiI
								The increase of water concentration facilitates its reduction to complex 1
					H 2		[RhI 4 (CO) 2 ] -11		CO, H 2 O
							WGSR
					2HI				2 HI, CO 2
						[RhI 2 (CO) 2 ] -
	CH 3 C	O				1		at high water content Rate determinig step
	CH 3 C	O O	+ LiI or H 2 O + HI	Rate determinig step at low water content without lithium iodide	MeCOI	H 2 O	MeI
			Fast at high water content or with LiI	[(MeCO)RhI 3 (CO) 2 ] -4	CH 3 COOH + HI		[MeRhI 3 (CO) 2 ] -MeOH 2
									Fast
	CH 3 COO -Li + , CO or CH 3 COO -H 3 O +	CO	[(MeCO)RhI 3 (CO)] -3
		O
	CH 3 C	O	+ Li	H 2 O	2 CH 3 COOH + LiI		CH 3 COO -Li + + HI	MeOH	MeI + H 2 O
	CH 3 C	O				CH 3 COOH			CH 3 COO -Li +
		Scheme 15 : Proposed rhodium catalytic cycle for methanol carbonylation to acetic acid

In

PPN][RhI 3 (COMe)(CO) 2 ] 4:

  NMR (CD 2 Cl 2 ): 3.15 ppm (s, CH 3 ); 3.13 (s, CH 3 ). Calcd for C 78 H 66 Rh 2 I 6 N 2 O 4 P 4 : C: 42.81, H: 3.02, N: 1.28. Found: C: 42.62, H: 2.92, N: 1.33.[PPN] 2 [RhI 3 (COMe)(CO)] 2 3 (1g, 0.915 mmol) is dissolved in CH 2 Cl 2 and CO is bubbled through the solution at room temperature (Sc. 12). The reaction is fast but under nitrogen atmosphere, [PPN][RhI 3 (COMe)(CO) 2 ] 4 slowly gives back [PPN] 2 [RhI 3 (COMe)(CO)] 2 3. [PPN][RhI 3 (COMe)(CO) 2 ] 4 can be isolated as crystals suitable for X-ray analyses by adding hexane in the Schlenk under a CO atmosphere, making a bilayered mixture. (0.70g, 65 %) IR (CH 2 Cl 2 ): νCO= 2083.5, 1704.2 cm Cl 2 ): 216.85 ppm, COCH 3 , (d, J Rh-C = 18.23 Hz); 177.7 ppm, CO, (d, J Rh-C = 54 Hz). Scheme 11 : Preparation of [PPN] 2 [RhI 3 (COMe)(CO)] 2 3

	[PPN][RhI 2 (CO) 2 ]	CH 3 I r. t	[PPN][Rh(CH 3 CO)I 3 (CO)]
	[-1 .	13 C NMR
	(CD 2 1 H NMR 2.97 ppm
	(s, CH 3 ).

1 H

[PPN][RhI 4 (CO) 2 ] 11:

  [PPN][RhI 2 (CO) 2 ] 1 (1g, 1.05 mmol) is dissolved in CH 2 Cl 2 and an excess (10 equivalents, 2.36g of a HI solution in water at 57%) of HI is added. The solution is stirred for 2h at room temperature and the solvent and the excess of HI are removed under vacuum to obtain a red powder.Yield: 100% (1.25 g). IR (CH 2 Cl 2 ): νCO= 2090 cm Cl 2 ): 172.6 ppm, CO, d, J Rh-C = 47.24 Hz

	-1 ;	13 C NMR (CD 2

Synthesis of iridium compounds [PPN][IrI 2 (CO) 2 ] 5.

  This compound was prepared from IrI 3,4 (2 g, 3.21 mmol) in a mixture of DMF (150 mL) and water (1.5 mL)(Sc. 2). The solution was stirred under a gentle CO bubbling at 160°C for three hours. The solution initially dark red turned to yellow due to the presence of [NH 2 Me 2 ][IrI 2 (CO) 2 ]. [PPN]Cl (1.84 g, 3.21 mmol) was then added and the solution was cooled to room temperature. Cold water (750 mL, 1-3°C) was then slowly added in order to precipitate [PPN][IrI 2 (CO) 2 ] 5. The yellow powder was filtered and washed three times with 50 mL of cold water. This product was dried under vacuum and crystallized at -18°C in a bilayered mixture of CH 2 Cl 2 /n-hexane (1:1). After a few hours yellow needle-shaped crystals were obtained (2.67 g, 80%). IR (CH 2 Cl 2 ) ν(CO)/cm ): m/z = 503. Anal. Calcd for C 38 H 30 IrI 2 NO 2 P 2 : C, 43.86; H, 2.91; N, 1.35. Found: C, 43.69; H, 2.81; N, 1.38.

	[PPN][Rh 2 (CO) 2 ]	HI r. t	[PPN][Rh 4 (CO) 2 ]
		-1 : 2046, 1967.	13 C {	1 H} NMR (CD 2 Cl 2 ): δ 169.9 (Ir-CO).
	MS (FAB

-Scheme 14 : Preparation of [PPN][RhI 4 (CO) 2 ] 11.

[PPN][

IrI 3 (CH 3 )(CO) 2 ] 6

  This compound was prepared starting from [PPN][IrI 2 (CO) 2 ] 5 (1 g, 0.96 mmol) dissolved in 10 mL of dichloromethane. Methyl iodide (5.45 g, 38.4 mmol) was added to the medium and the solution was stirred at room temperature for 1 hour The solution is filtered. The solvent was removed under vacuum to obtain a the yellow-orange powder (1.08 g, 95%). IR (CH 2 Cl 2 ) ν(CO)/cm -1 : 2100, 2048. Calcd for C 39 H 33 IrI 3 NO 2 P 2 : C, 39.61; H, 2.81; N, 1.18. Found: C, 39.51; H, 2.72; N, 1.23. Preparation of [PPN][IrI 2 (CO) 2 ] 5. [PPN][IrI 2 (CO) 2 ] + CH 3 I CH 2 Cl 2 [PPN][Ir(CH 3 )I 3 (CO) 2 ] Scheme 3 : Preparation of [PPN][IrI 3 (CH 3 )(CO) 2 ] 6.

	IrI 3,4 Mer,trans-	DMF, H 2 O CO 1 bar, 160°C Scheme 2 : 181 [NH 2 (CH 3 ) 2 ][IrI 2 (CO) 2 ] 1) PPNCl 2) H 2 O, -[NH 2 (CH 3 ) 2 Cl [PPN][IrI 2 (CO) 2 ]

1 H NMR (CD 2 Cl 2 ): δ 2.15 (s, CH 3 ).

13 C { 1 H} NMR (CD 2 Cl 2 ): δ 156 (s, Ir-CO), -16.6 (s, CH 3 ). MS (FAB -): m/z = 646. Anal.

[PPN][Ir(COCH 3 )I 3 (CO) 2 ] 8.

  This complex was prepared by addition of H 2 O (2 g, 0.11 mol) to a solution of[PPN][IrI 3 (CH 3 )(CO) 2 ] 6 (0.5 g, 0.42 mmol) in 30 mL of methanol. The solution was placed in a glass reactor and stirred for 1 hour under 5 bar of CO at room temperature. Then 20 mL of n-hexane were added by a liquid feed-line overpressurized with CO and the reactor was cooled to 0°C. After 2 hours w e observed the formation of orange crystals (0.38 g, 75% yield) suitable for X-ray analysis (Sc.5), corresponding to mer,trans-[PPN][Ir(COCH 3 )I 3 (CO) 2 ]. IR (CH 2 Cl 2 ) ν(CO)/cm Cl 2 ): δ 2.76 (s, CH 3 ). Cl 2 ): δ 201.5 (COCH 3 ), 162.7 (s, Ir-CO), 52.2 ppm (s, CH 3 ). Crystal data of complex mer,trans-[PPN][Ir(COCH 3 )I 3 (CO) 2 ] 8: Ir(CH 3 )I 3 (CO) 2 ] H 2 O/CH 3 OH/CH 2 Cl 2 CO 5 bar, r.t., 1 h mer-trans-[PPN][Ir(CH 3 CO)I 3 (CO) 2 ] Scheme 4 : Preparation of mer,trans-[PPN][Ir(COCH 3 )I 3 (CO) 2 ] 8.

	-1 : 2114, 2070, 1655. [PPN][182 1 H NMR (CD 2 13 C { 1 Z 4 Radiation type Mo K; Wavelength (Å) 0.709300 Density 1.96 M (g.mol -1 ) 1210.59 µ (mm -1 ) 5.628 Temperature (K) 180 Size (mm) 0.19x 0.23x 0.40 Colour orange Shape block Cell from 5785 Reflections Theta range 3 to 32 Diffractometer type X-Calibur Scan type Phi Omega Absorption type analytical Transmission range 0.14 0.49 Reflections measured 39257 Independent reflections 13532 Rint 0.05 Hmin, Hmax -21 21 Kmin, Kmax -27 26 H} NMR (CD 2 Formula C 40 H 33 I 3 Ir N O 3 P 2 Lmin, Lmax -22 19
	Crystal Class	Monoclinic
	Space Group Refinement	P 2 1 /c on F
	a (Å) R-factor	14.605(1) 0.0397	alpha	90
	b (Å) Weighted R-factor	18.580(1) 0.0436	beta	95.168(6)
	c (Å) Delta Rho min	15.176(1) -1.53	gamma	90
	Volume (Å 3 ) Delta Rho max	4101.4(5) 2.22

GENERAL CONCLUSION

In this PhD thesis, we studied iridium and rhodium catalyzed methanol carbonylation system for the production of acetic acid. We focused our attention on the acceleration of the rate determining step of the iridium catalytic cycle by addition of co-catalyst. We also studied the effect of water and iodide salt on the determining step of the rhodium catalytic cycle. For this study, we used different analytical tools such as high-pressure infrared spectroscopy and high-pressure NMR spectroscopy.

Concerning the iridium catalytic cycle study, we showed that protic and polar solvent played a major role on the rate determining step and on the nature of the obtained isomer from the methyl iridium species [Ir(CH 3 )I 3 (CO) 2 ]

-to the acetyl iridium species [Ir(COCH 3 )I 3 (CO) 2 ] -. First, two stable isomers of the acetyl species exist in solution, the fac,cis-[Ir(COCH 3 )I 3 (CO) 2 ] -and the mer,trans-[Ir(COCH 3 )I 3 (CO) 2 ] -and second, the increase of polar and protic solvent concentration in the medium, as methanol or water, increased the rate of the reaction rate but also the ratio of mer,trans-[Ir(COCH -. Thus, we proposed two reactions pathways for this reaction: a nucleophilic attack of the acetate ions on the acetyl part of complex [Rh(COCH 3 )I 3 (CO)] 2 2-or a coordination of an acetate ion on this complex followed by a rapid elimination.

Reaction of the rhodium acetyl species with an acetic acid/water mixture appears to be more complicated and the kinetic data obtained didn't allowed us to propose a mechanistic pathway.

Thus, looking at these results, we can believe that addition of a promoter which is able to form acetate ions, more efficiently than lithium iodide, in the catalytic medium from acetic acid or methyl acetate could be a good co-catalyst for the low water rhodium-catalyzed methanol carbonylation since a large amount of LiI is needed in the Celanese process. This promoter, which could be a salt or a metal, must not catalyze other side reaction and have to be stable in the reactor as well as in the flash tank. In a dried 20 mL Schlenk tube under argon equiped with a stirring magnet and placed in a water bath at 25°C, complex 9 (100 mg, 9.15.10 -2 mmol) is disolved in 5 mL of anhydrous dicloromethane.

Complex 10 is obtained by bubbling CO through this solution for 5 secondes. Then, 5mL of a solution of acetic acid/sodium acetate or acetic acid/water is added through a septum seal with a 10 mL glass seringe and the chronometer is started. Infrared spetra were recorded with the same CaF 2 cell on a Fourrier

Transform spectrophotometer Perkin-Elmer GXII. The instruments are aligned before each experiments and the time between each spectrum is as short as possible (usually of 2 or 3 minutes). The reaction has been monitored in transmitance by following the increase in intensity (calculated by the computer) of the 1987 cm -1 band characteristic of complex 7 formed in the medium. We have chosen to take into account the height of the band rather than the area to avoid errors due to the spectrum noise. The height of the band was calculated as shown in figure 1. The corrected height is the difference between the transmitance of the chosen limit of the band represented by base 1 and base 2 in figure 1, and the maximum transmitance of the band. In order to obtain absorbance values suitable for the determination of the reaction rate, according to the Beer's law, we have changed the Beer's law as follows.

A = -log 10 (T)

Then, reaction order have been calculated as described in section III-3.

Synthesis of rhodium compounds

[PPN][RhI 2 (CO) 2 ] 1:

Crystal data of 1.

Empirical formula C38H30I2NO2P2Rh

Formula weight 951.28