Pierre-Alain R Breuil

Lionel Magna

Hélène Olivier-Bourbigou

"Role of Homogeneous Catalysis in Oligomerization of olefins : focus on selected examples based on Group 4 to Group 10 transition metal complexes"

Homogeneous olefin oligomerization plays a pivotai role in the field of petrochemistry. Through catalysts, technology and process developments, market requirements in terms of productivity, selectivity and sustainability have been addressed. Over more than 50 years, an intensive research has been devoted to the design of new Group 4 to Group 10 transition metal complexes and to the study of their reactivity towards olefins leading to severa! breakthroughs of prime importance for academy as for industry. Since the early sixties, IFPEN contributed to bring innovative industrial solutions to different targets from gasoline production to alpha-olefin on purpose processes with over 100 production units built worldwide . Based on nickel, titanium, zirconium or chromium, the catalytic systems for such processes and their next generation are subject to a continuous research where the adaptation of the ligand architecture to the nature of the metal and its mode of activation, play a crucial role to control the reaction selectivity and the catalyst lifetime. lnteresting relationships between the complex structure and their reactivity have been drawn and will be discussed on selected examples.

1-INTRODUCTION AND SCOPE

Oligomerization reactions are widely used on an industrial scale to upgrade light olefin streams coming from different hydrocarbon forming processes (steam cracking, FCC, Fischer-Tropsch ... ) into heavier olefins that find applications in the field of energy or petrochemistry. This reaction is catalytically achieved by means of different technologies such as heterogeneous acid-catalysis which is essentially applied to the production of fuels or by homogeneous catalytic systems mainly applied to provide high added value chemicals [START_REF] Forestiere | Oil Gas Sei[END_REF].

1.1-Ethylene oligomerization :general market and current commercial technologies

A few new industrial homogeneous-catalyzed processes have emerged in the past 10 years. An especially growing area of homogeneous catalysis is the oligomerization of short chain alkenes, an important reaction mainly for the synthesis of second generation intermediates (or base materials) for chemical industry. The oligomerization of ethylene to Linear Alpha Olefins (LAOs, even-carbon numbered olefins ranging from C4 to C30+ carbon atoms) dominates the market of homogeneouscatalyzed oligomerization. The main applications for the LAOs are as co-monomers for polyethylene production, for oxo-alcohols used in detergent and plasticizers and for the production of poly-alphaolefins for the synthetic lubricant pool. The total world production for LAOs accounted for 3.5 Mt/y in 2012 for a total capacity of 4.3 Mt/y (with the exception of 1-butene production from refinery streams). The global annual average growth for LAOs is estimated at 3.3% (2012-2018) but depends largely on the region with a higher growth in developing countries. Globally, co-monomer grade LAO consumption (C4-C8) is the largest and fastest-growing application, particularly for the production of linear low-density polyethylene resins (LLDPE). Controlling the amount of short-chain branching in the polyethylene polymer through co-monomer addition determines the density of the resin and modifies the processing and mechanical properties of the polymer. As examples, HDPE (High Density PE) uses 2-4% of co-monomer (C4-C6), while LLDPE (Linear Law Density PE) uses 8-10% of comonomer (C4-C8). 1-Butene is still the most used co-monomer, in the commodity end-use applications of LLDPE (more flexible and resilient PE) and to a less extent HDPE. 1-hexene and 1-cetene are becoming increasingly attractive because of the excellent properties imparted ta the copolymer product, including greater stress-crack resistance and tear resistance. For example, LLDPE with 1-hexene leads ta tougher film than with 1-butene incorporation and can be produced in a thinner thickness (preferred for packaging application PE) . The incorporation of 1-octene gives the highest quality products with good surface finish, good transparency and improved resistance ta tearing (produced in solution or slurry-based PE processes). The global LAO supply is still dominated by the 3 majors, Chevron Phillips Chemical (CPChem), lneos and Shell, who operate with different catalytic systems, with more or less flexibility (Table 1). These processes are named "full-range processes" because they produce a broad distribution of olefins.

The oldest processes use aluminum under harsh reaction conditions of temperature and pressure due ta the law activity of aluminum. The first commercial production of LAOs was operated in 1966. The one step process using triethylaluminum catalyst developed by the Gulf Oil Company (now CPChem) produces alpha olefins with a broad Schul z-Fiory carbon number distribution. The distribution of olefin chain lengths can be changed (in a certain limit) by altering the reactions conditions. The two-step process (stoichiometric and catalytic steps), known as the Ethyl process (now lneos) also uses triethylaluminum but leads ta a Poisson distribution with a relative narrow distribution of olefins (recycle of butenes ta produce higher olefins) and an increased branching olefins in higher fraction (C14-C18). lt is the less flexible process in term of product distribution. ln these two processes, the olefin streams produced are separated by distillation in quite similar way (in concept) . However, an extra demand has appeared for higher quality alpha olefins. This demand is driven by the metallocene polyethylene catalyst and process developments in which the short cha in LAOs are used as co-monomers . For this purpose, the alpha-olefin quality upgrade can be achieved by adding a distillation column in the full-range processes. Shell developed the Shell Higher Olefin Process in the 1970s (first commercial use in 1977). ln the SHOP process, ethylene is oligomerized ta a broad Schulz-Fiory distribution of olefins. Compared ta the other full-range processes, this process is the most complex but it is quite exceptional because the distribution of oligomers may be adjusted through isomerization and metathesis consecutive steps ta meet the required linear internai olefins chain length (applications for detergents and plasticizers). The processes catalyzed with zirconium based complexes have emerged later. ldemitsu Kosan Co . Ltd . developed an oligomerization process catalyzed by a system based on a zirconium(IV) complex associated with an chloroalkylaluminum derivative and a Lewis base . This process was commercialized in 1989. IFP Energies nouvelles (IFPEN) and SABIC-Linde bath separately developed processes based on a Ziegler-Natta catalytic system composed of a zirconium precursor, a proprietary ligand and an aluminum co-catalyst. The SABIC-Linde technology was operated at SABIC in 2009 . ln Table 1, the additional estimated new capacities for LAOs using these existing technologies are also mentioned. The applications of the additional capacities are mainly for the polyethylene market but other applications, such as poly-alpha-olefins, ail drilling fluids and surfactants can also contribute to the growth.

1.2-Main recent industrial developments in the field

Today, the oligomerization of ethylene ("full-range" processes) is the predominant route to produce LAOs. However, as the demand for short-chain C4 to C10 linear alpha olefins is growing faster than that for the ClO+ range, significant recent progresses have been made to control the product distribution to shorter alpha olefin distributions (flexibility of the most recent Zr-based processes) or even to produce selective/y one main alpha-olefin such as 1-butene, 1-hexene or 1-octene. Transition metal homogeneous catalysis again opens the door to the development of new selective oligomerization processes. These selective processes are called "on purpose" technologies. Sorne of these technologies have recently been commercialized or industrialized (Table 2). lt is now generally accepted that the mechanism for the selective ethylene transformation into 1hexene and 1-octene goes through a metallacycle formation as illustrated in Scheme 1 (route A) . On the other hand, the generalized mechanism for the "full-range process" occurs via a degenerated polymerization route as shawn in Scheme 1 (route B). Discussion is still open for 1-butene production .

Metanacydoheptane

Route A Route B Scheme 1: Ethylene oligomerization mechanisms-metallacycle pathway (route A), degenerative polymerization (route B)

Today, there is an increased availability of natural gas [START_REF] Armor | [END_REF], especially in North America with the shale gas exploration. The large production of shale gas in the US had the effect of reducing the US natural gas priees, making ethane supply abundant, increasing the use of lighter feed in the crackers and contributing to the construction of new ethane crackers. Globally, this has contributed to a growing priee gap between crude ail and natural gas. This dynamic makes lighter feedstock derived from natural gas {like ethane) more attractive input for cracking operations, compared to heavier feedstock derived from crude ail (like naphtha). The shift toward greater use of these lighter feedstock has resulted in the production of significantly less C3 and C4 fractions which could generate a shortfall for certain major intermediates. If most 1-butene is today still supplied from refinery sources (extraction from C4 fractionL one can expect that in the future, the "on purpose" production via selective dimerization processes will become predominant.

1.3-IFPEN achievements in olefin oligomerization

Since the early sixties [4L IFPEN has been deeply involved in the search of innovative homogeneous transition metal based catalytic systems. ln 1977, the first Dimersol industrial plant was put onstream in the United States producing gasoline by oligomerization of propylene coming from a Fluid Catalytic Cracking C3 fraction. This reaction was catalyzed by a homogeneous Ziegler-Natta nickelbased catalyst. ln the 1980s, a new challenging activity was to selectively dimerize ethylene into 1butene, the most popular key ethylene co-monomer for manufacturing LLDPE and HOPE to a less extend. An efficient soluble titanium-based complex was designed. The first AlphaButol industrial unit was started up in 1986 in Thailand. The demand and annual growing rates for LAOs, motivated the development of a "full-range" ethylene oligomerization process. Ziegler-Natta type systems based on zirconium complexes were chosen for this purpose leading to the AlphaSelect process. ln the mid-1990s, a research was initiated at IFPEN in one of the most promising recent area in liquid phase homogeneous catalysis: the use of non-conventional, non-aqueous ionie liquids for biphasic catalysis. This new technology, trade-named Difasol, was successfully applied to the nickel-catalyzed butene or propylene selective oligomerization/dimerization making possible a better use of the catalyst associated with a more efficient process scheme. The market demand for 1-hexene drave our research, in the 1990s, to the design of a selective trimerization catalyst. An efficient catalytic system based on chromium associated with a law investment cast process were developed and optimized to produce high purity 1-hexene. The first AlphaHexol unit was commercialized in 2012 [5].

Ethylene is mainly originating from steam cracking. Fluctuating steam cracking traditional ethylene priees as weil as consumer orientation toward renewable materials initiated our search for new resources able to bridge the gap between bio-based and traditional petrochemicals. One of these resources is bio-ethanol. Using bio-ethanol feed based on non-food biomass (also called 2"d generation resourceL enables to produce ethically acceptable and renewable plastics with the same properties and recyclability as existing polyethylene grades. A process chain encompassing ethanol to ethylene technology and "on purpose" alpha-olefin processes (e.g. AlphaButol or AlphaHexol) was fitted in to develop a complete process portfolio to turn bio-ethanol in 100% bio-polyethylene containing 100% bio co-monomers.

1.4-Scope of the review

Since the early days, important work has been devoted to the design of new transition metal complexes and to the study of their reactivity towards olefins, mainly ethylene. However, during these last years, interest grew in developing new generation of catalysts with the aim of a greater control of the selectivity towards the desired linear alpha olefins. The architecture of the ligands, which have to be adapted to the nature of the metal, plays a crucial role. The recent past years have seen a growth in the design of versatile multidentate ligands. Severa! studies have clearly demonstrated that even mi nor variations on the ste rie and electronic properties of these ligands and the geometrical constraints they impose on the metal can lead to great changes in catalytic reactivity such as oligomerization vs polymerization, initial activity (TOF), productivity (TON) as weil as catalyst stability and lifetime. lnteresting relationships between molecular structures of the complex precursor and their reactivity have in some cases been drawn. However, it should be noted that most of the catalysts used in oligomerization are based on Ziegler-Natta type systems, a multi-component combination of a metal precursor and a suitable activator such as an alkylaluminum derivative. ln the open literature, the molecular structures generally depict the catalyst precursor, which has to be activated to form the catalytically active species. This pre-catalyst activation is determining and directs the following catalytic steps. There is generally only limited direct information on the nature and the structure of the active species. The introduction of MAO as a suitable activator for transition metals (Gr4 to Gr 10) oligomerization was a major breakthrough but it also introduced an important issue with regards to the elucidation of the nature of the active species involved.

Few reviews dea ling specifically with olefin oligomerization were published [START_REF] Mcguinness | [END_REF]7,8,9]. The aim of this present review is not to give an exhaustive overview of the recent developments in the field of oligomerization. This review highlights some selected examples based on our own achievements in the light of the major industrial developments. The catalytic systems are classified according to their industrial importance, across different families of ligands. ln the group 10, nickel is the most described and used oligomerization catalyst with a considerable number of publications. lt is applied in three different industrial processes currently in operation for different purposes. ln the review, we choose to take the nature of the nickel active species as the main theme to exemplify its reactivity and use. Group 6, particularly chromium, has seen a particular growth in research activity this last years. The chromium chapter focuses on the industrial developments and the key ligand design developed. Metals from group 4, particularly titanium, are illustrated with a "cyclopentadienyl/phenoxy" ligand oriented research. The new developments regarding iron are then presented. Finally, tungsten even if it was not applied in industry in the oligomerization field up to now is discussed. Despite some very recent examples tantalum [10] as weil as vanadium [11] catalysts are not treated here. lt is worth underlining that no heterogeneous or supported catalyzed processes emerged for ethylene oligomerization despite the numerous studies published in this field from bath academia and industry.

Il-NICKEL INDUSTRIAL DEVELOPMENTS AND FUTURE CHALLENGES

The nickel catalyzed olefin oligomerization has a long history [START_REF] Olivier-Bourbigou | Applied Homogeneous Catalysis with Organometallic compounds, A Comprehensive Ha nd book in Three Volumes "Dimerization and selective oligomerization-Selected examples of industrial applications[END_REF]. The discovery of the "nickel effect" by Wilke [13] was the beginning of the major Ziegler's breakthrough who observed that nickel salts cou Id modify the outcome of AIRrcatalyzed "growth reaction". Sin ce this first discovery in the 1960s, nickel is still playing a central role in the tate-transition metal catalysts for olefin oligomerization and polymerization and continues to be one of the most studied metal in this field bath by academie and industrial laboratories [14]. The interest has mainly been focused on the ligand design. A myriad of multifunctionalligand backbones has been reported including phosphorous, nitrogen and/or oxygen heteroelements. A large number of nickel(ll) pre-catalysts were described opening to a very rich coordination chemistry. This chapter does not caver these developments which have already been reviewed [9,14]. With regards to the nature of the active nickel species, two main families have so far been recognized, according to the nature of the ligand bonding around the nickel metal: neutra/ and cationic nickel catalysts (Figure 1). Cationic nickel active species (type A) are electronically unsaturated and highly electrophilic. They involve nickel (Il), incorporate a donor ligand and are generally four coordinated with a square planar geometry. They are difficult to isolate because of their high reactivity. Cationic 7t-allyl nickel complexes have nevertheless been isolated by Wilke [15] and can be considered as a madel of this kind of active species. lt is worth noting that the neutral7tallyl nickel halide complexes are not active towards olefin. However, combined with Lewis acids such as aluminum halides, they are active homogenous catalysts for the dimerization of ethylene and propylene and the polymerization of butadiene. The transfer of the halide from the nickel to the aluminum results in the formation of an electrophilic cationic metal center and formation of a free coordination position on the nickel on which the olefin can coordinate. Neutra! nickel catalysts (type B) are less electrophilic than those of the cationic family. They commonly conta in anionic type ligands (oxygen or nitrogen based) and a Ni-C or Ni-H active bond in which the olefin will insert ta begin the catalytic cycle. These two kinds of nickel catalysts independently led ta the main still currently performed industrial nickel-catalyzed oligomerization processes: Dimersoi/Difasol and Phillips processes (Figure 1, type A) and SHOP process (Figure 1, type B). These industrial processes are first briefly described. Then, further developments on the design of ligands will be discussed.

::. ""'Ni/ 1 A ] Ni__.......- [ / L R ]+ _ 1 (L"" R '•L'/ <?
x/ """'-L' The industrial Dimersol process [16] was developed by IFPEN ta convert light olefins (ethylene, propylene and/or butenes) into liquid olefin oligomers for the production of high octane sulfur free gasoline (C6 olefins predominate) and for the production of C8 olefins used for isononylalcohols (INA) manufacture dedicated ta the plasticizer market. Sorne gasoline Dimersol units are still in operation despite the planned restriction of olefin level in gasoline and the increasing demand of propylene for petrochemistry. The C8 olefins produced from butenes in Dimersol units constitute good quality feeds for oxonation reaction because of their law branching index (production of INA).

The INAs produced by this route are of special interest and competitive alternatives ta the 2-ethylhexanol (propylene oxonation) the demand of which is decreasing in sorne regions because of the health issue of the DEHP (DiEthyiHexyiPhthalate) phthalate plasticizers. The Dimersol reaction is catalyzed, in the liquid phase, by a homogeneous nickel complex which results from the interaction of a Ni(ll) precursor with a chloroethylaluminum compound. The nickel active species is of type A (Figure 1). The alkylaluminum co-catalyst has a dual raie: ta alkylate the Ni complex ta form the Ni-H bond after (3-H elimination and ta capture the anionic ligand from the Ni ta generate the counter-anion of the cationic Ni complex. The active species is formed inside the oligomerization reactor under olefin atmosphere. The most frequently encountered mechanism for nickel catalyzed oligomerization is the degenerated polymerization mechanism weil described by Cassee and Arlman (Scheme 1). The reaction products are mainly composed of dimers due to the high rate of termination step. The reaction is non-(regio)selective and leads ta a mixture of the different dimeric isomers. ln the case of propylene or butenes conversion, the formed dimers are mainly mono methyl branched internai olefins (methylpentenes and methylheptenes, respectively). lsomerization of the double bond occurs as a side reaction catalyzed by the active Ni-H (insertion of the dimer in Ni-H, followed by (3-H elimination). The heavier product formation mainly originates from consecutive co-dimerization of the monomer with the formed dimers. These consecutive reactions are favored at high monomer conversion. ln this type of nickel catalysis, the main challenges are focused ta reach high conversion and high selectivity for the dimers. The type of reactor technology (CSTR vs plug flow) has a significant impact on the oligomer by-product formation which results from consecutive reactions [START_REF] Olivier-Bourbigou | Applied Homogeneous Catalysis with Organometallic compounds, A Comprehensive Ha nd book in Three Volumes "Dimerization and selective oligomerization-Selected examples of industrial applications[END_REF]. lt has ta be borne in mind that beside the addition of ligands, the process design can impact the formation of these by-products and that sorne advances can a Iso be proposed ta optimize the dîmer yield by that way. This has been one of the drivers of the development of biphasic catalysis.

The biphasic Difasol process: Ni{II}/Chloroaluminate ionie liquids ln the early 1990s, biphasic liquid catalysis appeared as a highly attractive approach ta solve sorne of the main issues of homogeneous catalysis such as the catalyst recovery and recycle. The cationic nature of the active nickel species and the use of chloroalkylaluminum compounds as activators prompted us to investigate acidic chloroaluminate ionie liquids as possible medium ta perform the reaction. At that time, chloroaluminates were mainly applied as electrolytes (17]. lt was the first time that these ionie liquids were used as a combination of solvent and co-catalyst for a chemical reaction [START_REF] Chauvin | Guibard 1[END_REF]. The addition of ethyl aluminum dichloride (EtAICI 2 ) to the acidic chloroaluminate was found to form anionic mixed alkylchloroaluminate anions that inhibit the formation of higher oligomers due ta a cationic side reactions [19]. Compared to the homogeneous single-phase reaction, the biphasic process in chloroaluminates, named Difasol, led to a higher selectivity into octenes (> 90 wt. % of octenes relative to the total products even at 80% butene conversion). ln such system, consecutive side reactions (octenes + bute nes) are minimized as a consequence of the lower solubility of octenes compared to butenes in the ionie liquid. Moreover, the reaction could be performed with diluted feeds (butenes + isobutene or butane) with the same performances in terms of productivity. The demonstration of the stability of chloroaluminates and nickel under catalytic conditions was achieved via a continuous long term pilot run. The product separation operated by setting was efficient and no significant loss of ionie liquid in the product phase could be detected [START_REF] Olivier-Bourbigou | Handbook of Green Chemistry-Green Catalysis[END_REF]. The Difasol technology [START_REF] Forestiere | Multiphase Homogeneous Catalysis[END_REF] can extend the field of applications of the Dimersol to the less reactive longer chain olefins such as CS fractions allowing the production of decenes or nonenes through co-dimerization of C4 and CS olefins.

The advantages brought by the biphasic approach can be ali the more important since costly phosphine ligands are used in excess without recycling. This is the case of the sterically demanding basic tricyclohexyl phosphine involved in the homogeneous Ni-catalyzed propylene dimerization to favor the formation of 2,3-dimethylbutenes (2,3-DMBs, tail-to-tail dimers). 2,3-DMB-1 and 2,3-DMB-2 are key industrial intermediates for fine chemical synthesis (musk fragrance or insecticide) [22,23].

We switched this reaction into chloroaluminates and operated successfully the catalysis in a continuous biphasic way [24]. The main issue was ta maintain the phosphine effect with time white optimizing the phosphine consumption. This was achieved by the addition of small a mount of a weak organic base which acts as a buffer of chloroaluminate acidity. A new biphasic continuous flow process for the regio-selective dimerization of propylene into 2,3-DMB-1 was developed by IFPEN. Total hexene selectivity was hold steady at 75-80 wt. % hexenes/total products and 2,3-DMB-1 was hold steady at 70-75 wt.% relative to the total hexene content [19].

The Phil/ips process: Ni{II}/P(n-Bu}JfEtA/C/ 2

Ni homogeneous Ziegler-Natta systems based on Ni(ll) and a chloroalkylaluminum were very early disclosed ta catalyze the oligomerization of olefins, such as ethylene. ln these early days, there was a lot of patents in this field assigned to different companies [8]. However, nickel catalysts were not highly selective for the formation of dimers (e.g. butenes from ethylene) affording tao important undesirable and wasteful quantities of side products such as higher oligomers. ln the tate 1960s, Phillips Petroleum company [25] disclosed a process which converts ethylene selectively into 2butene. A typical catalyst composition consists of bis(tri-n-butylphosphine)nickel dichloride and ethylaluminum dichloride. The Al to Ni molar ratio has an important impact on the reaction selectivity. lt is described that ethylene dimerization takes place in a circulating loop reactor under mild reaction conditions (P = 10-1S bars and T= 35-45•C). High velocities are maintained in the reactor ta minimize fouling and temperature rise across the reactor.

Toward alternatives to monophosphine ligands

Monophosphines have been largely studied in nickel-catalyzed oligomerization field and the weil described phosphine effect has been applied in two main industrial processes for the production of 2,3-DMB and 2-butene . However, one main issue of these systems is the loss ofthe phosphine effect with time which results from the de-coordination of the ligand from the nickel metal center, particularly when excess of Lewis acid aluminum compounds are used as co-catalyst.

To circumvent this issue, different strategies have been developed. One of them consists in using bidentate neutra! ligands (L-L or L-L') which combine interesting properties in catalysis and stabilization of the metal. A large number of ligands such as P-P, P-0, P-N, ... have been reported [9] . Nickel usually forms four-coordinated complexes with these ligand systems white a few five coordinated complexes have been reported. If we make a focus on diphosphines, we can see that they have largely been evaluated in nickel catalyzed oligomerization. However, examples of dissymmetrical diphosphines and their corresponding nickel complexes are scarce maybe due to their tedious synthesis. We recently described a metal-induced rearrangement strategy from iminobisphosphine ligands to symmetrical and non-symmetrieal diphosphinoamine nickel complexes (Scheme 2) [26]. Upon activation with MAO, these unprecedented complexes oligomerize ethylene to small chain oligomers (mainly C4-C8) with high productivity, highlighting the prime importance of introducing alkylphosphine moieties in such pre-catalysts. An interesting approach to generate the active Ni-H cationie species is via oxidative addition of a Bronsted acid to a Ni(O) precursor [27]. This combination promotes ethylene oligomerization in organic solvent but with very law activity. However, the reaction activity can be enhanced by using ionie liquids as solvents. We were nevertheless surprised to observe that in [1-butyl-3methylimidazolium][NTf2] ionie liquid (NTf 2 =bis[(trifluoromethane)sulfonyl]amidure), ethylene was transformed into butenes without any addition of acid. The formation of active nickel complex was explained by an oxidative addition of the imidazolium cation to the Ni(O) to form in situ a [(NHC)Ni-HnNn2r species (NHC=N-heterocyclie carbene) [28] . NHC carbene are versatile ligands that share many of the coordination properties with the phosphines with whieh they are frequently campa red. They present stronger sigma-donating properties that can stabilize tate transition metal complex. Nickel complexes bearing 1,3-dialkylimidazolin-2-ylidene carbene have been reported for the olefin oligomerization [29,30] (Figure 2) . Upon activation with chloroalkylaluminum derivatives in toluene, these complexes are inactive to sparingly active. On the other hand, they are highly active in acidic chloroaluminate ionie liquids. However, disappointingly, they do not present any regio-selectivity with propylene unlike the [NiCI 2 (PCy 3 h] complex in the same conditions.

R2 f-2 R1 R2 R3 R2, ,R2 NiBr 2 (DME) (1 eq.) 0 p 4-BrBz Ph Ph 9, ~f'p ' R3
~N,......Me Guy Bertrand and co-workers have developed a new type of versatile carbenes: the cyclic alkyl amino carbenes (CAAC} [32]. These ligands are more electron donating and bulkier than classical NHC and phosphines. The synthesis of the corresponding iminium salts is quite simple [33,34] so that we anticipated that CAAC could be perfect candidates for their use as ligands on nickel complexes to form new efficient catalysts in propylene dimerization. The [NiCI 2 (PCy 3 h] complex, partially soluble in toluene, can be used as starting material for "Ni-CAAC" synthesis. For example, the addition of 2 equivalents of CAAC to [NiCI 2 (PCy 3 hl complex allowed to isolate a new bis(CAAC}nickel(ll) complex, soluble in heptane (Scheme 4), the molecular structure of which was determined by single-crystal Xray analysis (Figure 3). The carbene ligands are situated trans to each other and the geometry is slightly distorted square planar [35] .

IN~ /1 R R / Ni'\._/ 1 J N /NJ Me R = iPr or Me
//"è,

~~o-DIPP

Vj-( /c1

NiCiz(PCy3b -T 0 -1 u-en_e_.

CI /Nit? Starting from the pale green insoluble [NiCI 2 ,2Pyridine] complex, a new mixed complex [NiCI2(CAAC)(Pyr)] could be isolated in low yield despite many attempts of synthesis optimization (Scheme 5) . The X-ray structure of this violet complex shows a distorted tetrahedral geometry [36] (Figure 4). -.

/ \} •~ ~ _:t~ ~ ~ !""• ' C< ---~ % L i Q.~ 1 ~' • -~ Ç' "' -~ t "r " -• ' g () è ~,g: • , tb b Q e 0 {t <;L <Jd DIPP 1 0""" /N # Ni 1 '
Cl Cl The two "Ni-CAAC" complexes were evaluated in propylene dimerization in homogeneous or biphasic system using acidic chloroaluminate ionie liquids. Upon activation with EtAICI 2 , both complexes present an activity but are less active than [NiCI 2 {PCy 3 h) used in similar operating conditions.

Moreover, the "Ni-CAAC" complexes do not display any regio-selectivity and form methylpentenes as the major isomers.

To reduce deactivation pathway with monodentate carbene, one strategy consists in attaching a donor function to the NHC to form a heterobidentate (L-L') ligand which could undergo hemilabile behavior. Ether-functionalized NHC ligands have recently been studied [37) . The corresponding biscarbene nickel(ll) complexes have been isolated and characterized by X-ray analysis. No inter-or intra-interaction between the ether function and the metal was observed. These complexes promote ethylene oligomerization upon activation with EtAICI 2 but they did not lead to remarkable activity.

_rOM

• ~N ~ f Cl R N~ 1 l ~ c,ryJ N ,, " R = DIPP, Mes, n-Bu N~ M e r

Toward nitrogen ligands : Versipol complexes and analogues

For a long time, nickel complexes, as other tate transition metal complexes, have been considered to be less prone to polymerization and to preferentially lead to olefin oligomerization because of their propensity to generate (3-elimination, in contrast to Ti and Cr which were good polymerization catalysts. However, the discovery in the tate 1990s that, upon activation with MAO, a-diimine nickel{ll) complexes were highly active catalysts for ethylene oligomerization as weil as for polymerization [38], changes the simple picture of the nickel and triggered a true exploration of the tate-transition metal catalysts for olefin polymerization . ln oligomerization, the activity of these systems is very high (activity up to 1 370 000 gc 2 H 4 /(moiN;.h) upon activation by MMAO, Schulz-Fiory distribution of olefins with a-selectivity up to 94%). Moreover, a-dimine ligands are versatile and easy to synthesize which made these systems very popular. The Schulz-Fiory distribution of the olefins can be modulated in a certain limit (K values range from 0.59 to 0.81). The alpha selectivity of the olefins depend on the operating conditions but are generally lower (96% for the highest) than the quality required for the LAO. The isomerization ability of cationic Ni hydride cannat be completely avoided.

Figure 6: Example of "Ni-diimine" complexes evaluated in ethylene oligomerization

Other functionalized nitrogen-based ligands (pyrazole, imidazole, oxazole, ... ) with interesting coordination chemistry have also attracted attention for Ni-catalyzed oligomerization but none of them really differentiate in term of terminal olefin selectivity (> 95% alpha olefin) [14,39]. Nevertheless, pyridine functionalized ligands remains quite interesting. Reliance reported few years aga the use of pyridine-oxime derivatives (Figure 7). Associated to nickel and activated by MAO or Et 2 AICI, these ligands were surprisingly found to be particularly active and highly selective for the production of 1-butene (up to 92 % of C4 and 99.5% 1-butene/C4) [40]. However, no patent or further experimental work was disclosed. Organometallic nickel complexes combining soft phosphorous and hard anionic oxygen atom in a chelate P-0 ligand were described and first isolated by Keim in 1978 [41,42]. These catalysts are synthesized by mixing a phosphorus ylide (typically Ph 3 P=C(H)-C(=O)Ph) in presence of Ni(CODh and a coordinating phosphine. They are isolable and suitable madel of the SHOP type catalysts. lt is worth mentioning that SHOP catalysis is operated in a biphasic system using 1,4-butanediol as the catalyst solvent. This process was the first commercial catalytic process to take benefit from two-phase, but non aqueous, liquid/liquid technology. The organometallic SHOP type catalysts have played a predominant role in the development of well-defined catalyst precursors for olefin oligomerization.

~R

To rationalize the ligand effects, it was suggested to break dawn the catalyst in two: a chelate part and an organo part (43] {Figure 8) . ln toluene, they produce alpha-olefins {under 50 bars of ethylene at 50°C, activity up ta 60 000 gc 2 H 4 /{moiN;.h), 99% of the olefins are linear of which 98% are terminal) while in hexane polyethylene is formed. These catalysts are also capable of producing polyethylene when combined with a phosphine scavenger, which demonstrates their versatility [44]. Recently, the impact of a number of parameters on the catalytic properties of these complexes has been reported (45] .

c: R Ph Ph Since we were interested in the short chain olefins, we have attempted ta modify the electronic density at the nickel center in arder to facilitate ~-elimination during the growing chain. The strategy was ta create an intramolecular hydrogen bonding between the enolate oxygen atom and an orthosubstituent on the aromatic ring of the P,O chelate, with the aim to generate an electron withdrawing effect {Figure 9) [46,47] . We could demonstrate that the intramolecular hydrogen bon ding allows a fine tuning of the selectivity of the complexes. ln comparison to typical SHOP madel complex, the alpha-olefin distribution significantly shifts towards the lower C4-(8 olefins of which 95-99% are linear a-olefins, while the activity remaining unchanged.

Toward alternative anionic ligands

Du ring the following two decades, there was no significant progress in the design of different type of anionic ligands and corresponding neutra! nickel catalysts. Just at the beginning of the century, the discovery of the versatile salicylaldiminato catalysts [48] open the way ta investigations on neutra! nickel complexes {type B). However, the studies have focused on the ethylene polymerization with the aim ta control the microstructure of the polymer.

Ill-CHROMIUM CATALYSTS FOR SELECTIVE ETHYLENE TRI-AND TERAMERIZATION

The ability of chromium complexes ta oligomerize selectively ethylene was discovered at Union carbide and reported in 1967 [49]. Manyik, Walker and Wilson observed the predominance of 1hexene in the liquid fraction during ethylene polymerization with chromium tris(2-ethylhexanoate) activated with partially hydrolyzed triisobutylaluminum, suggesting the presence of severa! active species and eventually mechanisms. This breakthrough triggered the search for selective trimerization and later tetramerization catalysts leading ta industrial successes.

Although the first evidence for selectivity in 1-hexene with chromium-based catalysts was brought with a partially hydrolyzed alkylaluminum as activator, in other words, an aluminoxane, the first industrially relevant catalytic systems commercialized were developed with well-defined alkylaluminum and 1 or chloroalkylaluminum compounds. While Chevron Phillips Chemical Company LP (CPChem) reported the use of pyrrolide ligand in 1991 [50], IFPEN developed a catalytic composition based on an aryloxide ligand in 1998 (Figure 10) [51]. For bath proprietary anionic ligands, their association ta a chromium(lll) precursor lead ta selectivities in 1-hexene above 99% in the C6 fraction, affording a polymer grade co-monomer. CPChem implemented this technology through two joint ventures with Qatar Chemical Company Ltd. in Mesaieed (Qatar) and Saudi Polymers Company in Al Juba il (Saudi Arabia), producing 47 000 t/y and 100 000 t/y, respectively [START_REF] Nexant | Developments in LAO Comonomer Technologies for Polyethylene[END_REF]. ln 2014, CPChem a Iso announced the start-up of a 1-hexene production unit of 250 000 t/y in Baytown, Texas (USA) [53]. ln the meantime, Axens licensed in 2012, a production unit in Primorsk region (Russia) based on the 1-hexene IFPEN catalytic system [54]. ln 2009, SABIC, Linde and the Rostock University disclosed a catalytic system formed by a chromium(lll) precursor, R 2 P-N(R)-P(R)-NHR as ligand and triethylaluminum as activator for the selective ethylene trimerization ta 1-hexene (Figure 10) [55]. The evaluation of such catalytic composition on the pilot plant scale should be ongoing [START_REF] Meiswinkel | [END_REF]. Although multiple coordination modes and reactivities toward the alkylaluminum and the chromium precursor used are possible, these three catalytic systems are believed ta proceed via the wellknown metallacyclic mechanism ta afford such selectivities in 1-hexene. Having a doser look ta the side products formed is in line with such hypothesis, indeed, mainly decenes are produced, while octenes are kept ta a minimum. Decenes are suggested ta be formed by co-trimerization of 1-hexene with two molecules of ethylene through a chromacycloheptane [57]. Detailed studies of these systems and other examples of anionic monodentate or multidentate ligands in association with alkylaluminum have been described with in the literature and exhaustively reviewed [START_REF] Mcguinness | [END_REF]58]. ln 2002, BP Chemicals published highly active trimerization catalytic system comprising a chromium source and a neutra! functionalized diphosphinoamine ligand (>10 6 gc 2 H 4 /(gc,.h), Figure 11) when associated ta methylaluminoxane [59,60]. Few years later, Sasol researchers showed that with a nonfunctionalized diphosphinoamine ligand, exceptionally high selectivities in 1-octene along with high activities could be obtained [START_REF]The Alphabutol process is licensed by Axens, a subsidiary company from IFP group[END_REF]4 wt. % of 1-octene compared ta ali products formed, 2.7.10 5 gc2H4/(gc,.h)) [61,62]. Ta satisfy the growing need in 1-octene, Sasol rapidly announced the construction in 2010 of a tetramerization unit producing 100 000 t/y of 1-octene and 1-hexene in Lake Charles, Louisiana (US) [63]. The start-up of the unit was planned for late 2013, no further information are, however, accessible. Numerous diphosphinoamine and several other diphosphine ligands have a Iso been reported in the literature for the selective tetramerization [START_REF] Mcguinness | [END_REF]58].

Ry!yR' R'YR'

IV-TITANIUM FOR SELECTIVE ETHYLENE Dl-AND TRIMERIZATION

Very early, titanium-based homogeneous catalysts have opened the door ta selective ethylene dimerization ta 1-butene (64,65]. ln the mid-1980s, IFPEN and SABIC jointly developed the first commercially viable process for this reaction, the AlphaButol process [66]. Today, 30 AlphaButol units have been licensed for a cumulated 1-butene production capacity of 708 kt/y, nearly 25% of the world's 1-butene consumption as co-monomer in polyethylene [START_REF]The Alphabutol process is licensed by Axens, a subsidiary company from IFP group[END_REF]. The catalytic system is based on a proprietary titanium{IV) precursor and an alkylaluminum co-catalyst which are bath injected in the reactor ta generate the active species [START_REF] Forestiere | Oil Gas Sei[END_REF]4]. Careful choice of this catalyst composition and reaction conditions lead ta selectivity for 1-butene up ta 93% (purity of 1-butene higher than 99.5%). This field was later on the subject of interest for researchers in industry and academia. Nevertheless, if we except the initial statement of Chauvin et al. [START_REF] Bre | [END_REF], few attempts ta investigate the reaction mechanism induced by "Ti(OR) 4 /AIR{ system were undertaken [69]. ln its first proposai, Chauvin and co-workers identified titanacyclopentanes as possible intermediates for this reaction. Substituted titanacyclopentanes were then generated by reacting 1,4-dilithiobutanes with [Ti{OBu) 4 ] precursor.

The nature of the hydrocarbons formed by thermal decomposition of these compounds and their comparison with the CG by-products formed under real dimerization experiments, suggested successive formation in the catalytic cycle of metallacycle intermediates. Recently, several experimental [70] and theoretical studies (71] conducted by McGuinness and co-workers suggest that metallacycles are not likely responsible for the selective formation of dimers. According ta these studies, a conventional Cossee-Arlman mechanism with very fast ~-H elimination could be the preferential route ta explain the primary product 1-butene and the secondary reaction products. Despite its apparent simplicity, the mechanism involving "Ti{OR) 4 /AIR 3 " system remains under discussion.

Titanium complexes also appeared in the last 10 years as good catalysts for the selective trimerization of ethylene ta 1-hexene [START_REF] Mcguinness | [END_REF]58]. The first remarkable example was described in 2001 by Hessen and co-workers [72] with a system based on a cyclopentadienyl ligand bearing an aromatic group {Figure 12, 1). At 3o•c and 10 bar of ethylene, [(fl 5 Cp-CMerC 6 H 5 )TiCI 3 ] upon activation with MAO was described as very active {~6.10 6 gH 1 =/moiTi/h) and selective for 1-hexene production {86% of C6 with more than 99% of 1-hexene). The thermal stability of the catalyst remains modest as increasing the reaction temperature ta 8o•c decreases the overall catalyst productivity and selectivity. The same authors carried out systematic studies based on structure performance relationship (73]. They demonstrated the delicate interplay of the ste rie and electronic properties of the ligands ta reach catalysts presenting high activity and high selectivity for 1-hexene (examples Il, Ill and IV, Figure 12). If none of these Cp-Ti{IV) complexes was operated at an industrial scale (ta the best of our knowledge), their discovery represents a real breakthrough in the field of selective oligomerization using Ti complexes.

Il

Ill Figure 12: Examples of 11 Cp-Ti{IV)" complexes for selective ethylene trimerization Detailed theoretical studies were also conducted in arder to shed light on the mechanism [74,75,76]. After activation of the catalyst precursor [(!1 5 Cp-CMe 2 -C 6 H 5 )TiCI 3 ] with MAO to generate a cationic Ti(ll) species, a Ti(II)/Ti(IV) metallacycle mechanism was predicted to operate. Mechanistic investigations a Iso pointed out the critical raie of the a rene side arm of the Cp ligand. This part of the ligand has the capacity to modify its coordination mode to the titanium center from 11 DFT studies tried to rationalize selectivity for 1-hexene taking into account the capacity of the Rgroup grafted on the Cp ring to dissociate from the metal center [78]. Assuming that, whatever the nature of the R-group, the catalyst enters the metallacyclic mechanistic pathway, the authors have presented the dissociation energy of the arene group as a descriptor to explain 1-hexene selectivity.

A madel is proposed that predicts the formation of 1-hexene when the dissociation energy is larger than 15 kcal/mol. ln the case of smaller dissociation energies, multiple insertion reactions are likely to occur producing polyethylene as the major product.

Shortly after the discovery of [(!1 5 Cp-CMerC 6 H 5 )TiCI 3 ]/MAO system, substitution of the arene group by chelating heteroatoms was proposed. ln 2003, Huang et al. replaced the pendant arene with a thienyl group (Figure 13, V) [79]. Selectivity for 1-hexene appeared at a very high leve! {84%) with nevertheless apparent poor activity compared with the a rene analogue. Substitution of the sulfur by a more coordinating ether group {Figure 13, VI) renders the system quite inactive [80]. The same group has recently proposed a new evolution of this catalyst by introducing indenyl group instead of cyclopentadienyl [81]. This evolution is assumed by the authors to improve the activity compared with the corresponding cyclopentadienyl complex {Figure 13, VIl). Viewing these first examples, the ligand hemilability [82] appeared as a key parameter to promote selective ethylene oligomerization rather than polymerization. Owing to the expected reversible coordination of the pendant group, such ligands could stabilize highly reactive electrophilic metal centers until the substrate coordinates and replaces the pendant group. lntroduced as alternative to cyclopentadienyl ligands for the development of post metallocenes group 4 complexes [83], aryloxybased ligands have also raised a great deal of interest in that field, partly due to their versatile coordination mode and their potential to bring 5 electrons to the metal center {Figure 14) [START_REF] Bradley | Aikoxo and Aryloxo derivatives of meta/s[END_REF][START_REF] Gibson | [END_REF]

. R R R 1 1 1 ,,,,Q,,,, ,,,,,, 0 ,,,,,, 0 M
'' ~ ''M M M - M M bridging ).! 3 bridging ).! 2 terminal R R R 1. 1. 1 _.........o.:
""o: Depending on the structure of the aryloxy, mononuclear as weil as binuclear complexes were obtained (Figure 15). The hemilabile behavior of the aryloxy ligand in IX (Figure 15) resulting from reversible coordination of the -NMe 2 arm was demonstrated by variable-temperature 1 H NMR spectroscopy. Upon activation with MAO these catalysts appeared poorly active with respect to ethylene with a selectivity oriented towards polymers (>95%) [87]. However, when activated with A1Et 3 (3 eq./Ti) at 20 bar and 60 oc, these complexes exhibited interesting activity (up to 2100 g/gTi/h) for the selective dimerization of ethylene to 1-butene {92% butenes; 99+% 1-butene). Noticeable differences in catalyst activity were observed when the hemilabile part was modified. This work was extended later to functionalized bis(aryloxy)-Ti(IV) complexes of general formula [(ArO-LhTi(O;PrhJ with the same catalytic tendencies [88]. ln parallel with this study, investigations on functionalized imido Ti(IV) complexes were also undertaken [89]. Upon activation with MAO, ali the catalysts evaluated appeared selective for ethylene polymerization. ;Pro,,,, for selective ethylene dimerization ln the field of selective ethylene trimerization using titanium catalyst, another very recent breakthrough is the work published by Fujita and Mitsui company [90,91]. Since many years, Fujita and co-workers developed a ligand oriented research with the use of phenoxy-imines type ligand in group 4 polymerization {named FI systems) [92]. ln the course of their investigations, they found that very specifie phenol imine ligand (Figure 16) can convert ethylene into 1-Hexene with high selectivity and quite high productivity. To the best of our knowledge, this system has been implemented at the industrial scale at lshihara (Japan) [START_REF]Chemical Economies Handbook:Linear alpha-Oiefins[END_REF]. From this study, and the results published in the corresponding patent [93], it emerged that the nature of the bridging spacer linked to the hemilabile ether group is crucial for determining the selectivity for 1-hexene. Variation of 1-hexene selectivity from selected examples extracted from [93] is presented in Figure 16.

(j) v M M M M-0-R = 109° M-0-R = 120° M-0-R = 180°
N R N -! ~~ _...C l :n / l 'cr Cr tBu !Bu 1 0% 0% 25% 50% 75% 100%
% wt of 1-hexene in the products distribution The authors proposed that the reaction mechanism goes through a metallacycle formation from a cationic Ti(ll) intermediate [90]. lnterestingly, catalyst productivity shows a second-arder dependence on ethylene pressure, suggesting that the rate-determining step is the formation of the Ti(IV) metallacyclopentane intermediate. Along the same line, Bercaw et al. recently reported the synthesis and characterization of the cationic [FI-TiMe 2 nMeB(C 6 Fshr complex generated from the most selective Fujita's pre-catalyst. This complex also proved to be an effective pre-catalyst for the ethylene trimerization reaction [94,95].

More recently, McGuinness et al. published titanium catalysts, obtained by combination of phenoxyimine moiety from the FI catalysts and the arene group characteristic of Hessen catalysts [96]. A series of Ti{IV)-catalysts were prepared and tested for the ethylene oligomerization and polymerization reactions (Figure 17). Predominantly these catalysts formed polyethylene with small amount of 1-hexene in sorne cases. catalysts towards ethylene oligomerization reaction [97]. From ali the catalysts considered in this study (Figure 18), complex bearing a thioether as donor group gave the best results in term of selectivity for 1-hexene, however polyethylene was still the main product of the reaction.

V-ZIRCONIUM CATALYSTS FOR UNSELECTIVE ETHYLENE OLIGOMERIZATION

Along with group 4 complexes, zirconium-based systems appeared very early as efficient catalysts for olefins polymerization [83]. Cyclopentadienyl-based complexes, once activated with MAO, are among the most active and selective zirconium catalysts for both ethylene and propylene polymerization . ln comparison to this literature, examples of zirconium catalysts for ethylene oligomerization remains scarce. Processes operating with this metal were nevertheless industrially developed to produce mainly C4 to C12 olefins in order to maximize the amount of a-olefins in the co-monomer range.

Processes developed by SABIC-Linde (AiphaSablin [START_REF] Meiswinkel | [END_REF]), ldemitsu [START_REF]Chemical Economies Handbook:Linear alpha-Oiefins[END_REF] or IFPEN/ Axens (Al phaSe lect [START_REF] Forestiere | Oil Gas Sei[END_REF]4]) are by far the most widely described. These processes operate in the liquid phase using proprietary soluble Zr-based catalytic systems activated in situ by an alkylaluminum alkyl co-catalyst, affording ethylene oligomers having a Schulz-Fiory type distribution. The propensity of zirconium to modulate product distribution makes processes based on this metal versatile and useful to access specifie a-olefins distribution (Table 1). The role of the non-MAO aluminum co-catalyst was described as a key parameter to control the product distributions. Detailed study on this aspect was published in 2002 by Shiraki and co-workers [98] . He describes particularly the role of organic additives on the performances of the three components "ZrCI 4 -TEA-EASC" oligomerization catalyst. The effect of organic additives was discussed on the basis of the ir ionization potential {IP) and electron density (ED). As exemplified, additives with small IP (tetrahydrothiophene) improve the purity of the a-olefins. Heterocompounds additives presenting large electron density (tetrahydrofurane) have detrimental effect on the polymer amount produced.

Up to now, attempts to selectively oligomerize ethylene to 1-butene, 1-hexene or 1-octene with zirconium catalysts were quite unsuccessfull [7]. Shortly after the discovery of Cp-based titanium catalysts for selective trimerization of ethylene to 1-hexene [72], theoretical calculations were performed to evaluate the potential of zirconium and hafnium analogs to selectively produce 1hexene and/or 1-octene [99,100,101]. Calculations predict 1-hexene as the major product for zirconium catalysts.

VI-IRON BASED CATALYSTS FOR UNSELECTIVE ETHYLENE OLIGOMERIZATION

The ability of iron catalysts to form carbon-carbon bonds from unsaturated compounds was initially reported by Hata in the 1960s, focusing on dienes as butadiene or isoprene [103,104,105]. Mainly monodentate donor ligands as PPh 3 have been investigated in Ziegler-Natta type systems comprising an iron{lll) precursor with an alkylaluminum as AIEt 3 . Bipyridine [106] followed by diimine ligands [107,108] were then described by Misono and tom Dieck, respectively, for the cyclodimerization of butadiene to cyclooctadiene and vinylcycloohexene. Ali these homogeneous systems are suggested to lead to an iron(O) active species as co-catalysts having reductive properties are mainly used such as AIEt 3 , or Grignard reagents. Until the late 1990s, the reactivity of iron towards mono-olefins was not reported to the best of our knowledge. The breakthrough independently reported by the groups of Brookhart [109] and Gibson [110] triggered the "iron age" for the transformation of olefins, especially the oligomerization and polymerization of ethylene. This surprising reactivity arase from the fine association of a tridentate bis(imino)pyridine ligand with an iron(ll) pre-catalysts activated by a methylaluminoxane, leading to exceptional activities along with an excellent alpha selectivity for the olefins produced (Figure 19). Moreover, the easy tuning of the ligand structure led to a broad range of products from law molecular weight linear alpha olefins to high molecular weight linear polyethylenes. The technology was commercialized under the trademark of Vers ipol by DuPont which offers the technology for licensing [111] for high density polyethylene production and a range of olefin oligomers (Schulz-Fiory distribution). However, no commercial unit was announced yet. lt seems that operating this kind of highly active catalytic system is still challenging and involve the development of novel approach to resolve the technical challenge [112]. Detailed recent reviews caver ali the complex modifications that have been studied [113,114], but also bring to the foreground severa! limitations of the ironcatalyzed ethylene oligomerization . While the most active systems are based on an iron(ll) precursors bearing a tridentate neutra! ligand and activated by a methylaluminoxane, they present by-product and stability issues, particularly in temperature . Considering the different iron(ll) precatalysts reported in the literature, the donor-functionalized diimine-and phenantroline-based complexes appear among the most promising systems when activated by (modified) methylaluminoxane, leading to wax-or polymer-free distributions of oligomers (Figure 19) [115,116]. IFPEN was interested in the development of mono-anionic tridentate ligand and obtained N,NH,N ligands based on a 1,2-dihydro-1,10-phenanthroline scaffold from condensation of 2-acetylpyridine with 8-aminoquinoline derivatives [119]. ln presence of a base as Buli, these ligands were coordinated on an iron(lll) precursor leading to a pseudo-square pyramidal coordination geometry (Figure 21) [120]. While little to no activity was observed in presence of MAO at 40°C and 30 bar of ethylene, at 80°C, a steady ethylene uptake was noticed over 2 hours. Ethylene was transformed ta short-chain oligomers with up ta 63 wt. % of butenes compared ta ali products formed. Polymer production {12 wt. % / ali products) was also observed accompanying the product distribution that may suggest multiple active species. The iron{lll) complex coordinated by the tridentate ligand under its neutra! form and the corresponding iron{ll) complexes appeared ta be inactive under ethylene in presence of MAO, highlighting the synergistic combination of the anionic ligand with the oxidation state of the iron precursor. Starting from the iron(ll) precursor bearing the N,NH,N ligand, we investigated an oxidative path ta access iron{lll) pre-catalysts. ln solution in acetonitrile and with bubbling 0 2 , the pink solution containing the iron{ll) complex progressively turned ta a dark purple mixture [121]. The characterization by IR of the complex formed showed the absence of proton. By analogy, we performed the same oxidation reaction on an iron{ll) 1,3-bis(2'-pyridylimino)isoindoline complex and the 11-oxo-bridged diiron(lll) complex could be isolated and characterized by XRD (Scheme 6). Such species has already been described although the synthesis was performed with an equimolar amount of the ligand and FeCI that the cationic alkyl iron{ll) species is formed in the activation process, we highlighted in this experimental and theoretical study the mechanisms favoring or inhibiting ethylene oligomerization. A weak interaction in the ion pair is preferred as observed for the well-defined co-catalyst and a representative MAO model leading to active catalyst, whereas trivalent aluminum sites as in AIMe 3 may favor the adduct formation leading to catalyst inactivity. Recently, we extended this approach to diol and bis{phenol) compound. We observed that in association with AIMe This hypothesis was supported by a DFT study from Tobisch who described a mono-imido tungsten complex as an effective catalyst when chelated by a Lewis acid [135). The bis(imido) tungsten compound associated to a Lewis acid was also investigated and may be considered as an efficient system for olefin dimerization [136). This study highlighted as weil that one imido may be released leading to the active mono-imido tungsten catalyst. To confirm or infirm the bis(imido) tungsten complex activity, Hanton, Dyer et al. reported, few years later, the synthesis of well-defined bimetallic complexes [PhN=WMe 2 (=N(Ph)AIMe(X)(11-CI))] (with X = Cl or Me, Figure 25) [137).

However, these complexes proved ta be inactive in presence of ethylene. The in situ generated catalyst although ill-defined remains the most efficient system to promote olefin dimerization. A detailed catalytic study from Sasol led to selectivity above 99 %for dimerization of 1-pentene to 1nonene depending on the catalyst components and their stoichiometry [138). Having a closer look ta the products formed, the authors proposed that the Cossee-Arlman mechanism is effective, supported by a C 2 H 4 /C 2 D 4 co-dimerization experiment leading ta full isotopie scrambling. lt should however be noted that a second arder dependence on substrate is a Iso described that still questions the mechanism ta consider. Although a series of patents was published in 2007 by Sasol [139), relative ta olefin dimerization for methyl branched compounds production, no press release or announcement for a process commercialization was published.

VIII-CONCLUSION

Since its discovery 60 years ago, the transition metal homogeneous catalyzed oligomerization of light olefins has remained a tapie of prime importance in academia and industry. The oligomerization of ethylene ta Linear Alpha Olefins is one of the main growing area of homogeneous catalysis owing ta the increasing market demand for LAO, most notably as co-mono mers for PE. lt is one of the applied catalysis a rea that has seen the emergence of the main scientific breakthrough and developments of novel industrial processes such as for the first time the ethylene tetramerization. One of the outstanding feature of oligomerization is that many transition metals, representative of group 4 to group 10, have shawn their ability to be effective catalyst. Despite an abundant literature, only sorne of them display exceptional activity and selectivity, highlighting that selective ethylene dimerization, trimerization and tetramerization are far ta be trivial and well-understood reactions. This opens avenues for cutting edge research of a broad scientific community. Moreover, the demand for the selective formation of short chain alpha olefins poses challenges at the interface of different science: coordination, organometallic chemistry, catalysis, modeling and process design .... From the early days, IFPEN has recognized the potential of oligomerization and followed it from basic research ta developments. Armed with this experience and in the light of the industrial achievements, we attempted in this report to show, with selected examples, the specificity and deficiency of each metal and the corresponding catalytic systems. For example, the same metal can react via different mechanisms and drive the transformation to polymerization or oligomerization depending on operating conditions or co-catalyst applied. The challenge continued to a greater control of the selectivity and to the possibility to design "on purpose processes". The right combination of metal-ligand-activator remains the key of the discovery even if the development of new ligands has been essential. There is no single rule for ligand design and subtle changes can switch the reaction outcome. One may anticipate that in the forthcoming years, studies on the fundamental and applied aspects, will allow the finding of new oligomerization catalyst generation with improved efficiency and sustainability.

  (a) Typ1cal d1stnbut1 on , m brackets poss1ble fl ex 1b1hty (b) poss1ble d1stnbut1on of LAOs after ohgom emat1on and before 1somemat1on and metathesis processing

Figure 1 : 1 -

 11 Figure 1 : The two main families of Ni active catalysts

11 , 1 Scheme 2 :

 1112 Scheme 2 : Symmetrical and non-symmetrical diphosphinoamine nickel complexes

Figure 2 :Scheme 3 :

 23 Figure 2: Examples of non-functionalized "Ni-NHC" complexes evaluated in catalysis

Scheme 4 :

 4 Scheme 4: Synthesis of [(CAAChNiCI 2 ]

Figure 3 :

 3 Figure 3: ORTEP view of [(CAAChNiCI 2 ] (CCDC 1030813). H atoms omitted for clarity. Thermal ellipsoids include 50% of the electron density.

Toluene DIPP = 2 , 6 -diisopropylphenyl Scheme 5 : 10 "

 26510 Scheme 5 : Synthesis of [(CAAC){Pyr)NiCI 2 )

Figure 4 :

 4 Figure 4: ORTEP view of [(CAAC){Pyr)NiCh) {CCDC 1030814). H atoms omitted for clarity. Thermal ellipsoids include 50% of the electron density

Figure 5 :

 5 Figure 5 : Examples of functionalized "Ni-NHC" complexes

Figure 7 : 2 -

 72 Figure 7: "Ni-oxime" complexes

Figure 8 :

 8 Figure 8: Suitable organometallic madel for SHOP catalyst {Type B)

Figure 9 :

 9 Figure 9 : Fine tuning of P-0 ligand with the occurrence of H-bonding {SF constant obtained in toluene, 0.10 mmol catalyst, 60-95°(, 60 bar ethylene, 90-300 min)

Figure 10 :

 10 Figure 10 : Ligand precursors for the selective ethylene trimerization processes

Figure 11 :

 11 Figure 11: Diphosphinoamine ligands for tri-and tetramerization of ethylene.

Figure 13 :

 13 Figure 13: Functionalized 11 Cp-Ti{IV)" complexes

Figure 14 :

 14 Figure 14: Coordination modes of aryloxy type ligands

Figure 15 :

 15 Figure 15: Examples of [(Ar0-L) 0 Ti{O;Pr) 4 • 0 ] for selective ethylene dimerization

Figure 16 :

 16 Figure 16: From polymerization to selective ethylene trimerization with tridentate FI-type catalysts

Figure 17 :

 17 Figure 17: Examples of functionalized aryloxy-imine Ti( IV) complexes ln another publication McGuinness et al. reported the investigations of analogues of Fujita titanium

Figure 18 :

 18 Figure 18: Fujita's analogues for ethylene trimerization and polymerization

  Experimental investigations performed later by McGuinness et al. did not confirmed these predictions as ali the zirconium-based catalysts evaluated produced only polyethylene [102].

R1Figure 19 :

 19 Figure 19: Bis(imino)pyridine, donor-functionalized diimine-and phenanthroline-based precursors for iron-catalyzed ethylene oligomerization .

R2

Figure 20 :

 20 Figure 20: Bidentate N,N ligands coordinated on iron(ll) precursor.

Figure 21 :

 21 Figure 21 : Anionic and neutra! N,N,N ligands on iron{lll) and iron( li) precursors.

3 .Scheme 6 :

 36 Scheme 6: Synthesis of 11-oxo-bridged diiron{lll) complexThe proposed 11-oxo-bridged diiron(lll) bearing a 1,2-dihydro-1,10-phenanthroline (Figure22) obtained by oxidation could transform ethylene upon MAO activation at 80oC and 30 bar, affording a C4 ta C 12 distribution of oligomers along with polymer {11 wt. %)[123].

Figure 22

 22 Figure 22 Proposed 11-oxo-bridged diiron(lll) complex.

Figure 24 :

 24 Figure 24 : Well-defined tungsten precursors for ethylene dimerization.

  ......./ '\ _....Al-

Figure 25 :

 25 Figure 25: Bimetallic bis(imido)complex.

Table 1 :

 1 LAOs by ethylene oligomerization processes -technology and market survey[START_REF]Chemical Economies Handbook:Linear alpha-Oiefins[END_REF]

	Company	Type of catalyst	Typical LAO	World capacity	Announced new capacities
	(industrial)		distribution (wt. %)	(2012, kt/y)	(> 2012, kt/y)
	CPChem	AIEt 3	C4-C1o =54 (49) (a)
		(1 step)	c12-C14= 18 (18)	1053	140
			c16-C2o+ = 28 (33)
	lneos	AIEt 3	C4-C 10 = 70-77
		{2 steps)	C12-C 14 = 21-28	565	375
			c16-C2o+ = 2
	Shell	Ni/P-O	C4-C10 = 54 (32)

  1

Table 2 :

 2

	LAO produced	Process/ company	Catalyst type	Capacity (kt/y) (bl
	1-Butene	AlphaButol /Axens	Ti/AIR 3	708
	1-Hexene	CPChem	Cr proprietary/ AIR 3	397
	1-Hexene	AlphaHexol/ Axens	Cr proprietary 1 AIR 3	50 (c)
	1-Hexene	Mitsui	Ti proprietary /"MAO"	30
	1-0ctene/1-Hexene	Sa sol	Cr proprietary/ "MAO"	100

"On purpose" processes(al for the selective production of 1-butene, 1-hexene and 1-octene (a) only commercialized processes are cited he re (b) include planned capacities {c) total capa city for 2 units AlphaButol et AlphaHexol

  Tungsten-based catalysts were described since the 1960s as active for the metathesis of olefins[127]. lnterestingly, Menapace and coworkers from Goodyear showed in 1975 that for WCI 6 in presence of aniline a reaction switch from metathesis to dimerization of 2-pentene is observed depending on the AI/W ratio[128]. Applied to ethylene, excellent selectivities in 1-butene {> 98% compared to ali products formed) were afforded by refluxing WCI 6 in presence of 2 equivalents of aniline in chlorobenzene and further activated by Et 2 AICI {AI/W = 80) at 34 bar and 40°C [129]. More sterically hindered 2,6-dimethylaniline introduced in similar conditions as described above led to significant enhancement of the activity with up to 2.8.10 4 gc 2 H 4 /{gw.h) wh ile maintaining the 1-butene selectivity to 98% [130]. ln the meantime, Goodyear filled several patents related to propylene dimerization describing catalyst additives. Prior to the introduction of the aniline compound, the tungsten precursor is mixed with an carboxylic acid [131], a phenol [132] or a diketone [133) molecule leading to active catalysts in association with ethylaluminum sesquichloride {EASC) for example, however no peculiar selectivity is observed . With such approach, uncertainties remain regarding the nature of the catalyst precursor, hampering comprehension and to formulate hypothesis on the activation step and the catalytic mechanism in operation. Starting from WOCI 4 , IFPEN synthesized and isolated the mono-imido complex [{2,6-Me2C6H3)N=WC14] {Figure 24) [134). Moderate activities were observed over more than 4 hours {up to 3.4.10 3 gc 2 H4/{gw.h)) affording good selectivity for ethylene dimerization at 40°( and at 60 bar when activated by Et 2 AICI or EtAICh at a AI/W molar ratio of 40.Two catalytic mechanisms may be considered for high selectivity in 1-butene. They may be initiated whether by the formation of metal alkyl species leading to a Cossee-Arlman type mechanism or by the reduction of the tungsten precursor followed by the concerted coupling of two molecules of ethylene. Thus IFPEN prepared and isolated the law oxidation state imido complex, stabilized by PMe 3 , [PhN=WCh{PMe 3 h) {Figure 24), inactive in presence of ethylene alone but that led to ethylene dimerization in presence of 4.5 equivalents of AICI 3 , suggesting that the metallacyclic mechanism may be in play.

	~ Q N N Cl Il Cl Cl......._ Il /Cl ' w...... _.,w., Cl/ "-cl Me3P t PMe3 PMe 3	3 , they form well-defined
	tri-aluminum complexes as described by Ziemkowska {Figure 23) [125,126]. ln presence of iron{ll) or
	iron{lll) complexes, active catalysts are generated affording Schulz-Fiory distribution of oligomers
	comparable to the ones observed with MAO.
	\1 o-<:>-o 1\	~ / \,t O o<I>~ o/ )a-v '-...Al ~••• , ~~
	Figure 23 : Well-defined co-catalysts for the iron-catalyzed oligo
	VIl-TUNGTEN BASED CATALYSTS FOR SELECTIVE ETHYLENE DIMERIZATION