Rx Résolues Ensuite

Je Remercie

Caroline Toppan

Pierre Lavedan

Marc Vedrenne

Benoît Champin

Paul-Louis Fabre

Olivier Reynes

David Lafarge

Je Remercie Marta

Laura

Antonio

Tatsuya

Natalia Amos

Noemi Sophie

Dr Nicolas Mézailles

Dr Noël Nebra-Muñiz

S S Burant

J Iyengar

M Tomasi

N Cossi

N J Rega

M Millam

J E Klene

J B Knox

V Cross

C Bakken

J Adamo

R Jaramillo

R E Gomperts

O Stratmann

A J Yazyev

R Austin

C Cammi

J W Pomelli

R L Ochterski

K Martin

V G Morokuma

G A Zakrzewski

P Voth

J J Salvador

S Dannenberg

A D Dapprich

Ö Daniels

J B Farkas

Nickel

Cy Cy

du bon fonctionnement du laboratoire avec bonne humeur

Keywords: br, broad, s, singlet, d, doublet, t, triplet

Amor Rodríguez Iglesias d'avoir accepté de juger ce travail de

microscope using Stalke's X-Temp 2 device. [1] Phi-and omega-scans were used. Space groups were determined on the basis of systematic absences and intensity statistics. Semi-empirical absorption correction was employed. [2,3] The structures were solved by direct methods (SHELXS-97), [4] and refined using the least-squares method on F². All non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were refined isotropically at calculated positions using a riding model with their isotropic displacement parameters constrained to be equal to 1.5 times the equivalent isotropic displacement parameters of their pivot atoms for terminal sp 3 carbon and 1.2 times for all other carbon atoms.

The thermal ellipsoids determined by single crystal X-ray diffraction in this thesis are at the 50 % probability level. The following color code indicating the respective atom type in the depicted molecular structures is used throughout the manuscript: hydrogen: white, carbon: grey, nitrogen: blue, oxygen: red, phosphorus: orange, nickel: dark green, aluminium: light pink.

Theoretical calculations

Theoretical calculations were performed by Dr. Nicolas Mézailles (Toulouse) at the DFT level theory using the Gaussian 09 program package. [5] Basis sets and functionals are detailed in the respective context. Cross couplings reactions have advanced to a major tool in synthetic organic chemistry and offer a very efficient method for C-C or C-heteroatom bond formation. [1] Cross coupling reactions find important applications among others in the pharmaceutical [1,2] and agrochemical industries as well as in material science for the preparation of polymers [3] , liquid crystals [4] and advanced materials. Richard Heck, Ei-ichi Negishi [5] and Akira Suzuki [6] were rewarded in 2010 with the Nobel Prize for their pioneering work in the field. More than 40 years after their discovery, the research of always more efficient catalytic systems, which couple ever more challenging substrates still remains a very active field.

Metal catalyzed cross coupling reactions consist in the reaction between an electrophile, commonly an aryl halide and an organometallic species that acts as a nucleophile, as shown in Scheme 1.1. The most active systems known to date involve palladium catalysts. [7,8] Scheme 1.1: General scheme of cross coupling reactions.

Among them, the Negishi reaction employs zinc derivatives as nucleophiles, which present several advantages. First of all, zinc derivatives have a relatively low toxicity and a very large functional group tolerance compared to organolithium and Grignard reagents. [9,10] Furthermore, a broad scope of zinc compounds is now available. Knochel especially contributed to the development of synthetic strategies to access a wide variety of derivatives, which are increasingly getting commercialized. [9] Additionally, Negishi couplings usually require mild conditions [9,10] and can be performed with several transition metals such as palladium and nickel, but also less frequently used iron and cobalt.

Advances in palladium catalyzed C sp2 -C sp2 Negishi cross couplings

Since the first report of a palladium catalyzed cross coupling between aryl halides and zinc derivatives by Negishi in 1977, [11] the field has attracted a lot of attention. [9] Scheme 1.2: First palladium catalyzed Negishi cross coupling. [11] Great progress has been made in the Csp2-Csp2 coupling of aryl chlorides starting from the early 2000s. In 2001, Fu reported the first general procedure for the Negishi cross coupling between aryl chlorides and aryl zinc chlorides. [12] The commercially available and air stable palladium catalyst [Pd(P(tBu)3)2] successfully promotes the coupling of ortho substituted, heteroaryl and vinyl substrates at only 2 mol% of catalyst loading in a mixture of THF and NMP at 100 °C, with TON greater than 3000. [12] Cl + ZnCl 2 mol% [Pd(P(tBu) 3 ) 2 ] THF/NMP, 100 °C

= 76 -97 % R 1 R 1 R 2 R 2
Scheme 1.3: First general procedure for the palladium catalyzed Negishi cross coupling of aryl chlorides with zinc derivatives reported by Fu. [12] The following procedures developed by Buchwald [13,14] and Organ [15,16] count to the most efficient and general systems known to date for the palladium catalyzed Negishi coupling of aryl chlorides.

The [Pd2(dba)3]/RuPhos system [13] and the palladacycle precatalyst supported by the dialkylbiaryl monophosphine XPhos [14] designed by Buchwald couple a wide range of otherwise challenging heteroaryls and functionalized substrates under mild conditions.

[Pd2(dba)3]/RuPhos allows for efficient production of biaryls at 0.01 mol% catalyst in THF at could even be dropped down to 0.025 -0.050 mol% at RT with the XPhos ligated palladacycle for the coupling of aryl halides, as shown in Scheme 1.4. [14] Scheme 1.4: Highly efficient Negishi cross coupling catalyzed by a XPhos palladacycle precatalyst designed by Buchwald. [14] While Buchwald used a phosphine derived catalyst, Organ synthesized in 2006 the easily affordable, air stable and highly active NHC based palladium precatalyst PEPPSI-IPr, [15] which enables Csp2-Csp2, Csp2-Csp3 and Csp3-Csp3 cross couplings of a very broad scope of iodide, bromide, chloride, triflate, tosylate and mesylate substrates. between aryl and heteroaryl bromides and chlorides and aryl zinc derivatives. [15,16] The reaction has a large functional group tolerance and affords the corresponding biaryls in high yields. [15,17] The related PEPPSI-IPent precatalyst disclosed in 2010, can even couple some aryl and heteroaryl bromides and chlorides at 0 °C and tolerates strongly sterically hindered substrates yielding tetra ortho substituted biaryls. [16] All the examples presented above support that the palladium catalyzed Negishi cross coupling has advanced to a very powerful tool in the 2000s, that is now widely applied in modern synthetic organic chemistry.

1.2 Nickel catalyzed C sp2 -C sp2 Negishi cross couplings

1.2.

Advantages and challenges of nickel chemistry

Palladium mediated reactions and especially cross coupling processes have been extensively investigated and are well understood but the corresponding nickel chemistry remains far less explored.

Concern can be raised about the cost and low abundance of palladium. Nickel is therefore often considered as a cheap alternative to noble metal chemistry. Indeed, bulk nickel costs about 2000 times less than palladium [18] and most Ni(II) precursors are two orders of magnitude cheaper than their palladium analogues. Nevertheless, some Ni(0) sources such as commonly used [Ni(COD)2] can still be quite expensive.

Nickel as a first row transition metal possesses a small atomic radius and usually short metalligand bonds. Overall, nickel has a rather hard character compared to its heavier group 10 counterparts. [18] Furthermore, as a late transition metal, nickel is very electron rich and much more electropositive than palladium. This makes oxidative additions more favorable, [18,19] even allowing insertion of nickel in otherwise quite unreactive derivatives such as alkyl halides, esters and ethers. [18,20] Additionally, Ni-C bond dissociation energies (BDE) lie in general lower than Pd-C BDEs, also resulting in low activation barriers for reductive eliminations. [19] However, the eliminations are often only slightly exothermic. In contrast reductive eliminations on palladium complexes require higher activation energies but are strongly exothermic, rendering the processes highly favorable and irreversible. [19] Moreover, more oxidation states are readily available to nickel complexes. Whereas most palladium cross coupling mechanisms are polar and involve exclusively Pd(0)/Pd(II) or Pd(II)/Pd(IV) intermediates, stable nickel compounds are commonly found in oxidation states ranging from 0 to +III. [18,19] Nickel has also a greater propensity to homolytic bond cleavage than palladium, allowing SET and radical processes. [18,19] A wide variety of mechanisms has thus been disclosed, with Ni(I)/Ni(III) cycles being predominant for the couplings of aryl halides with mono-phosphine substituted nickel complexes. [21,22] Ni(0)/Ni(II) cycles are observed for cross couplings with phenol derivatives [23,24] as well as for the amination of aryl chlorides using [(BINAP)Ni(η 2 -NC-Ph)]. [25] Even Ni(0)/Ni(I)/Ni(II) reaction pathways have been reported for the Negishi cross coupling of alkyl halides [26,27] and an only Ni(I) based reactivity for the reductive cleavage of aryl ethers with silanes. [28] Nickel mediated reactions are therefore primarily cost-efficient compared to palladium catalysis but even more importantly they provide an extremely rich and active chemistry, despite the carcinogenic character of numerous nickel complexes. It might be difficult to predict, to control and to understand nickel chemistry but when it is fine-tuned it turns out very efficient and can be able to perform challenging reactions. [19] 1.2.2 Advances in nickel catalyzed C sp2 -C sp2 Negishi cross couplings

The first nickel catalyzed Csp2-Csp2 Negishi cross coupling was reported as early as 1977. By derivatives in good to excellent yields within 1 -2 h. at RT. [11] Since then the field has been ever growing [10] and this paragraph will focus only on the coupling of aryl halides.

Scheme 1.6: First nickel catalyzed Negishi cross-coupling reported in 1977. [11] Research aimed at using cheap and largely available electrophilic coupling partners as well as lowering the catalyst loadings.

On one side, lots of efforts were especially oriented towards the coupling of cheaper and readily available aryl chlorides in replacement for aryl bromides and iodides. Aryl chlorides are known to be more difficult to couple because of the poorer reactivity of C-Cl bonds compared to C-Br and C-I bonds. Miller reported in 1998 the first Negishi coupling of aryl chlorides catalyzed by 2 mol% of [Ni(acac)2]/dppf. [29] Contributions by de Vries [30] and Kappe [31] followed in 2002 and 2004.

On the other side, nickel catalyzed Negishi reactions have become increasingly efficient in the last ten years, now being able to compete with their palladium counterparts. [10] Thorough ligand optimization initiated by Knochel in 2005 -2006 allowed for the drastic lowering of the catalytic charge. [32,33] The combination of 0.05 mol% of [NiCl2], 0.2 mol% of diethylphosphite (EtO)2P(O)H and 0.2 mol% of DMAP efficiently catalyzes a broad scope of aryl, heteroaryl and alkenyl bromides, sulfonates and activated chlorides in a mixture of THF and Nethylpyrrolidinone (NEP) at RT. The products are obtained in good yields with only small amounts of homocoupled side product within 1 -48 h. [29,30] Scheme 1.7: Negishi cross coupling procedure developed by Knochel using a combination of [NiCl 2 ],

(EtO) 2 P(O)H and DMAP. [32,33] Extensive investigations led by Wang between 2007 and 2012 further disclosed the ability of pincer complexes to catalyze Negishi cross couplings involving chloro arenes and chloro pyridines. [34] . The first efficient and systematic coupling of aryl chlorides, presented in Scheme 1.8, was reported in 2007. 0.01 mol% of nickel amido pincer complexes in a 1:1 mixture of THF and NMP were sufficient to synthesize the corresponding biaryls in good to excellent yields at 70 °C. The reaction tolerates a large variety of functional groups except nitro and aldehyde substituents. Higher catalytic charges as well as higher temperatures were required to couple sterically hindered ortho derivatives with zinc reagents in good yields. [35] Subsequently, Wang reported Negishi couplings between aryl chlorides and aryl zinc chlorides under similar conditions using various nickel pincer complexes [36 -39] , while Chen disclosed the first nickel NHC catalyzed Negishi cross coupling. [40] and phenyl zinc chloride derivatives. [35] In 2011 Frech also described a very general and efficient procedure for the nickel catalyzed Negishi cross-coupling using diarylzinc reagents, in which both aryl moieties are involved in the reaction. [41] The highly active aminophosphine based nickel catalyst is generated by mixing NMP solutions of [NiCl2] and 1,1',1''-(phosphanetriyl)tripiperidine in air at RT. A very broad scope of activated, non-activated and sterically hindered ortho substituted aryl bromides and chlorides and even heterocyclic substrates were all coupled within 2 h. at 60 °C using 0.1 mol% catalyst. The catalytic loading could even be lowered down to 0.01 -0.05 mol% and/or RT conditions for several substrates. High conversions and high yields were almost commonly obtained. Compared to other systems no inert atmosphere is required as well as no excess of ligand or other additives making it the most efficient system nowadays. [41] X + R 1 X = Br, Cl an aminophosphine nickel catalyst. [41] More recently, the Negishi cross coupling between aryl fluorides and zinc reagents has also been achieved by Nakamura by combining [Ni(acac)2] with a diphosphine bearing an alkoxide moiety (POP) [42] as well as by Love [43] and Wang [44] using respectively [Ni(PEt3)2Cl2] and [Ni(PCy3)2Cl2].

The systems presented above count to date to the most general and efficient ones for the nickel catalyzed Negishi cross-couplings. Yet, the reaction mechanisms underlying these processes remain largely unknown.

Mechanisms of nickel catalyzed Negishi cross-couplings

Several mechanistic studies on nickel catalyzed Negishi cross couplings have been undertaken.

However, they still remain scarce partially due to the tendency of nickel to undergo SET and to generate paramagnetic species, which complicates the investigations. As previously described (1.2.2) a large variety of ligands, nickel precursors and complexes are able to promote efficiently Negishi couplings and provide a great multiplicity of possible mechanisms to achieve the transformation. Nevertheless, the general mechanism for nickel catalyzed Negishi processes should always involve as for palladium at least three elementary steps, which are oxidative addition, transmetallation and reductive elimination.

The mechanism of the nickel catalyzed Csp2-Csp2 coupling between the mono-phosphine substituted oxidative addition product [(PEt3)2Ni(Ar)(Br)] and aryl bromides was examined as early as 1979 by Kochi. [21] The reaction is initiated by a SET from the trans-arylnickel(II) halide complex to a free aryl bromide and proceeds then through Ni(I) and Ni(III) intermediates as shown in Scheme 1.10. [21] Scheme 1.10: Radical chain mechanism for the coupling of aryl halides in the presence of monophosphines. [21] During the first step of the mechanism, the active catalyst [(PEt3)3Ni I . [21] Additionally, Colon and Kelsey investigated in 1986 the nickel catalyzed coupling of aryl chlorides to symmetric biaryls in the presence of a reducing metal such as zinc. [22] Monodentate triarylphosphines and especially PPh3 turned out to be the best ligands for this reaction, where zinc plays a major role as quantitative coupling cannot proceed in its absence. [22] Scheme 1.11: Nickel catalyzed coupling of aryl chlorides in the presence of zinc. [22] The mechanism of the reaction slightly differs from the previous one but still predominantly involves Ni(I) and Ni(III) intermediates. The key step of this reaction is the reduction of an arylnickel(II)halide by zinc to prepare the catalytically active arylnickel(I) species [(PPh3)3Ni I (Ar)]. The reaction does not seem to involve any transmetallation between nickel complexes but instead zinc mediated reduction processes. [22] A simplified catalytic cycle is depicted in Scheme 1.12.

Reduction

Red. Elim.

Ox. Add.

Zn Ar Cl

Ar Ar Ar Cl Scheme 1.12: Ni(I)/Ni(III) based mechanism for the coupling of aryl chlorides to biaryls in the presence of zinc. [22] Furthermore, Wang [35] and Frech [41] also postulated radical chain processes and Ni(I)/Ni(III) based mechanisms for Negishi cross couplings relying respectively on nickel amido pincer [35] and [NiCl2]/1,1',1''-(phosphanetriyl)tripiperidine [41] catalysts.

As mechanistic investigations on nickel catalyzed Negishi cross coupling processes are still relatively rare, [10,45] the studies on closely related Csp3-Csp3 couplings are also presented here.

Two systems involving different types of pyridine containing ligands have been thoroughly investigated.

First of all, Vicic promoted alkyl-alkyl Negishi couplings between alkyl iodides and alkyl zinc bromides via the terpyridine nickel complex [(terpy) Ni(Me)]. [46] Scheme 1.13: Vicic conditions for the C sp3 -C sp3 Negishi cross coupling catalyzed by [(terpy) Ni(Me)]. [46,47] The analysis of the side products of the reaction evidences a dimerization process of the employed alkyl halides. Radical clock experiments converting iodomethylcyclopropane into olefinic products, additionally point towards a radical based mechanism. [46] The isolation and characterization of several [(terpy)Ni] complexes lead to the proposed catalytic cycle in Scheme 1.14. [26,46,47] Scheme 1.14: Catalytic cycle proposed on the basis of experimental investigations for the coupling of alkyl iodides with alkyl zinc bromides using [(terpy)Ni(Me)] as catalyst. [26,46,47] EPR measurements and DFT calculations evidence that catalytically active paramagnetic [(terpy)Ni(Me)] is a ligand based radical best described as a Ni(II)-methyl cation with a reduced anionic ligand rather than a Ni(I) complex. [26] In the presence of an alkyl halide, SET generates [(terpy)Ni II (Me)] + X -together with a free alkyl radical. The transmetallation of [(terpy)Ni II (Me)] + I -with alkyl zinc reagents leading to dialkyl Ni(II) [(terpy)Ni II (Me)(R)] intermediates could be ruled out. [26,47] Recombination of the radical with the nickel complex leads to Ni(III) intermediate [(terpy)Ni III (Me)(R)], from which the cross coupled product can be reductively eliminated. [(terpy)Ni I (X)] [46] which is generated alongside in the process reacts then with the transmetallating alkyl zinc derivative to regenerate [(terpy)Ni(Me)]. [26] Overall, the oxidative addition occurs after the transmetallation. Ni(I), Ni(II) and Ni(III) intermediates are all involved in this catalysis. DFT calculations performed by Phillips support the feasibility of this catalytic cycle with relatively low activation barriers. [48] Besides, Fu developed a powerful procedure for the coupling of alkyl electrophiles with zinc derivatives relying on a nickel/pybox catalyst. [27,49,50,51] Based on these results, Cárdenas studied in 2007 the [Ni(Py)4Cl2]/(S)-(sBu)-pybox catalyzed cascade cyclization-cross coupling of secondary iodoalkanes with alkyl zinc halides and provided strong experimental and computational evidence for a radical based mechanism. [52] Proofs include among others stereoselectivity studies, radical scavenging and radical clock experiments. [52] In addition, the mechanism of the [Ni]/pybox catalyzed stereoconvergent Negishi arylation of racemic propargylic bromides, experimentally investigated by Fu in 2014, [27] displays strong similarities with the mechanism earlier reported by Vicic. [26] Overall this literature survey shows that most nickel catalyzed Csp2-Csp2 and Csp3-Csp3 Negishi cross couplings tend to favor SET and radical processes which mainly generate Ni(I) and Ni(III) intermediates over polar Ni(0)/Ni(II) mechanisms.

Research objectives

The following chapter will be dedicated to the investigation of the Negishi cross coupling between aryl chlorides and phenyl zinc chloride derivatives catalyzed by [(dcpp)Ni] based complexes (dcpp = 1,3-bis(dicyclohexylphosphino)propane). The chelating bis-phosphine dcpp is the ligand of choice for this study. Indeed, strongly donating bis-dialkylphosphines are well known to stabilize unsaturated Ni(0) complexes unlike bis-diarylphosphines like dppe (dppe = 1,2-bis(diphenylphosphino)ethane) and dppp (dppp = 1,3-bis(diphenylphosphino)propane). Moreover, the propyl bridge enforces a large bite angle and provides an enhanced flexibility of the ligand around the nickel center, which further helps stabilizing Ni(0) complexes and intermediates. for mono-phosphine based cross couplings with aryl halides. [21,22] Scheme 1.17 represents both possible catalytic cycles. This mechanistic investigation would help to shed light into complex and unpredictable nickel chemistry and to improve the design of potentially new catalysts for even more efficient processes. This is achieved by performing stoichiometric reactions and following the procedure developed by Matthieu Demange in his thesis. [1] 31 P{ 1 H} NMR shows the fast disappearance of the free dcpp ligand II-1 at δ = -6,7 ppm. The product is gathered as an orange powder in 94 % yield. As Matthieu Demange reported, no NMR signal is observed in THF for the poorly soluble [(dcpp)NiCl2] complex II-3, suggesting that it has a predominant tetrahedral geometry. [1] However, NMR data could be recorded in CH2Cl2, in which the complex is fully soluble, showing that in this solvent the nickel center is in a square planar environment. The two geometries thus possess quite similar energies and the equilibrium between both can be shifted by changing the solvent. Single crystals for X-ray diffraction analysis were grown by diffusing petroleum ether in a CH2Cl2 solution of [(dcpp)NiCl2] II-3. The square planar geometry is favored in the solid state, as evidenced by the sum of the angles around the nickel center being equal to 360.21 °. The resolved structure presented above in Figure 2.1 is comparable to Matthieu Demanges one. [1] 2.1.2 Synthesis of Ni(0) complex [(dcpp)Ni(toluene)] II-4

Chelating [(bis-phosphine)Ni(arene)] complexes are known since Jonas [2] and Pörschke [3] reported their synthesis. The Ni(0) complexes are obtained either from Ni(II) or Ni(I) precursors which are reduced by alkali or alkali earth metals in the presence of aromatic compounds, as shown in Scheme 2.2. By following this procedure, Jonas managed to prepare among others monometallic [(dcpe)Ni(C6H6)] and [(dcpp)Ni(C6H6)], [2] which are characterized only by elemental analysis and Pörschke obtained bimetallic trigonal planar [[(dtbpe)Ni]2(C6H6)] [3] with the more sterically hindered tert-butyl substituted phosphines. ]. [3] In a similar way, the two electron reduction of [(dcpp)NiCl2] II-3 by KC8 in toluene, reported by Matthieu Demange, affords the targeted Ni(0) complex [(dcpp)Ni(toluene)] II-4 together with small amounts of [(dcpp)NiH]2 II-5. The yield of the 16 electron complex [(dcpp)Ni(toluene)] II-4 could be optimized up to 70 % by working with concentrated solutions, which favor the precipitation of the product. [1] As ] II-4. [1] Interestingly, Nobile already reported the reduction of [(dcpp)NiCl2] II-3 by stoichiometric amounts of sodium sand in toluene in 1992 and suggested the formation of the divalent 14

electron complex [(dcpp)Ni] on the basis of elemental analysis. [4] However, the experimental data and especially the 31 P{ 1 H} NMR rather indicate the formation of [(dcpp)NiH]2 II-5 as the main compound. Indeed, [(dcpp)NiH]2 II-5 is characterized by a singlet at δ = 25.0 ppm in 31 P{ 1 H} NMR and a quintet at δ = -10.8 ppm in 1 H NMR. [5] The small amounts of nickel ]. [3] Single crystals for X-ray diffraction analysis were grown from a concentrated ] II-4 and can be kept as a solid at RT over long period of time. [6] Scheme 2.4: Synthesis of [(dcpp)Ni(naphthalene)] II-6. [6] 2.2. Negishi cross coupling

Testing [(dcpp)Ni(toluene)] II-4 in Negishi cross coupling

The catalytic activity of the highly reactive [(dcpp)Ni(toluene)] complex II-4 was tested in Negishi cross coupling reactions. Cheap and commercially available aryl chlorides were chosen as electrophiles. The coupling of aryl chlorides still remains challenging because of the larger bond dissociation energy (BDE) of C-Cl bonds (95 kcal.mol -1 ) [7] compared to C-Br (79 kcal.mol -1 ) [7] or C-I bonds (64 kcal.mol -1 ), [7] which makes the oxidative addition to a metal(0) complex more difficult.

In a first test 4-chlorotoluene II-7 is reacted with phenylzinc chloride II-8 in the presence of cannot compete with Wang's nickel amido pincer complexes, [8] it still gets close to the performance of Frech's [NiCl2]/1,1',1''-(phosphanetriyl)tripiperidine system [9] and figures among highly efficient catalysts for the Negishi cross coupling of aryl chlorides. At this point it is important to verify that the oxidative addition product [(dcpp)Ni(Ph)(Cl)] II-14 is not involved in side reactions leading to homocoupling. Furthermore, it is essential to determine whether a transmetallation or a single electron transfer (SET) is more favorable at this stage of the reaction.

Mechanistic investigations

Kochi reported in the late 1970s the biaryl formation from the coupling between the monophosphine substituted oxidative addition product [(PEt3)2Ni(Ar)(Br)] and aryl bromides.

The reaction is initiated by a single electron transfer (SET) from the trans-arylnickel(II) halide complex to the aryl bromide and proceeds then through Ni(I) and Ni(III) intermediates. This SET process is kinetically more favorable than the transmetallation/reductive elimination sequence. [10,11] Colon and Kelsey analogously showed in 1986 that the nickel catalyzed coupling between two arylchlorides in the presence of a reducing metal such as zinc involves the reduction of the arylnickel(II) halide by the metal into an arylnickel(I) active species. [12] Therefore, the possibility of a redox reaction between the Ni Alternatively to the comproportionation, Ni(I) complexes can be obtained from the reaction between Ni(II) precursors and reducing agents such as sodium and potassium naphthalenide [13] or KC8. [14] The one electron reduction of [(dcpp)NiCl2] II-3 by 1 eq. of KC8, reported by Matthieu Demange, produces the desired [(dcpp)NiCl]2 dimer II-14 in 65 % yield. [1] Scheme 2.10: Synthesis of [(dcpp)NiCl]2 II-16. [1] The X-band EPR spectrum of paramagnetic [(dcpp)NiCl]2 II-16 was recorded at RT. A g-factor of 2.154 and a hyperfine coupling constant Aiso( 31 P, n = 2) = 72.2 G were determined. These parameters are in good agreement with the data previously measured by Hillhouse for the related [(dtbpe)NiCl]2 complex. [15] In addition, the crystal structure of the dimer II-16 could be resolved by X-ray diffraction analysis. [1] Two [(dcpp)nickel]-fragments are linked together by two bridging chlorides. Both Ni(I) centers are in a tetrahedral environment and no interaction is observed between both metallic atoms. The distance between both nickel centers (3.3134 Å) is greater than the sum of their van der Waals radii (3.26 Å). Hence, these stoichiometric reactions show that all elementary steps of the catalytic cycle:

oxidative addition, transmetallation and reductive elimination are facile at RT and in agreement with the mild catalysis conditions. Ni(0) and Ni(II) intermediates could be observed and/or isolated from the independent reaction steps and are favored over Ni(I) complexes. These results are in contrast to previously reported nickel catalyzed Negishi cross couplings, which are shown to involve Ni(I)/Ni(III) cycles with different ligand systems. [11, 16 -18] Nevertheless, in parallel to these investigations, Hartwig demonstrated in 2014 that the use of [(BINAP)Ni(η 2 -NC-Ph)] (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) in the catalytic coupling of aryl chlorides with amines and ammonia also enforces a Ni(0)/Ni(II) reaction pathway. [19,20] 2.3. ] during the course of the catalysis, [19] the dcpp ligand prevents the decomposition of the catalyst.

DFT calculations

The mechanism of the reaction was additionally investigated by DFT calculations. This study further supports the feasibility of the Ni(0)/Ni(II) catalytic cycle presented above in 2.3.3. The calculations were performed with the Gaussian09 suite of software, [21] using the B3PW91 functional, [22,23] the 6-31G* basis set for all non-metallic atoms [24] and the all-electron Def2-TZVP basis set for nickel and zinc. [25] Single point calculations allowed to take into account the solvent's effects and the dispersion interactions. In this paragraph the numbering of the molecules will be replaced by the letters A -H used for the calculations. The associative mechanism involves an 18 electron transition state TS AB2 , which features both the departing toluene and the incoming aryl chloride as η 1 ligands. Evolution to complex C

[(dcpp)Ni(PhCl)] occurs through the η 1 coordinated intermediate B 2 . The overall activation barrier for this pathway reaches 22.8 kcal.mol -1 and makes it therefore less favorable by 3.9 kcal.mol -1 than the previously described dissociative mechanism.

However, the entropy is inaccurately taken into account by DFT calculations. As the entropy increases in the dissociative mechanism but decreases in the associative mechanism through the transition state TS AB2 , it is overall difficult to draw any conclusion on the most favorable pathway.

Catalytic process

In the next step, the catalytic process itself was computed as depicted in Scheme 2.15. The Afterwards, the organozinc reagent PhZnCl(THF)2 coordinates to D and forms the heterobimetallic complex E, which lies 9.0 kcal.mol -1 lower in energy. Explicit THF molecules have been taken into account in order to maintain the zinc atom in a tetrahedral geometry. In complex E, the zinc is surrounded by a phenyl, a chloride, a THF molecule and a bridging chlorine atom from D. It is the strength of this bridging Zn-Cl bond that renders the coordination process exergonic. states with analogous short interactions have already been calculated for Pd-Zn [26,27,28] and Pd-Au/Sn-Au [29] species and were linked to lower activation energies. They are however absent in Pd-Sn transition states. [29] The transition state leads to F which lies 14.6 kcal.mol -1 higher than E. F is another heterobimetallic complex with a phenyl ring bridging the nickel and the zinc atoms. The reductive elimination of biphenyl from G is facile as the transition state TS GH lies only 13.2 kcal.mol -1 higher than G. In the Ni(0) complex H, the biphenyl remains coordinated to the nickel center in a η 2 fashion similar to the coordination observed for toluene in complex A.

Overall, each catalytic cycle is exergonic by 52.5 kcal.mol -1 . The rate determining step is the transmetallation with a moderate activation barrier of 21.8 kcal.mol -1 in good agreement with the mild reaction conditions. The transition state TS HB2 is located 29.7 kcal.mol -1 above H, which makes it 6.8 kcal.mol -1 higher in energy than the 14 electron intermediate B found for the dissociative pathway.

Product to reagent exchange

Therefore, an associative ligand exchange seems out of reach.

Overall, the low activation barrier (22.9 kcal.mol -1 ) evidences that the liberation of the biphenyl product and the regeneration of [(dcpp)Ni(PhCl)] C is a favorable and facile process in agreement with the mild reaction conditions, that allows to engage into further catalytic cycles.

Hence, the DFT calculations support the experimental observations and corroborate the presence of an active Ni(0)/Ni(II) catalytic cycle. The activation energies for all elementary steps are reasonably low, in excellent agreement with the mild catalytic conditions. The highest barriers that need to be overcome reach respectively 21.8 kcal.mol -1 and 22.9 kcal.mol -1 for the transmetallation and the product to reagent exchange, which makes them the rate determining steps.

Conclusion and perspectives

The synthesis of the Ni (0) It would be interesting to study the impact of the strongly donating bis-phosphine ligand dcpp on other nickel catalyzed reactions such as the Suzuki-Miyaura cross coupling. Efforts will be especially directed towards the investigation of the mechanism in order to determine whether the reaction performs similarly to the Ni(0)/Ni(II) Negishi cross coupling. Florian D'Accriscio extensively studied the Suzuki-Miyaura cross coupling during his PhD and showed that the Ni(0)/Ni(II) mechanism is highly substrate dependent. [6] Furthermore, the [(dcpp)Ni]-platform could be tested towards other challenging coupling reactions such as borylations or cyanations.

Experimental part 2.6.1 General remarks

All reactions were carried out under an atmosphere of dry argon using standard Schlenk techniques or in a nitrogen-filled MBraun LabStar glovebox. Dichloromethane, Et2O, pentane, THF and toluene were taken from an MBraun SPS-800 solvent purification system, freezepump-thaw degassed and stored over 4 Å molecular sieves. C6D6 and CD2Cl2 were degassed and stored over 4 Å molecular sieves.

All the chemicals were purchased in reagent grade purity from Acros, Cytech and Sigma-Aldrich and were used without further purification.

2.6.2 Synthesis of the dcpp ligand II-1 [1] The synthesis of the dcpp ligand II-1 was upscaled following a procedure described in Mathieu Demange's thesis. [1] n-BuLi (1.6 M in hexanes, 37.9 mL, 6.05×10 -2 mol, 1 eq.) is added at -78 °C to a 90 mL THF solution of dicyclohexylphosphine (12.0 g, 6.05×10 -2 mol, 1 eq.). The reaction mixture is allowed to come back to RT over 2 h. The solution turns yellow and a white precipitate falls out. 31 ] II-3 [1] The dcpp ligand II-1 (1.09 g, 2.50×10 -3 mol, 1 eq.) is added at RT to a THF suspension (30 mL)

of [(DME)NiCl2] II-2 (0.78 g, 2.50×10 -3 mol, 1 eq.). The reaction mixture is stirred overnight at RT and an orange solid fall out of the solution. Matthieu Demange reported a different workup: KCl and graphite are eliminated by filtration.

The solution is taken to dryness affording exclusively [(dcpp)Ni(toluene)] II-4 as a yellow powder in 83 % yield (170 mg). [1] 1 H NMR (300 MHz, C6D6): 0.86 -2.21 (m, 53H, Cy + CH2 + CH3), 7.01 -7.57 (m, 5H, Ph) ppm.

13 C{ 1 H} NMR (75 MHz, C6D6): δ 20.9 -39.2 (Cy + CH2 + CH3), 128.5 -136.2 (Ph) ppm.

31 P{ 1 H} NMR (121.5 MHz, C6D6): δ 15.4 (s) ppm.

Synthesis of [(dcpp)Ni(Ph)(Cl)] II-14

Chlorobenzene II-11 (10.2 µL, 1.0×10 -4 mol, 1 eq.) is added at RT to a 5 mL toluene solution is finally gathered as a yellow solid in 89 % yield (54.1 mg). Single crystals for X-ray diffraction analysis were grown from a THF solution layered with pentane at -25 °C.

Synthesis of [(dcpp)

NiCl] 2 II-16 [1] [(dcpp)NiCl2] II-3 (283 mg, 5.00×10 -4 mol, 1 eq.) and KC8 (78 mg, 5.00×10 -4 mol, 1 eq.) are suspended in 5 mL of toluene. The reaction mixture is stirred overnight at RT, KCl and graphite fall out of the solution. The toluene is filtered away, [(dcpp)NiCl]2 II-16 is further extracted with THF and taken to dryness. The paramagnetic Ni(I) complex II-16 is gathered as a light red product in 65 % yield (172.5 mg). Small pieces of lithium previously crushed with a hammer (307 mg, 4.42×10 -2 mol, 3 eq.) are added to a 15 mL THF solution of chlorobenzene II-11 (1.632 g, 1.45×10 -2 mol, 1 eq.) at -20 °C. The reaction mixture is stirred for 5 h. between 0 °C and -20 °C. The solution turns yellow to brown and a green precipitate is formed. In a second Schlenk ZnCl2 (1.98 g, 1.45×10 - 2 mol, 1 eq.) is solubilized in 8 mL of THF and cooled down to -20 °C. The phenyllithium is then cannulated to the ZnCl2 solution while staying at -20 °C. The reaction is further stirred overnight at RT and the solution turns light yellow. The volatiles are removed under reduced pressure and PhZnCl.LiCl II-8 is redissolved in 10 mL of THF. The concentration of the PhZnCl.LiCl solution II-8 is determined by titration with iodine, following the procedure developed by Knochel. [30] 2.6.4.2 General procedure for Negishi cross coupling

X-Band

In a typical experiment, the precatalyst [(dcpp)Ni(toluene)] II-4 is weighted in the glovebox and introduced in a Schlenk. THF is then added, followed by the appropriate amounts of aryl chloride and organozinc reagent. The reaction mixture is heated to 60 °C and its progression followed by GC-MS spectrometry. The volatiles are evaporated upon completion of the reaction and the product is extracted with dichloromethane. The organic phase is washed twice with water. The collected organic phases are dried over magnesium sulfate, filtered and concentrated on the rotary evaporator to afford the crude product. In the last step the product is purified by column chromatography.

A representative example is given below: The concentration of CO2 in the atmosphere has been growing exponentially since the beginning of the industrial area in middle of the 19 th century and has recently reached 400 ppm, which represents 2750 billion tons. [1,2] In percentage, CO2 is the fourth gas in the atmosphere after dinitrogen, dioxygen and argon and is especially known as the main greenhouse gas, which is responsible for global warming and climate changes.

Nowadays, the global industry is still primarily based on fossil fuels such as petroleum, coal and natural gas and releases 30 Gton.year -1 of CO2. Most organic products are also still synthesized through successive oxidations of hydrocarbons. [1,2] The conversion of CO2 into valuable chemicals could help reducing greenhouse gas emissions and developing sustainable alternatives to the fossil fuel based energy and chemical productions. First efforts were made in the late 1970s to functionalize CO2 into value added chemicals, but this research field especially attracted a lot of attention in the last 15 years.

Thermodynamic and kinetic stability of CO 2

CO2 is an abundant, cheap and non-toxic renewable resource, which costs only 15 -20 €/ton. [1] It has a linear structure with double bonds between the carbon and the oxygen atoms. Though the thermodynamic and kinetic stability of CO2 limits its reactivity. Indeed, most targeted chemicals lie higher in energy than CO2, which makes the reactions endergonic and activation barriers are commonly very high. [3] Therefore, catalytic processes, which generate highly energetic intermediates, are developed in order to overcome these difficulties. A lot of efforts are currently directed towards the reduction of CO2 into formic acid, [4,5] methanol [4,5,6] or methane [4,7] and the reductive functionalization of CO2 into value added chemicals such as amides or aminals. [8,9] Numerous homogeneous transition metal and organocatalyzed reactions have been described in the literature. [10,11] 3.1.3 Industrial production of chemicals from CO 2 CO2 already finds three main applications in large scale industrial processes for the synthesis of urea, carbonates and salicylic acid. [1,2] First of all, the Bosch-Meiser reaction provides 120 Mton.year -1 of urea, [12] which is used for the synthesis of fertilizers and polymers. During this process, ammonia and CO2 are first quickly converted to an intermediate carbamate, which slowly generates urea and water at high temperatures and high pressures, as shown in Scheme 3.1. [13] Scheme 3.1: Production of urea by the Bosch-Meiser process. [13] Furthermore, carbonates can be generated by the reaction between epoxides and CO2 (Scheme

3.2).

Polycarbonates are essential in the polymer industry, whereas cyclic carbonates make good solvents for lithium-ion batteries. [14,15] Scheme 3.2: Production of linear and cyclic carbonates from the reaction between epoxides and CO 2 . [14,15] Finally, salicylic acid is synthesized from phenol and CO2 in the Kolbe-Schmitt reaction by successive basic and acid treatments. The esterification of salicylic acid by acetic anhydride further affords aspirin ® . [16] Scheme 3.3: Synthesis of salicylic acid from phenol and CO 2 . [16] 3.1.4 Design of new reactions involving CO 2

Nowadays, 180 Mton.year -1 of CO2 are used for the production of chemicals, which represents only 0.6 % of the total amount of CO2 released in a year. From an academic and industrial point of view, an equally interesting challenge is the catalytic production of α,β-unsaturated acrylic acid from ethylene and CO2. Acrylic acid is currently produced in a two stage process by the catalytic oxidation of propylene over a mixture of metal oxides, as presented in Scheme 3.4.

Propylene is first reacted with molecular oxygen to acrolein, which is further oxidized to acrylic acid. [17,18,19] The global market for acrylic acid reaches 5.4 Mton.year -1 and is particularly relevant for the polymer industry. Sodium acrylate can be functionalized into water absorbent polymers commonly used in diapers. Yet, most acrylic acid is converted into related esters, which after polymerization with various monomers affords textiles, adhesives and coating materials. [19] Scheme 3.4: Two step synthesis of acrylic acid through the oxidation of propylene by dioxygen. [19] Nevertheless, it would be desirable to develop an economically attractive and more sustainable synthesis of acrylates. This chapter describes the advances on the synthesis of acrylate derivatives from ethylene and CO2. Several transition metals have been considered to perform this reaction, [20] however only nickel based chemistry [21] will be discussed here. A sustainable production of acrylates can be initiated by the oxidative coupling of ethylene and CO2 at a transition metal. Hoberg was the first to achieve this reaction in 1983 on a Ni(0)

complex. [22] Under a pressure of ethylene and CO2, and in the presence of a bidentate ligand, either dcpe or bipyridine, [Ni(cdt)] is converted to oxanickelacyclopentanes, as shown in and 1635 cm -1 respectively. [22] Addition of different alkenes to the nickelalactones produces a mixture of metallacycles and demonstrates that the oxidative coupling is reversible.

Furthermore, hydrolysis of the nickelalactones provides propanoic acid and addition of CO generates succinic anhydride. [22] Ni(cdt) CO 0) complexes. [22] Following this methodology, several nickelalactones with different amine, phosphine [23] or mixed P,N ligands [24] and various alkenes [24,25,26] and dienes [27 -30] were synthesized by Hoberg and Walther. [31 -35] Noticeably, in 1987 the square planar, 16 electron amine substituted nickelalactone [(DBU)2Ni(κO,κC-OC(O)CH2CH2)] could be crystallized. [36] A DFT study from 2004 by Papai using the B3LYP/SDDP methodology shows that the oxidative coupling between ethylene and CO2 at bipyridine and bis-phosphine Ni(0) complexes proceeds through a single step C-C bond formation from [(L2)Ni(C2H4)] and CO2. [37] In the case of [(bipy)nickelalactone] the oxidative coupling has a low activation barrier and is an exergonic process by 16.6 kcal.mol -1 . [37] Yet, a later DFT study at the M06L level of theory on a broad set of bis-phosphine nickelalactones reveals that the stability of the nickelalactones is correlated to the ligand's bite angle. [38] If the bite angle of the chelating ligand is smaller than 92 °, the nickelalactone is stable. However, when the bite angle becomes larger, the nickelalactones are increasingly difficult to synthesize through oxidative coupling and are more prone to decomposition. [38] 3.2.1.1 Alternative synthesis of nickelalactones More convenient strategies have been developed to synthesize hardly accessible nickelalactones by oxidative coupling in order to investigate their reactivity.

The first nickelalactones were obtained by Uhlig in good yields through the decarbonylation of nickel anhydride complexes, as shown in Scheme 3.7. [39] Cyclic anhydrides can easily undergo oxidative addition on Ni(0) complexes affording oxanickelacycles, which decompose upon heating and loss of CO to more stable nickelalactones. Addition of phthalic, [39] succinic [39] and glutaric anhydrides [40] to [(bipy)Ni(COD)] or [(bipy)Ni(Et)2] [41] has successfully generated the corresponding [(bipy)nickelalactones]. Widely used [(tmeda)nickelalactone] is also commonly synthesized through this reaction. [42] Scheme 3.7: Synthesis of [(bipy)nickelalactone] from [(bipy)Ni(COD)] and succinic anhydride. [39] Numerous nickelalactones have further been synthesized by simple ligand substitution, especially from [(tmeda)nickelalactone] and [(py)2nickelalactone]. [43,44,45] Stronger σ donating chelating bis-phosphines easily displace the diamine and the labile pyridines under mild conditions. By using this strategy, Walther largely contributed to broaden the scope of available nickelalactones. [43,44,45] A few substitution reactions can be found in Scheme 3.8. Scheme 3.8: Synthesis of [(bis-phosphine)nickelalactones] through ligand substitutions. [43,44,45]

Stoichiometric production of acrylates

The procedure to synthesize nickelalactones is well established. However, the conversion of nickelalactones to acrylate derivatives still remains difficult. The most straight forward pathway consists in β hydride elimination, followed by reductive elimination liberating the acrylate. Yet, large Ni-Hβ distances and strong Ni-O bonds make the process inefficient.

Theoretical investigations and first reports of β hydride eliminations

A DFT study at the B3LYP/LANL2DZ level of theory, realized by Buntine in 2007 on [(DBU)2Ni] complexes, explores the feasibility of such a reaction process. [46] The oxidative coupling of ethylene and CO2 to acrylates is divided in three elementary steps, namely oxidative coupling, β hydride elimination and reductive elimination (see Scheme 3.9). complexes. [46] First of all, the formation of the nickelalactone is exergonic by 4.1 kcal.mol -1 with respect to the starting materials. The thermodynamic stability of the nickelalactone drives the reaction despite a high activation barrier reaching ΔG ≠ = 29.0 kcal.mol -1 , as similarly reported by

Oxidative Coupling

Papai. [37] Afterwards, the direct β hydride elimination from the five membered cycle proves to be impossible. [37,46] Geometry constrains keep the β hydrogen atoms away from the nickel center. However, partial elongation of the Ni-O bond releases sufficient ring tension to allow β agostic interactions and the formation of a [(hydrido)(acrylate)Ni]-complex, which lies 9.2 kcal.mol -1 above the nickelalactone. The kinetic barrier of ΔG ≠ = 35.2 kcal.mol -1 is the highest to overcome during the cycle, [46] which was also later confirmed by calculations at the M06L level of theory carried out on bis-phosphine nickel complexes. [38] Finally, the liberation of acrylic acid through reductive elimination and the regeneration of the catalyst [(DBU)2Ni(C2H4)] can proceed through several pathways but is always endergonic by at least 12.9 kcal.mol -1 with a minimal activation barrier of ΔG ≠ = 24.9 kcal.mol -1 . Therefore, all three calculated kinetic barriers for each elementary step are high but not unbridgeable. [38,46] The main hurdle to the production of acrylic acid is the overall unfavorable thermodynamics of the system with a positive ΔG = 10.2 kcal.mol -1 . [46] Despite all the reported obstacles, β hydride eliminations on nickelalactones were already observed by Hoberg in the late 1980s while heating DBU complexes and recovering unsaturated carboxylic acids upon hydrolysis. [26,36] Furthermore, in 2006 Walther et al. managed to convert for the first time a nickelalactone into a nickel acrylate complex, as depicted in Scheme 3.10. [44] The dppm ligand induces β hydride elimination under mild conditions and affords after rearrangement a dimeric Ni(I) complex, displaying a Ni-Ni single bond and bearing a bridging acrylate in 65 % yield. [44] Scheme 3.10: First conversion of a nickelalactone into a nickel acrylate complex by means of a dppm ligand. [44] Even though isolated cases of β hydride elimination from nickelalactones had been described, [26,36,44] there were no general procedures to generate free acrylates before 2010.

Strategies were especially directed towards weakening the Ni-O bond to cause β hydride elimination.

Cleavage of nickelalactones by strong electrophiles

The first approach relies on the cleavage of nickelalactones by electrophiles.

In 2010, Rieger showed that the reaction between [(dppp)nickelalactone] and electrophilic methyl iodide (MeI) in CH2Cl2 at RT leads to the formation of methyl acrylate, which was observed after hydrolysis by NMR spectroscopy and ESI mass spectrometry. [47] The yield of the reaction could be improved up to 33 % in neat MeI, although the solubility of [(dppp)nickelalactone] is limited in this solvent. In situ IR measurements confirm the disappearance of the C=O absorption bands of the nickelalactone at 1627 cm -1 and 1322 cm -1 and the formation of a C=O absorption band at 1732 cm -1 , originating from the new acrylate. [47] Scheme 3.11: Reaction between [(dppp)nickelalactone] and methyl iodide in CH 2 Cl 2 at RT leading to the formation of methyl acrylate. [47] The reaction consists in the methylation of the nickelalactone, which opens the cycle and potentially generates a flexible methyl propanoate nickel complex. The β hydrogens become available for agostic interactions. Subsequent β hydride elimination releases methyl acrylate and forms a Ni(II) hydrido iodo complex. [47] A simplified hypothetical catalytic cycle is presented in Scheme 3.12.

Scheme 3.12: Simplified hypothetic catalytic cycle for the production of methyl acrylate from ethylene, CO 2 and methyl iodide. [47] In the following years, the reaction conditions have been optimized by varying either the ligand on the nickelalactone and the related palladalactone [48,49] (chelating P-and N-donor ligands)

or the methylating reagent (MeOTf, CH3CH2I, CF3CH2I). [49,50] In addition, reaction intermediates have been identified. Methyl propanoate nickel complexes

[(L)Ni(CH2CH2C(O)OMe)(X)
] (X = I, OTf) could be detected by IR spectroscopy [48] and were further isolated. [49] The presence of nickel hydride complexes [(L)Ni(H)(X)] has also been confirmed by 1 H NMR spectroscopy. [49] However, the reaction still faces several problems such as the competition between reductive and β hydride elimination producing iodopropanoates, [48] the formation of acidic side products (HI), that can degrade reaction intermediates and the use of methyl iodide that easily undergoes oxidative addition on Ni(0) complexes like [(dppp)Ni(C2H4)]. [47] In 2014, Xu and Sautet proposed a more detailed mechanism for this reaction on the basis of DFT calculations using a XYG3/XO3 approach. [51] The cleavage of the nickelalactone through methyl iodide proceeds through a SN2 reaction, which requires less than ΔG ≠ = 20. preventing catalytic applications. [51] This DFT study clearly shows that the activation barriers for the production of acrylates in the presence of methyl iodide are lower and within experimental reach compared to the one previously calculated in the absence of any kind of additives. [46] Ligands have minimal influence on the energetic situation of the main reaction pathway. Nevertheless, by tuning the ligand side reactions such as ligand decoordination, dissociation or lactone rearrangements can be inhibited. Interestingly, the polarity of the solvent also plays a key role in destabilizing side reaction intermediates. [51] The potential catalytic cycle can be found in Scheme 3.13. Scheme 3.14: Hypothetic catalytic cycle for the base assisted production of methyl acrylate computed by Limbach and Hofmann. [52] The reaction between (dtbpe)nickelalactone and MeI most likely proceeds through an SN2 reaction pathway, with a low activation barrier of ΔG ≠ = 17.4 kcal.mol -1 in agreement with the mild reaction conditions reported earlier. [47,48,49] The methylation occurs at the carbonyl oxygen of the nickelalactone, providing the cationic nickel compound [53] Even a neutral nickelalactone is basic enough to carry out the deprotonation with an activation barrier of ΔG ≠ = 15.8 kcal.mol -1 , which was also verified empirically. The problematic step is still the release of the acrylate and the regeneration of the catalyst, which would require ΔG ≠ = 24.0 kcal.mol -1 , but has not been achieved so far. [52] In an energetically less favorable side process, the cationic nickelalactone might be converted to the open chain nickel propanoate complex [(dtbpe)Ni(CH2CH2C(O)OMe)] + , which further undergoes β hydride elimination. Methyl acrylate is then released along the formation of [(dtbpe)Ni(H)(I)]. This decomposition mechanism as well as the loss of hydroiodic acid, lead to side products such as propanoates, Ni(I) and Ni(II) iodo complexes, which have been observed experimentally. [52] 3.2.

Cleavage of nickelalactones by bases

The second approach for the production of acrylates is based on the activation of nickelalactones by Lewis acids, followed by base induced β hydride elimination, as suggested by Limbach and Hofmann's [52] and Pidko's [38] DFT calculations. This concept was experimentally established by Bernskoetter in 2013, through the use of strongly Lewis acidic B(C6F5)3 in combination with a mild neutral base to promote the formation of acrylate derivatives. [54] When [(dppf)nickelalactone] is reacted with B(C6F5)3 at RT, the formation of a 5 membered Lewis activated γ nickelalactone is observed, which is converted over time to the thermodynamically more stable 4 membered Lewis activated β nickelalactone, as shown in Scheme 3.15. Additionally, the reaction between [(dppf)Ni(η 2 -acrylic acid)] and B(C6F5)3 also leads spontaneously to a mixture of both activated lactones. The generation of the four membered β nickelalactone most likely originates from β hydride elimination on the γnickelalactone-B(C6F5)3 adduct, that forms a non-detected nickel hydride acrylate complex, and is followed by a 2,1 insertion of the acryl borate moiety. [54] Scheme 3.15: Isomerization of the [(dppf)-γ-nickelalactone]-B(C 6 F 5 ) 3 adduct into the [(dppf)-βnickelalactone]-B(C 6 F 5 ) 3 adduct through a transient nickel hydride acrylate intermediate. [54] Afterwards, the nickel acrylate complex could be afforded in the presence of neutral organic bases like the phosphazene BTPP or DBU. Deprotonation might occur in the α position of the four membered β nickelalactone through the transient nickel hydride acrylate complex. [54] Scheme 3.16: Generation of the [(dppf)Ni(η 2 -acrylate)] complex through deprotonation of the [(dppf)β-nickelalactone] by phosphazene BTPP. [54] The η 2 coordinated acrylate is further expelled upon addition of ethylene. However, subsequent exposure to CO2 did not provide the corresponding [(dppf)nickelalactone] and close the cycle. [54] In a later contribution from 2014, Bernskoetter similarly evidenced the role of the weaker Lewis acidic sodium cation, that assists the isomerization of 5 membered activated γ-nickelalactone into 4 membered β-nickelalactone and might help β hydride elimination processes. [55] The sodium cation acts thermodynamically by stabilizing and releasing ring strain on the β-lactone and kinetically by lowering the activation barrier of the isomerization compared to a system without any Lewis acid. [55] Hence, two strategies have been successfully established to convert nickelalactones into acrylate derivatives in stoichiometric reactions using either strong electrophiles [47 -50] or a combination of a Lewis acid and a base. [54,55]

Catalytic production of acrylates

The first catalytic formation of acrylates starting from ethylene and CO2 was achieved only in 2012 by Limbach et al. [56] The endergonic carboxylation of ethylene (ΔGR 0 = 10.2 kcal.mol - 1 ) [46] can be made exergonic by the addition of a base (ΔGR 0 = -13.4 kcal.mol -1 for NaOH), generating an acrylate salt instead of acrylic acid. [57] Indeed, the use of strong bases such as alkoxides (NaOtBu) or amides (NaHMDS) enabled for the first time efficient conversion from poor regime to release the sodium acrylate and avoid side reactions between alkoxides and CO2. [58] Nevertheless, TON of 10 could be achieved. [56] Scheme 3.17: First stepwise catalytic production of sodium acrylate from ethylene and CO 2 . [56] In 2014 Vogt improved the TON of this reaction up to 21 in a one pot catalysis using a combination of LiI, Et3N and Zn dust. [59] The mechanism is slightly different as LiI promotes the β-hydride elimination under release of HI. Here again the use of an alkali metal cation proved to be of primary importance as it activates the nickelalactone and significantly lowers the activation barrier for β-hydride elimination. Et3N is used to trap the liberated acid and prevent the formation of lithium propanoate. The zinc powder reduces Ni(II) complexes back to catalytically active Ni(0) intermediates. [59] Finally, in 2014 Limbach [60] managed to reach TON of 107, the highest reported to date, by tuning the base and replacing sodium tert-butoxide by sodium 2-fluorophenolate. Phenolates are basic enough to carry out the deprotonation but poorly nucleophilic as not to react with CO2.

The methodology was also successfully tested on ten membered macrocyclic diphosphine nickelalactones [61] and could be expended to various alkenes and dienes. [60] + CO acrylates from ethylene and CO 2 . [56,59,60] Related palladium catalysis was also achieved in 2015 by Limbach following a similar methodology. However, maximum TON only reached 29 [62] and was further optimized by Schaub to 106 by the end of 2015. [57] 3

.3 Research objectives

The stoichiometric and especially catalytic synthesis of acrylates from nickelalactones still necessitates major improvements to render the process viable for industrial application. The aim of this research project is therefore to synthesize new nickelalactones and to explore innovative pathways for the generation of acrylate derivatives. Transmetallation can be a promising alternative for the synthesis of valuable chemicals under mild conditions from ethylene and CO2.

In the early 2000s, Mori already treated nickelalactones, synthesized from the oxidative coupling between alkynes [63,64] or 1,3-dienes [65,66,67] and CO2, with zinc derivatives such as Me2Zn, Et2Zn or PhZnCl, as illustrated in Scheme 3.19. In a first step the nickelalactone is opened to an acrylate or propanoate complex and in a second step the product is reductively eliminated from the nickel. When the zinc reagent possesses β hydrogens, β hydride elimination on the transferred alkyl moiety can precede reductive elimination. [63,66] In all cases carboxylic acids were obtained in high yields and good regio and stereoselectivities. Some reactions could even be performed catalytically. [64,66,67] Scheme 3.19: Alkylative carboxylation of alkynes using CO 2 and Me 2 Zn. [63] Rovis also investigated in two contributions [68,69] the reactivity of nickelalactones towards zinc derivatives and achieved decarbonylative cross coupling of cyclic anhydrides [68] and reductive carboxylation of styrenes [69] (Scheme 3.20) producing in both cases carboxylic acid derivatives. Though preliminary mechanistic investigations suggest that the later reaction, presented in Scheme 3.20, does actually not proceed through nickelalactones but rather through nickel hydride intermediates. [69] Reductive carboxylation of styrene using CO 2 and Et 2 Zn. [69] These reports all demonstrate that CO2 can be successfully reduced and integrated into valuable chemicals through the use of a transmetallating reagent. However, acrylates have never been generated through this synthetic approach.

Hence, in this work, nickelalactones obtained from the oxidative coupling between ethylene and CO2, will be directly subjected to various oxophilic transmetallating reagents, to attempt the synthesis of acrylates. Zinc derivatives do not seem suitable as they favor reductive elimination over β hydride elimination after cleaving the nickelalactones. [63 -69] Nevertheless, by changing the nature and the properties of the transmetallating reagents, it might be possible The main part of this work is committed to large bite angle chelating bis-phosphine nickelalactones. After successful synthesis of the nickelalactones, these will be subjected to various oxophilic transmetallating reagents such as boranes, silanes or aluminium derivatives to induce ring cleavage at the nickel center.

Moreover, the chemistry of related chelating bis-NHC complexes has been targeted. New procedures will be described for the synthesis of yet unknown [(bis-NHC)nickelalactones].

Changing the electronic and steric properties of the ligands might help to bypass the high CO2 pressures sometimes required for the generation of nickelalactones through oxidative coupling and ease β hydride elimination processes. The reactivity of the new [(bis-NHC)nickelalactones] can then be tested either under reported catalytic conditions [59,60] or towards transmetallating reagents to generate acrylates.

Functionalization of CO 2 with boranes

The oxidative coupling between ethylene and CO2 at chelating bis-phosphine nickel complexes and subsequent reactivity with boranes will be discussed in this chapter. Chelating bisphosphines with large bite angles, already selected for this type of chemistry and previously investigated in our group (Chapter 2) [1] , constitute the ligands of choice for this study. moiety is displaced by one equivalent of dcpp ligand IV-1, while the second is displaced by applying a pressure of ethylene. The complex is gathered as a yellow powder in 64 % yield.

31 P{ 1 H} NMR shows a single resonance at δ = 25.5 ppm. The ethylenic protons are found at δ = 1.52 ppm in 1 H NMR and the corresponding carbon atoms appear at δ = 31.87 ppm in 13 C NMR. The peaks are strongly shifted upfield, due to the electron richness of the [(dcpp)Ni]fragment and the strong π back donation from the nickel center to the coordinated olefin. This symmetric complex could therefore also be described as a metallacyclopropane. Single crystals were grown by heating a THF solution to reflux and allowing it to slowly cool down back to RT. 1.392(39) Å [2] ) lies between a carbon-carbon single (1.54 Å) and double bond (1.34 Å), further evidencing the strong π-back donation from the nickel center to the ethylene. (1.8655(13) Å for [(dipyridine)nickelalactone], [4] 1.868(3) Å for [(DBU)2nickelalactone], [5] 1.854(2) Å for [(tmeda)nickelalactone] IV-5, [6] 1.8448(1) Å for [(2,2'-bipyridine)nickelalactone] [7] ) and in the range of the other crystalized [(bis-phosphine)nickelalactones] [2,8,9] (1.890(2) Å for [(dcpe)nickelalactone], [8] 1.889(2) Å for [(dtbpe)nickelalactone] [2] ). shows similar instability at RT. [10] However, at 60 °C [(dcpp)nickelalactone] IV-4 gives quantitative formation of [(dcpp)Ni(C2H4)] IV-3 within 3 d. At higher temperatures the equilibrium between both complexes is largely shifted towards [(dcpp)Ni(C2H4)] IV-3 [(dippf)nickelalactone] [10] also undergoes reductive decoupling upon heating. However, no formation of [(dcpp)2Ni] is observed unlike for aryl substituted [(dppe)], [2] [(dppp)] [2,9] and [(dppf)nickelalactones]. [10] [(dtbpe)Ni(C2H4)] under 40 bar of CO2 in chlorobenzene at 60 °C. [2] The ΔG ≠ of the transition state can be obtained by inserting the determined rate constant kobs in the Eyring equation (Equation 4.1).

Reactivity between [(dcpp)nickel]-complexes and boranes

As an alternative to strong electrophiles [6, 11 -14] and bases, [2, 15 -17] the use of boranes as transmetallating reagents will be tested in order to cleave [(dcpp)nickelalactone] IV-4 and to generate acrylate derivatives.

Choosing an appropriate borane

The oxidative coupling between ethylene and CO2 at the [(dcpp)Ni]-fragment should not be hindered by the presence of a borane, required to further convert [(dcpp)nickelalactone] IV-4

into value added chemicals. Hydroboration can be considered as a competitive side reaction during the oxidative coupling. In order to smoothly fulfill the oxidative coupling and the following transmetallation with a borane, the overall reaction pathway always needs to lie lower in energy than the activation barriers required for hydroboration processes. Therefore, it was first of all made sure that boranes did not directly hydroborate ethylene or [(dcpp)Ni(C2H4)] IV-3. Three boranes were tested: 9-BBN, catecholborane and pinacolborane. 9-BBN easily hydroborates ethylene in THF at RT without the assistance of any metal.

11 B{ 1 H} NMR in [d8]-THF displays a sharp peak at δ = 71.7 ppm, which is shifted to δ = 88.3 ppm when the product is taken back to CDCl3. This matches the value given for ethyl-9-BBN in the literature. [18] Catecholborane on the other hand does not hydroborate ethylene efficiently without any additives. Nevertheless, catecholborane reacts immediately with [(dcpp)Ni(C2H4)] IV-3 in THF at RT. Ethylcatecholborane is detected quantitatively by 1 H and 11 B{ 1 H} NMR as well as GC-MS spectrometry. These stoichiometric investigations suggest that 9-BBN and catecholborane are not suited for the reductive functionalization of ethylene and CO2. quintet at δ = -10.1 ppm in 1 H NMR [19] and single crystal analysis [20] give additional proof. An organic product must be simultaneously released in the course of the reaction but could not be properly identified at this point. This is also supported by 1 H DOSY NMR separating a complex with a diffusion coefficient of D = 7.6×10 -10 m 2 .s -1 from lighter organic products with a D = 1.6×10 -9 m 2 .s -1 . In order to follow the formation of the organic product and to gain mechanistic insights on the reaction, 13 C labelled [(dcpp)nickelalactone] IV-4( 13 C) has been synthesized. Scheme 4.8

describes the two step synthesis of the labelled complex. 13 C labelled succinic anhydride IV-10( 13 C) is reacted with [Ni(COD)2] IV-2 in tmeda according to Fischer's procedure. [3] Ligand substitution and decarbonylation afford 13 C[(tmeda)nickelalactone] IV-5( 13 C) in 80 % yield.

Afterwards, by following the procedure developed in 4.1.2, 13 C[(dcpp)nickelalactone] IV-4( 13 C) is isolated in 87 % yield.

Scheme 4.8: Two step synthesis of 13 C[(dcpp)nickelalactone] IV-4( 13 C).

The reaction has then been repeated with 13 C[(dcpp)nickelalactone] IV-4( 13 C). The reaction conditions could be optimized. It turns out that 3 eq. of pinacolborane IV-8 are required to complete the reaction. In addition, three set of signals are observed by 13 doublet at δ = 24.7 ppm could arise from 13 C natural abundance in the methyl groups of pinacolborane IV-8. Scheme 4.9: Reaction between 13 C[(dcpp)nickelalactone] IV-4( 13 C) and 3 eq. of pinacolborane IV-8. The NMR experiments hence suggest the formation of 13 C-propoxypinacolborane IV-11( 13 C)

as well as diboroxane pinBOBpin IV-12.

A spiking experiment has been conducted in order to provide further structure confirmation of the organic product. Propoxypinacolborane IV-11 can be independently synthesized from propanol IV-13 and pinacolborane IV-8 either in C6D6 or in [d8]-THF. Thus, in an independent experiment, the reaction between [(dcpp)nickelalactone] IV-4 and pinacolborane IV-8 is followed by1 H NMR using mesitylene as internal standard. The reaction is quantitative and the conversion reaches 98 % after 1 h. at RT. Upon addition of 5 eq. of propoxypinacolborane IV-11 the corresponding peaks and integrations increase consequently, proving that the organic product of the reaction is indeed propoxypinacolborane IV-11. The Despite earlier reports by Mori [21 -24] and Rovis [25] on the cleavage of nickelalactones by zinc reagents leading to carboxylic acid derivatives, there are no precedents in the literature for this reaction. This is the first homogeneous nickel mediated synthesis of propanol derivatives from ethylene and CO2. Until here, the reaction fits nicely with the expected results. The further reduction to propanol derivatives remains unknown. Three different mechanisms could be considered for the reduction of propanoic acid pinacolborane ester IV-14 to propoxypinacolborane IV-11.

Reduction of propanoic acid pinacolborane ester IV-14 through a metal hydride

The first pathway is inspired by the reduction of CO2 and carbonyls with boranes using metal hydride complexes. [26,27] Guan first reported in 2010 the catalytic hydroboration of CO2 to methoxide derivatives using a PCP pincer nickel hydride complex and catecholborane. With a 500:1 borane to nickel-hydride ratio, TOF of 495 h -1 could be achieved. [28] Scheme 4.13: Guan's catalytic reduction of CO 2 with catecholborane using a PCP pincer nickel hydride complex. [28] Later other research groups discovered similar reactivity driven by different metal hydride complexes. In 2012 Sabo-Etienne [29] and Stephan [30] describe the catalytic reduction of CO2 into methoxy-pinacolborane by ruthenium hydrides. Alkaline earth hydride complexes as the β-diketiminate magnesium and calcium hydrido-tris(pentafluorophenyl)borates [31] and an ambiphilic β-diketiminate gallium hydride [32] also exhibit analogous reactivity even though they perform less efficiently compared to the transition metals (TOF for Mg = 0.07 h -1 , Ca = 0.1 h -1 and Ga = 2.5 h -1 ). Guan thoroughly investigated the mechanism of these reductions both experimentally [28,33,34] and by DFT calculations. [35] The reaction consists of three interlinked catalytic cycles to reach the methanol derivatives. Each cycle requires one equivalent of borane and reduces CO2 by one step. In this way formatoborane, formaldehyde and methoxyborane are successively generated.

Ru

Formates and methoxyboranes have been observed by NMR spectroscopy and could be crystallized [28,29,33] , whereas formaldehyde remained an elusive compound for a long time.

Sabo-Etienne and Bontemps finally managed to trap formaldehyde with methanol [36] and 2,6bis(diisopropyl)aniline. [37] Scheme 4.15: Mechanism of CO 2 reduction by boranes catalyzed by metal hydride complexes. The cycle is drawn relying on Guan's experimental and computational investigations. [34,35] The reaction proceeds through successive C=O insertions in the metal-hydride bond, followed by borane mediated B-H / O-M σ-bond metathesis. Altogether a first hydride is transferred from the metal center to the C=O bond and then a second hydride is transferred from the borane to the metal center to regenerate the catalyst and release the product. [28,35] The metal-or main group-hydride catalyzed reductions of CO2 into formate derivatives [38,39] and of carbonyls into alcohol derivatives [40 -47] corresponds either to the first or the last step of the previously proposed catalytic cycle. The various literature reports support the same C=O insertion, σ-bond metathesis mechanism described above.

The only requirement for such a mechanism is the presence of a nickel hydride complex.

Isolated [(dcpp)NiH]2 IV-9 has been tested as a potential catalyst. However, the dihydride does not reduce propanoic acid pinacolbrane ester IV-14 in the presence of pinacolborane IV-8 even after extensive heating at 60 °C. Therefore, [(dcpp)NiH]2 IV-9 is considered as a resting state. In the 1970s, Yamamoto already described the oxidative addition of esters [48,49] and ethers [50] to Ni(0) complexes. There are two possible ways to cleave an ester as represented in Scheme 4.17. The cleavage depends on the nature of the ester and of the ligands and was determined by analyzing the outcome of the reaction. The oxidative addition in the acyl group is observed when [Ni(COD)2] is reacted with aryl carboxylates in the presence of phosphine ligands such as PPh3 or PCy3. The aromatic substituent is essential to achieve the cleavage in this position. [48,49] On the other hand, allyl or vinyl carboxylates preferentially undergo oxidative addition in the C(O)O-R bond. [48] This topic is still actively investigated as Love systematically studied in 2016 the behavior of esters and thioesters towards stoichiometric amounts of [(dtbpe)2Ni(C6H6)]. [51] Scheme 4.17: Oxidative addition of a) phenyl propanoate and b) allyl acetate to [Ni(COD) 2 ] in the presence of PPh 3 . [48,49] In the last ten years, several research groups have taken advantage of this reactivity to perform nickel catalyzed cross coupling reactions using phenol derivatives as electrophiles. Aryl esters are relatively cheap, stable and in part readily available and therefore good candidates to complement or replace aryl halides. In 2008, Garg [52] and Shi [53] reported the first [(PCy3)2NiCl2] catalyzed Suzuki-Miyaura cross couplings between aryl carboxylates and boronic acids or aryl boroxins, involving a Caryl-O bond activation. Subsequently, Itami developed the coupling between 1,3-azoles and aryl pivalates [54] depicted in Scheme 4. 18 and could isolate in 2013 the first oxidative addition product of naphthalene-2-yl pivalate to a [(dcpe)Ni(0)]-complex closely related to our [(dcpp)Ni]-system. [55] Martin extended the concept of Caryl-O bond activation to enantioselective processes [56] and silylations. [57] Similarly to aryl halides, a mechanism involving successive oxidative addition, transmetallation and reductive elimination is proposed for most systems. Stoichiometric [55] and computational investigations [58] support the feasibility of the oxidative addition of a Caryl-O bond to a nickel complex.

Scheme 4.18: Itami's nickel catalyzed coupling between benzoaxoles and a) naphthyl pivalate [54,55] or b) naphthalene-2-yl thiophene-2-carboxylate. [59] Interestingly, Itami managed to switch the reactivity in favor of the Cacyl-O bond activation by replacing the tert-butyl group of the aryl carboxylates by thienyl [59] or pyridine [60] substituents, leading to decarbonylative cross coupling reactions. Houk [61] and Fu [62] More recently, Houk and Garg finally disclosed the nickel catalyzed activation of Cacyl-O bonds of methyl esters in the presence of Al(OtBu)3. An extensive aromatic system must still be present in all the methyl esters to achieve the coupling with anilines in good yields. [63] O N + [63] Therefore, in order to test the reaction pathway suggested above, the highly reactive Ni Sigma-alkylborane nickel complexes have already been isolated, starting from nickel dihydride complexes [(R2P-(CH2)2-PR2)NiH]2 (R = Cy, iPr, tBu). [64] Moreover, the analogous oxidative addition of hydrosilanes to {[(dtbpe)Ni]2(C6H6)} is also known. [65] In addition, a recent study on the hydroboration of ketones mediated by [(bipy)Ni(COD)] also discusses the possibility of an alternative to the nickel hydride based mechanism relying on the oxidative addition of pinacolborane IV-8. [66] The Ni(II) complex [(dcpp)Ni(H)(Bpin)], resulting from the oxidative addition of for the nickel catalyzed reductive cleavage of aryl ethers with silanes. [67] A paramagnetic three coordinate Ni(I) silyl complex {(dtbpe)Ni[Si(Mes)2H]}, which is related to Martin's proposed active Ni(I) silyl catalyst [(PCy3)2NiSiEt3], could be independently isolated by Hillhouse. [68] Scheme 4.21: Generation of a Ni(I) boryl complex for the reduction of propanoic acid pinacol ester IV-14.

In order to elucidate this mechanism, a preliminary experiment has been performed. The reaction between [(dcpp)Ni(naphthalene)] IV-16 and pinacolborane IV-8 indeed leads to the formation of [(dcpp)NiH]2 IV-9, which is observed by 31 Nevertheless, considering the promising stoichiometric results on the nickel mediated synthesis of propanol derivatives from ethylene and CO2, the research was then oriented towards the development of a catalytic application.

Stepwise catalytic cycle

Next, the feasibility of a catalytic cycle has been investigated step by step. units. X ray diffraction analysis reveals that the nickel centers are bridged by two hydrogen atoms, forming 3 center 2 electron bonds. [20] [(dcpp)NiH]2 IV-9 presents overall the reactivity of Ni(0) complexes and is rather considered as a non-classical hydride, with a hydrogen molecule interacting with two Ni(0) units. [19,69] Indeed, Jonas and Wilke previously reported, that [(dcpp)NiH]2 IV-9 can be converted to [(dcpp)Ni(C2H4)] IV-3 within 20 min. at RT under 1 bar of ethylene, thereby providing a pathway to close the catalytic cycle. [19] Therefore, all elementary steps required for the reaction: oxidative coupling, transmetallation with subsequent reduction of the carbonyl and ligand exchange appear to be achievable under relatively mild conditions. Oxidative coupling needing harsher pressure conditions is presumably the rate determining step. The proposed catalytic cycle is represented in Scheme

Catalysis

The first catalytic attempt was run using 5 bar of ethylene, 30 bar of CO2 and 10 mol% of The complex is characterized by a singlet at δ = 27.0 ppm in 31 P{ 1 H} NMR as well as two stretching bands at 1918 cm -1 and 1980 cm -1 in FTIR spectroscopy. The decreased frequencies of the CO bands compared to free carbon monoxide (2143 cm -1 ) evidence the π-back donation from the nickel center to the CO ligands. Single crystals for X-ray diffraction analysis were grown directly from the THF solution at -25 °C. The structure is depicted below in Figure 4.13.

The nickel center is in a distorted tetrahedral environment. The C1O1 (1.151(5) Å) and C2O2

(1.144(4) Å) bonds are slightly elongated through the π-back donation and therefore weakened compared to free carbon monoxide (1.128 Å). Similar chelating bis-phosphine nickel biscarbonyls have also been crystallized. [59, 70 -74] Among them diphenylphosphino-2hydroxypropane nickel bis-carbonyl [71] presents very close structural parameters whereas phenylphosphinopropane nickel bis-carbonyl [70] has shorter Ni-C and C-O bonds due to weaker donation. Among the 1,2-(dialkylphosphino)ethane series, except for the smaller bite angles, [(dcpe)Ni(CO)2] [59] and [(dippe)Ni(CO)2] [72] also display comparable bond lengths and angles. Furthermore, the production of [(dcpp)Ni(CO)2] IV-23 from other catalytic steps has then been considered. The nickel bis-carbonyl complex IV-23 could be generated during the reduction of propanoic acid pinacolborane ester IV-14 into propoxypinacolborane IV-11. Decarbonylation pathways may prevail under catalytic conditions that require heating and elevated ethylene and CO2 pressures. Sabo-Etienne reported that the catalytic reduction of CO2 with pinacolborane IV-8 by a ruthenium hydride complex generates among others a ruthenium bis-carbonyl species that drops the catalytic activity. The decarbonylation, that likely originates from transient formaldehyde, highly depends on the catalytic loading and the reaction time. [29] Moreover, [(dcpp)Ni ( 2 JP, P = 28.2 Hz). [75] Upon heating at 60 °C [(dcpp)Ni(CO2)] IV-24 is slowly transformed into [(dcpp)Ni(CO)2] IV-23. Similar reactivity under identical conditions had already been previously reported by Garcia [76] and Hillhouse [77] for the [(dtbpe)NiH]2 complex.

[(dippe)NiH]2, though, forms in the presence of CO2 a more complex mixture of various carbonyl complexes, among which [(dippe)Ni(CO)2], along with phosphine oxides. [76] Scheme 4.25: Reaction between [(dcpp)NiH] 2 IV-9 and CO 2 in toluene.

The generation of [(dcpp)Ni(CO)2]

IV-23 is unlikely to happen at this stage of the catalysis, as the reaction performs slowly compared to hydrogen-ethylene exchange and requires more time than the experimental catalytic conditions (1 d.).

In light of all these stoichiometric reactions, the formation of [(dcpp)Ni(CO)2] IV-23 is likely to occur during the first step of the catalysis, i.e. the oxidative coupling, which is also the most challenging one. Minor amounts of [(dcpp)Ni(CO)2] IV-23 could potentially also be formed from side reactions at later stages of the catalysis. In order to favor the oxidative coupling between ethylene and CO2 over the reduction of CO2 into CO, the catalytic conditions have been varied. Unfortunately, the experiments yet remained unsuccessful. The results are summarized in Table 4.3. First of all, the ethylene/CO2 ratio has been modified. However, by decreasing the CO2 pressure from 30 bar to 20 bar while increasing the ethylene pressure from 5 bar to 9 bar, no improvement could be observed. It would be interesting to completely switch the pressure ratio and saturate the autoclave with ethylene. Vogt for example observes the best TON for the synthesis of acrylates promoted by LiI/Et3N with 5 bar of CO2 and 25 bar of ethylene. [14] Afterwards, the influence of the catalyst was investigated. It was noticed empirically that nickelalactones with smaller bite angles were easier to generate. [2] Therefore, by choosing chelating bis-phosphines with shorter spacers such as dcpe or dcpm, the oxidative coupling between ethylene and CO2 at the nickel will be facilitated. In the ideal case it would even become more favorable than the reduction of CO2 into CO. Though, under the chosen conditions, the mixture of [Ni(COD)2] IV-2 and dcpe IV-25 also affords preferentially [(dcpe)Ni(CO)2] IV-26, which is characterized by a singlet at δ = 63.4 ppm in 31 P{ 1 H} NMR. [78] On the other side the catalysis with [Ni(COD)2] IV-2 and dcpm IV-27 leads to a mixture of unidentified products in 31 P{ 1 H} NMR. Propoxypinacolborane IV-11 could not be identified in 1 H NMR. The catalytic activity of further complexes such as [(tmeda)nickelalactone] might as well be tested.

A last approach would consist in changing the solvent to dioxane or toluene for example.

Especially the use of apolar solvents would dramatically limit the solubility CO2 in solution and might prevent the reduction of CO2 into CO.

To date, the nickel mediated synthesis of propanol derivatives from ethylene and CO2 cannot be performed catalytically. However, several tracks presented above can still be explored to finally achieve the catalysis.

Conclusion

The pressure ratio or changing the catalyst's precursor, the reduction of CO2 into CO could not be avoided. More reaction conditions such as different solvents, different temperatures, new pressure ratios or new catalysts will be screened in order to try to overcome this problem.

Overall, this is the first report of the reductive functionalization of a nickelalactone into a propanoate derivative with a borane.

Experimental part 4.8.1 General Remarks

All reactions were carried out under an atmosphere of dry argon using standard Schlenk techniques or in a nitrogen-filled MBraun LabStar glovebox. Diethyl ether, pentane, THF and toluene were taken from an MBraun SPS-800 solvent purification system, freeze-pump-thaw degassed and stored over 4 Å molecular sieves. C6D6 and [d8]-THF were degassed and stored over 4 Å molecular sieves.

[(tmeda)nickelalactone] [3] IV-5 and [(dcpp)NiH]2 [19] IV-9 were synthesized according to published procedures. All the other chemicals were purchased in reagent grade purity from Acros, Cytech and Sigma-Aldrich and were used without further purification. Ethylene, CO and CO2 were purchased from Air Liquide.

4.8.2 Synthesis of organic products 4.8.2.1 Synthesis of the dcpp ligand IV-1 [75] The synthesis of the dcpp ligand IV-1 was upscaled following a procedure described in Mathieu Demange's thesis. [75] n-BuLi (1.6 M in hexanes, 37.9 mL, 6.05×10 -2 mol, 1 eq.) is added at -78 °C to a 90 mL THF solution of dicyclohexylphosphine (12.0 g, 6.05×10 -2 mol, 1 eq.). The reaction mixture is allowed to come back to RT over 2 h. The solution turns yellow and a white precipitate falls out. 31 P{ 1 H} NMR spectroscopy shows a single resonance at δ = -13.0 ppm which corresponds to the lithiated phosphine. At -78 °C, 1,3-dichloropropane (2.59 mL, 2.72×10 -2 mol, 0.45 eq.) is added to LiPCy2. The solution is kept for 15 min at -78 °C before it is stirred overnight at RT. The solvent is removed under reduced pressure and the residue is taken up in pentane in order to precipitate all the salt. The solution is filtered off and the product is dried under vacuum. The dcpp ligand IV-1 is isolated as a pale yellow gel in 97 % yield (11.52 mg).

Synthesis of propoxypinacolborane IV-11

A C6D6 or [d8]-THF solution (0.7 mL) of pinacolborane IV-8 (72.6 µL, 5.00×10 -4 mol, 1 eq.) is prepared in a J. Young tube. Propanol IV-13 (37.6 µL, 5.00×10 -4 mol, 1 eq.) is added and the reaction mixture is thoroughly shaken. Hydrogen gas evolution is observed and the reaction is finished after 15 min at RT. Propoxypinacolborane IV-11 is used in situ and has not been isolated.

Synthesis of propanoic acid pinacolborane ester IV-14

Propanoic acid (37.4 µL, 5.00×10 -4 mol, 1 eq.) is added to an excess of pinacolborane IV-8

(108.9 µL, 7.50×10 -4 mol, 1.5 eq.) in C6D6 (0.7 mL). The solution in the J. Young tube is thoroughly shaken and hydrogen gas evolution is observed. The reaction is finished after 45 min. at RT. Propanoic acid pinacolborane ester IV-14 is the main product of the reaction. It has not been isolated as it is degraded under vacuum. [36] 1 H NMR (300 MHz, C6D6): δ 0.84 (t, 3 JH, H = 7.5 Hz, 3H, CH3), 1.05 (s, 12H, C(CH3)2 pinB), 2.01 (q, 3 JH, H = 7.5 Hz, 2H, CH2) ppm. tmeda)nickelalactone] IV-5 [3] [(tmeda)nickelalactone] IV-5 is synthesized according to a published procedure. [3] [Ni(COD)2]

IV-2 (1.653 g, 6.00×10 -3 mol, 1 eq.) is suspended with succinic anhydride IV-10 (413.2 mg, 4.13×10 -3 mol, 0.69 eq.) in tmeda (8 mL). The reaction mixture is stirred overnight at RT. The solution turns first yellow then brownish and a precipitate is formed. The supernatant is filtered away, the product is washed 3 × with Et2O and dried under vacuum. If the product is not green but brown at this stage of the reaction, the Schlenk is opened to air for a few minutes until the product turns green. This process is slightly exothermic. The product is then stored back under inert atmosphere. [(tmeda)nickelalactone] IV-5 is finally collected as a green powder in 90.5 % yield (923.0 mg). (N(CH3)2 B), 56.6 (CH2 αA), 61.5 (CH2 αB), 180.4 (C=O) ppm. 13 C[(tmeda)nickelalactone)] IV-5( 13 C) is synthesized by following the same procedure.

Starting from [Ni(COD)2] IV-2 (1.00 g, 3.62×10 -3 mol, 1 eq.) and 13 C labelled succinic anhydride IV-10( 13 C) (251.4 mg, 2.42×10 -3 mol, 0.67 eq.) in 7 mL of tmeda, 13 C[(tmeda)nickelalactone] IV-5( 13 C) is gathered as a green powder in 80 % yield (480.9 mg). * The chemical shifts of the ethylenic protons and carbons were determined on 13 C[(dcpp)Ni(C2H4)] IV-3( 13 C). For the protons 1 H 1D-HSQC and 2D-HSQC experiments were performed and for the carbons a 13 C{ 1 H}{ 31 P} NMR was recorded.

Synthesis of (dcpp)nickelalactone IV-4

[(tmeda)nickelalactone] IV-5 (350 mg, 1.42×10 -3 mol, 1eq.) is reacted with the dcpp ligand IV-1 (C = 0.917 mol.L -1 in THF, 1.70 mL, 1.56×10 -3 mol, 1,1 eq.) in THF ( 15 IR (ATR, cm -1 ): ν = 1650 (m).

* The chemical shifts of the protons and carbons from the lactone were determined on 13 C[(dcpp)nickelalactone] IV-4( 13 C). For the protons 1 H 1D-HSQC and 2D-HSQC experiments were performed and for the carbons a 13 C{ 1 H}{ 31 P} NMR was recorded. 13 C[(dcpp)nickelalactone] IV-4( 13 C) is synthesized by following the same procedure. Starting from 13 C[(tmeda)nickelalactone] IV-5( 13 C) (250.0 mg, 1.00×10 -3 mol, 1 eq.) and dcpp IV-1 (C = 0.829 mol.L -1 in THF, 1.33 mL, 1.10×10 -3 mol, 1.1 eq), 13 C[(dcpp)nickelalactone] IV-4( 13 C) is isolated as a yellow powder in 87 % yield (494.1 mg). For the spiking experiment, propoxypinacolborane IV-11 (C = 0.714 mol.L -1 in [d8]-THF, 185.1 µL, 1.32×10 -4 mol, 5 eq.) has been added to the reaction mixture after 4 h. at RT. The evolution of the peaks and integrations was followed by 1 H NMR.

Catalysis

The catalytic experiments are performed in 4 mL vials from Supelco, closed by polypropylene hole caps with PTFE/silicone septa. Three of these vials can fit at the same time in the glass holder of the autoclave.

A catalytic amount of [(dcpp)nickelalactone] IV-4 (28.5 mg, 5.00×10 -5 mol, 5 mol%) is weighted into a vial equipped with a small magnetic stirring bar and dissolved in 2 mL of THF.

Pinacolborane IV-8 (580.4 µL, 4.0×10 -3 mol, 400 mol%) is added and the vial is closed. A small piece of cannula is inserted in the septum to equilibrate the pressures in the tube and in the autoclave during the reaction. The autoclave is then pressurized with the gas at the lowest pressure, followed by the gas at the highest pressure. Typically, ethylene is introduced first (5 of the reaction, the autoclave is cooled down to RT and depressurized. The reaction mixture is recovered in the glovebox and analyzed by NMR spectroscopy.

Functionalization of CO 2 with aluminium derivatives

After the successful synthesis of propanol derivatives from nickelalactones and boranes, the Accordingly, the reactivity of AlEt3 V-3 has been tested. The literature reports facile reduction of CO2 into diethylaluminium propanoate V-4 in aromatic solvents as benzene or xylene. [1,2] Nevertheless, the reaction does not take place in THF even under 6 bar of CO2 and heating to 60 °C for 1 d. Even though CO2 has a greater solubility in THF compared to benzene, the coordinating ability of THF hampers the reaction. By using a large excess of AlEt3 V-3 (25 wt% in toluene, 56.5 eq.) full conversion is reached within 30 min. Additionally, 1 H, 13 C and 2D HSQC and HMBC NMR reveal the formation of diethylaluminium propanoate V-4. When the reaction is repeated with 13 C[(dcpp)nickelalactone] V-1( 13 C), the 31 P{ 1 H} NMR spectrum is identical to the one obtained from the reaction without 13 C labelling. This proves that the ethylene moiety of [(dcpp)Ni(C2H4)] V-2 comes from the transmetallation with AlEt3 V-3 and not from the decomposition of [(dcpp)nickelalactone] V-1 itself.

Further evidence is given by the analogous reaction between [(dcpp)nickelalactone] V-1 and Al(nBu)3 V-5, for which [(dcpp)Ni(1-butene)] V-6 and di(n-butyl)aluminium propanoate V-7

are identified as products. Upon coordination of an additional AlEt3 V-3 molecule on the second oxygen atom of [(dcpp)nickelalactone] V-1, the transition state for lactone ring opening lies significantly lower in energy, in good agreement with the reaction conditions. This also rationalize the large excess of trialkylaluminium derivatives required to perform the reactions efficiently.

3 R = Et V-3 R = nBu V-5 + O O AlR 2 -H elimination + RE TM V-1 V-2 (R = Et) V-6 (R = nBu) V-4 (R = Et) V-7 (R =

Reactivity of tri(tert-butyl)aluminium V-8

It would be even more interesting to induce β-hydride elimination on the propanoate fragment to generate acrylate derivatives as shown in Scheme 5.7. In this sense, the use of bulky substituents could prevent the coplanar configuration between the nickel center, the α and the β carbons of the transferred alkyl chain required for β-hydride elimination. Therefore, the reactivity of tri(tert-butyl)aluminium V-8 was investigated.

Scheme 5.7: Possible synthesis of acrylate derivatives using AlR 3 compounds with substituents that do not fulfill the conditions for β-H elimination. Al(tBu)3 V-8 to 5 eq., 28 % yield of product are measured after 8 h. at 60 °C. It seems that an equilibrium is reached since the yield does not further increase upon time. Fortunately, the complex could be crystallized in THF at -30 °C. X-ray diffraction analysis reveals a [(dcpp)nickelalactone-Al(t-Bu)3] adduct V-9 in agreement with a 31 P{ 1 H} NMR chemical shift close to the one reported for free 13 C[(dcpp)nickelalactone] V-1( 13 C). Curiously, switching the solvent from THF to non-coordinating toluene does not produce more 13 C[(dcpp)nickelalactone-AlR3] adduct V-9( 13 C). On the other hand, adding stronger coordinating agents such as pyridine or the Dipp carbene, does not break down the adduct either.

No adduct had been previously observed by NMR spectroscopy for the reactions with AlEt3 V-3 and Al(nBu)3 V-5. The bulkier the alkyl substituents get, the slower the reaction proceeds disclosing intermediates.

Scheme 5.8: Reaction between 13 C[(dcpp)nickelalactone] V-1( 13 C) and eq. of Al(tBu) 3 V-8 in [d8]-THF. 1.780(8) Å. [3] Otherwise, the adduct presents structural parameters close to the ones determined for free [(dcpp)nickelalactone] V-1. The P1-Ni1-P2 bite angles of both complexes (V-1 and V-9) are comparable. Though, the Ni1-O1 and C3-O1 bonds are slightly elongated by 0.06 Å and 0.03 Å respectively. As expected the aluminium derivative withdraws electron density from the nickelalactone and weakens the bonds of the cycle. Ph Ph R = Ph, 5 min. = 70 % R = tBu, 1 h. = 92 % Scheme 5.9: Ogoshi's oxidative coupling between diphenylacetylene and aldehydes in the presence of Ni(0)/PCy 3 and AlMe 2 OTf. [3] The reaction in neat Al(tBu)3 V-8 (60.8 eq.) at 60 °C does not show any Lewis adduct but a new AB system in 31

Ph Ph + O R H Ni(COD)

Preliminary catalytic investigations

In light of the stoichiometric investigations presented above, the catalytic production of propanoic acid derivatives from the oxidative coupling between ethylene and CO2 has been considered. Commercially available AlEt3 V-3 has been chosen as transmetallating reagent. Unfortunately, a catalytic test under 8 bar of CO2 with 10 mol% of [(dcpp)nickelalactone] V-1

did not produce the expected diethylaluminium propanoate V-4. Instead, like for pinacolborane, 31 P{ 1 H} NMR revealed the formation of [(dcpp)Ni(CO)2] V-14, which poisons the catalysis.

Until the right conditions are found to shut down the reduction of CO2 into CO and to preferentially regenerate the metallalactone under mild conditions, the catalytic production of propanoic acid derivatives from ethylene and CO2 cannot be achieved.

Conclusion and perspectives

This is the first report of the activation of a nickelalactone through an aluminium derivative.

The reactions between [(dcpp)nickelalactone] V-1 and trialkylaluminium derivatives with βhydrogens (V-3, V-5, V-8) lead to the formation of propanoic acid derivatives.

Scheme 5.12: Synthesis of propanoates from [(dcpp)nickelalactone] V-1 and trialkylaluminium derivatives (V-3, V-5, V-8).

AlR3 compounds are good transmetallating reagents for the nickelalactone and favor β-hydride elimination over reductive elimination. Unfortunately, the β-hydride elimination on the transferred alkyl group is more favored than the elimination on the propanoate moiety that would generate acrylates. The steric properties of the aluminium derivatives could not be tuned in order to reverse the reactivity. Overall, these results stand in contrast to the reactivity of pinacolborane, which preferentially leads to reductive elimination on the nickel center and generates propanol derivatives after further reductions.

More extensive investigations will be required in order to cleanly switch the reactivity towards the synthesis of acrylates. A first empirical approach would consist in varying not only the steric but also the electronic properties of the trialkylaluminium reagents. Derivatives without accessible β-hydrogens, such as AlMe3, Al(neopentyl)3, Al(CF3)3 or Al(Ph)3, could be tested in this sense. Aluminium halides or mixed aluminium alkyl halides could also come into consideration. The second approach, based on the DFT calculations discussed above, would moreover substitute dcpp for ligands with smaller bite angles and less bulky substituents in order to favor a double coordination of the trialkylaluminium derivative, which induces the lactone ring opening and its subsequent reactivity.

Activation of CO 2 with chelating bis-carbene nickel complexes

6 Synthesis and reactivity of (bis-NHC)nickel complexes 6.1 State of the art

N-heterocyclic carbenes

The isolation of the first crystalline N-heterocyclic carbene (NHC) by Arduengo in 1991 [1] opened this research field. Since then, a large variety of NHC compounds has been developed

and their application are ever growing.

A classic free NHC consist in an imidazole moiety, in which the central carbon atom bears a lone pair and an empty orbital. NHCs have a singlet ground state. The HOMO is an sp 2 hybridized lone pair in the plane of the heterocycle whereas the LUMO is an unoccupied out of plane p-orbital. The two electronegative nitrogen atoms on both sides of the carbene center stabilize the molecule electronically in creating a push-pull environment. The nitrogen atoms give electron density from their lone pair into the empty p-orbital, while they inductively withdraw electron density from the filled sp 2 hybridized orbital, as depicted in Scheme 6.1. The cyclic structure of a NHC further enforces a bent geometry which favors the singlet ground state. Bulky substituents can be attached to the nitrogen atoms to kinetically stabilize the free carbene and especially avoid dimerization reactions generating olefins. [2,3,4] Scheme 6.1: Structure of a NHC and orbital interactions.

Molecular orbital diagram of a NHC.

NHCs are nucleophiles, which have been commonly employed as ligands for transition metal complexes. They are stronger two electron σ-donors than tertiary phosphines and weak πdonors and -acceptors. As a consequence, the metal-ligand bonds are particularly stable.

Therefore, NHC-metal complexes usually have a good thermal stability, are often resistant to oxidation and do not tend to ligand dissociation. [5] Furthermore, they are highly active in numerous homogeneous catalytic reactions. [2,5,6,7] Grubbs' second generation ruthenium catalysts for olefin metathesis are probably the most famous NHC-based complexes. [8] C-C and C-heteroatoms cross-coupling reactions have also been widely promoted by palladium-NHC complexes designed by Herrmann, [9,10,11] Nolan [12,13] and Organ [14] among others. In addition, very efficient hydrosilylations as well as hydrogenation reactions catalyzed by NHC complexes have been reported. [6,7] 6.1.2 Chelating (bis-NHC)nickel complexes Chelating bis-phosphine ligands are commonly used for efficient homogeneous catalysis. It is therefore of interest to investigate the reactivity of stronger σ donating analogous chelating bis-NHC ligands. Chelating bis-NHC ligands are composed of two carbene moieties linked together through a spacer. The chemistry of their corresponding complexes has been far less explored.

In particular, (bis-NHC) nickel complexes are still relatively rare.

Chelating [(bis-NHC)nickel(II)] complexes

The investigations on (bis-NHC) nickel complexes started with the ambition of synthesizing analogues of cis chelated [(bis-NHC)Pd(II)dihalides]. The reactions between bis-imidazolium salts or free carbenes and appropriate nickel precursors led surprisingly to two different types of (bis-NHC)Ni(II) complexes. The length of the bridge and the size of the substituents on the nitrogen atoms had a great influence on the outcome of the reaction, as shown in Scheme 6.2. [15] When small spacers or small substituents were used, cationic [(bis-NHC)2Ni] 2+ 2[X] -(X = halide) complexes were obtained. Hermann and Green independently described the first synthesis of these homoleptic complexes bearing two chelating bis-NHC ligands in 1999. [16,17] They favored methyl linkers and alkyl groups such as methyl, isopropyl, cyclohexyl or tertbutyl moieties. Later, Huynh [15] and Foley [18] respectively reported related benzimidazole derived and N-benzyl substituted cationic complexes, which successfully catalyzed either Kumada or Heck and Suzuki-Miyaura cross-coupling reactions. On the other hand, long spacers or bulky substituents preferentially gave the expected neutral [(bis-NHC)NiX2] complexes. The first example was provided by RajanBabu in 2000 who used the chiral binaphtyl backbone as a linker between both NHC units. The large size of the chelate ring enforced an unusual trans coordination on the nickel center. [19] Subsequently in 2001

Baker reported an ortho-cyclophane linked [(bis-NHC)NiCl2] complex with cis geometry. [20] Interestingly, by using longer classic spacers, as propyl or butyl bridges, Bouwman [21,22] and Huynh [15] also managed to access cis chelated [(bis-NHC)NiX2] (X = Cl, Br) complexes, which turned out to catalyze Kumada cross-couplings as well as the vinyl polymerization of norbornene. Switching the N-substituent from small alkyl groups to bulkier Dipp moieties, as

shown by Danopoulos, also led preferentially to neutral cis chelated bis-NHC dihalide nickel(II)

complexes. [23] Scheme 6.3 presents some of the aforementioned [(bis-NHC)NiX2] complexes. Scheme 6.9: General procedure for the synthesis of bis-imidazolium salts. The procedure reported by Hofmann et al. [27] for the [(LtBu)Ni(COD)] and [(LDipp)Ni(COD)] complexes was used as a guideline for the development of the new ethylene complexes.

However, their synthesis' require a long and complicated work-up involving water and provide the products in low to moderate yields (34 % and 50% respectively). Indeed, the limited solubility of the nickel alkene complexes in common organic solvents renders the salts' separation difficult at the end of the reaction. Therefore, it proved to be essential to carry out the deprotonation in Et2O to precipitate and to remove KBr before adding the nickel precursor.

In this way, the products' purification is restricted to the removal of the volatiles under vacuum and the extraction of free COD with pentane without important losses.

[(LtBu)Ni(C2H4)] VI-9 is isolated as a brown product in 75 % yield. The ethylenic protons are strongly shifted upfield in 1 H NMR, resonating as an AA'XX' system between δ = 0.99 ppm and δ = 1.18 ppm. The methylene protons are diastereotopic, probably due to conformational rigidity, and give rise to two doublets. The ethylenic carbons appear at δ = 27.7 ppm in 13 C NMR, which rather corresponds to the chemical shift of an alkane. The carbenic center is located downfield at δ = 203.7 ppm as expected. Single crystals for X-ray diffraction analysis were grown from a THF solution at -25 °C. The solid state structure has a distorted square planar geometry with a bite angle of 91.45( 14) °. The six-membered chelate ring adopts a boat shape conformation. The distance between the ethylenic C1 and C2 carbons of 1.419(5) Å lies between a single and a double C-C bond. This evidences a very strong π-back donation from the nickel center to the coordinated olefin in this complex. By switching the substituents on the nitrogen atoms to the bulkier Dipp units, [(LDipp)Ni(C2H4)] VI-10 could be obtained as a red powder in 71 % yield. The 1 H and 13 C NMR chemical shifts of the ethylene moiety are here again extremely shifted upfield. The olefinic protons resonate at δ = 0.14 ppm as a singlet and the corresponding carbons at δ = 25.2 ppm. In this complex both methylene protons are also equivalent owing to fast geometry fluctuations in solution. The quaternary carbene atoms appear at δ = 200.1 ppm.

Synthesis of [(bis-NHC)Ni(alkyne)] complexes

The methodology has also been extended to alkyne complexes. In this way symmetric NMR spectroscopy shows that the acetylene moiety in these complexes is strongly shifted downfield towards the olefinic area. The 13 as shown in Scheme 6.14. [30] [(LtBu)nickelalactone] VI-18 is obtained as a beige product in 83 % yield. Despite its limited solubility, NMR data could be recorded in CD3CN. The 1 H NMR reveals two inequivalent Ni-CH2 protons at δ = 0.47 ppm and δ = 0.74 ppm and two protons in α of the ester at δ = 2.03 ppm. 13 Afterwards, different parameters of the [(bis-NHC)nickelalactone] have been successively

varied. First of all, the substituents on the nitrogen atoms have been replaced by bulkier Dipp moieties. By following the same methodology, [(LDipp)nickelalactone] VI-19 can be isolated as a beige product in 86 % yield. 1 H NMR in MeOD reveals Ni-CH2 protons as a triplet at δ = 0.17 ppm and CH2 protons in α of the ester as a second triplet at δ = 1.52 ppm. 13 C NMR displays the quaternary carbene carbon atoms at δ = 176.7 ppm and δ = 186.0 ppm and the C=O at δ = 194.3 ppm. FTIR spectroscopy shows an absorption band at 1562 cm -1 .

Next, the spacer between both NHC units has been modulated. The methyl chain has been switch for a longer propyl chain. [ (LtBu) prop nickelalactone] VI-20 is gathered as beige product in 62 % yield. The spectroscopic features resemble the ones previously reported for the other nickelalactones. Among others the acyl resonates at δ = 189.2 ppm in 13 diphenylacetylene VI-11 and tmeda under 5 bar of CO2 according to Hoberg's procedure. [34] The diamine in this Ni(II) complex is also easily displaced by the free LtBu carbene yielding 86 % of the nearly insoluble yellow diphenyl substituted [(LtBu)oxanickelacyclopentenone] VI-22. is more stable than [(LtBu)Ni(C2H4)] VI-9 and cannot be reverted into the latter upon heating. Limbach's [35,36] and Vogt's [37] procedures to generate acrylates, employing either sodium phenoxide or a combination of LiI and Et3N, have been tested on [(LtBu)nickelalactone] VI-18 without any success. The approach described in Chapter 4 and 5 using various transmetallating reagents also proved to be inconclusive so far. Rapidly limitations were encountered, first of all because of the poor solubility of [(bis-NHC)nickelalactones] in the organic solvents commonly used for these reactions. Moreover, the thermodynamic stability of these complexes could be deleterious. Indeed, calculations show that [(bis-NHC)nickelalactones] may be more readily accessible than [(bis-phosphine)nickelalactones], but that does not means that further steps would be more favorable as well. Finally, the loss of the phosphorus probe made the monitoring of the reactions very difficult.

In order to better understand that chemistry, 13 C[(LtBu)nickelalactone] VI-18( 13 C) was synthesized in a two-step procedure described in Scheme 6.22. Starting from 13 C labelled succinic anhydride VI-25( 13 C), 13 C[(tmeda)nickelalactone] VI-17( 13 C) could be obtained in 80 % yield following Fischer's procedure. [48] Deprotonation of the LtBuH2Br2 imidazolium salt VI-5 and subsequent reaction with 13 C[(tmeda)nickelalactone] VI-17( 13 C) afforded 13 C[(LtBu)nickelalactone] VI-18( 13 C) in 79 % yield. With the 13 C NMR probe in hand and by optimizing the reaction conditions, new advances in the synthesis of acrylates or alcoholates from (bis-NHC)nickelalactones can be expected.

Conclusion and perspectives

The development of robust procedures led to the efficient synthesis of new [(bis-NHC)Ni( 0 0)]-complexes and to explore their reactivity for example towards the catalytic formation of benzonitriles [39 -44] or the arylcyanation of alkynes. [45 -48 ] In N-(2,6-diisopropylphenyl)imidazole [32] VI-2, and [(tmeda)nickelalactone] [38] VI-22 were synthesized according to published procedures. All the other chemicals were purchased in reagent grade purity from Acros and Sigma-Aldrich and were used without further purification.

CO, CO2 and ethylene were purchased from Air Liquide.

6.6.2 Synthesis of imidazoles and corresponding imidazolium salts 6.6.2.1 N-tert-butylimidazole VI-1

N-tert-butylimidazole VI-1 was prepared according to a modified published procedure. [31] Distilled water (200 mL) was placed in a 500 mL three necked flask connected to two dropping funnels and a water condenser. One dropping funnel was filled with 37 % formaldehyde (29.8 mL, 0.4 mol, 1 eq.) and 40 % glyoxal (46 mL, 0.4 mol, 1 eq.), the other with tertbutylamine (42.4 mL, 0.4 mol, 1 eq.) and 30 % aqueous ammonia (25.6 mL, 0.4 mol, 1 eq.).

The water was heated until boiling and then both solutions were added simultaneously drop by drop. After complete addition, the brown reaction mixture was stirred for further 30 min. at 100 °C. The organic products were extracted with 2 × 200 mL of CH2Cl2. The organic fractions were dried over Na2SO4 and CH2Cl2 was removed by rotary evaporator. The remaining mixture was purified by vacuum transfer. N-tert-butylimidazole VI-1 is obtained as a pale yellow liquid in 40 % yield (19.90 g) by heating up to 85 -100 °C. Crystals for X-Ray diffraction analysis were grown from a THF solution at -25 °C. IR (cm -1 ): ν (C=O) = 1619 (m).

6.6.5.2 13 C[(LtBu)Ni(CH 2 CH 2 COO-κC,κO)] VI-18( 13 C) 13 C[(LtBu)Ni(CH2CH2COO-κC,κO)] VI-18( 13 C) was synthesized following the same procedure by using 13 procedure. [34] [Ni(COD)2] VI-8 (500 mg, 1.81×10 -3 mol, 1 eq.) is mixed with diphenylacetylene VI-11 (322.8 mg, 1.81×10 -3 mol, 1 eq.) in a Fischer-Porter and suspended in 10 mL of THF. Tmeda (1.36 mL, 9.06×10 -3 mol, 5 eq.) is added to the reaction mixture which is then freeze-pump-thaw degassed. The residual factors are defined as follows: comparables en tous points aux nickelalactones connues dans la littérature. [13,20,26] The aim of this research project is to promote the formation of new C-C bonds and the production of valuable chemicals by using chelated nickel complexes.

R = ∑ |F | -|F | ∑|F | wR = ∑[w(F -F ) ] ∑[w(F ) ] ⋅ 7
The first part of this thesis is dedicated to [nickel(bis-phosphine)] complexes employed as catalysts for Negishi cross coupling reactions. Designed Ni(0) precatalyst [(dcpp)Ni(η 2toluene)] (dcpp = 1,3-bis(dicyclohexylphosphino)propane) promotes efficiently the Negishi cross coupling between aryl chlorides and phenylzinc chloride derivatives at low catalyst loadings (down to 0.2 mol% -1 mol%) under mild conditions (THF, 60°C). Mechanistic investigations relying on stoichiometric reactions and DFT calculations prove the involvement of Ni(0)/Ni(II) intermediates rather than Ni(I)/Ni(III) species during the catalysis.

The second part of this work deals with the oxidative coupling between ethylene and CO2 at bis-phosphine and bis-NHC chelated nickel complexes for the production of value-added chemicals. The equilibrium between [(dcpp)Ni(C2H4)] and [(dcpp)nickelalactone] has been investigated by kinetic studies. The subsequent cleavage of [(dcpp)nickelalactone] by pinacolborane leads to its reductive functionalization into a propanol derivative. Preliminary mechanistic and catalytic investigations have been undertaken. Moreover, new methodologies are provided for the synthesis of the first [(bis-NHC)Ni(C2H4)] and [(bis-NHC)nickelalactone] complexes.

Couplages de Negishi et Couplages Oxydants à Base de Complexes de Nickel

Ce projet de recherche porte sur la formation de nouvelles liaisons C-C et la production de produits chimiques valorisables grâce à l'utilisation de complexes de nickel chélatés.

La première partie de cette thèse est dédiée à des complexes bis-phosphines de nickel employés comme catalyseurs pour le couplage croisé de Negishi. Le précatalyseur [(dcpp)Ni(η 2 -toluene)] (dcpp = 1,3-bis(dicyclohexylphosphino)propane) permet de coupler efficacement des chloro-arènes avec des organozinciques en utilisant de faibles charges catalytiques (0,2 mol% -0,1 mol%) et des conditions douces (THF, 60 °C). Une étude mécanistique fondée sur des réactions stoechiométriques et des calculs DFT prouve la présence

Nickel Mediated Negishi and Oxidative Couplings

The aim of this research project is to promote the formation of new C-C bonds and the production of valuable chemicals by using chelated nickel complexes.

The first part of this thesis is dedicated to [nickel(bis-phosphine)] complexes employed as catalysts for Negishi cross coupling reactions. Designed Ni(0) precatalyst [(dcpp)Ni(η 2toluene)] (dcpp = 1,3-bis(dicyclohexylphosphino)propane) promotes efficiently the Negishi cross coupling between aryl chlorides and phenylzinc chloride derivatives at low catalyst loadings (down to 0.2 mol% -1 mol%) under mild conditions (THF, 60°C). Mechanistic investigations relying on stoichiometric reactions and DFT calculations prove the involvement of Ni(0)/Ni(II) intermediates rather than Ni(I)/Ni(III) species during the catalysis.

The second part of this work deals with the oxidative coupling between ethylene and CO2 at bis-phosphine and bis-NHC chelated nickel complexes for the production of value-added chemicals. The equilibrium between [(dcpp)Ni(C2H4)] and [(dcpp)nickelalactone] has been investigated by kinetic studies. The subsequent cleavage of [(dcpp)nickelalactone] by pinacolborane leads to its reductive functionalization into a propanol derivative. Preliminary mechanistic and catalytic investigations have been undertaken. Moreover, new methodologies are provided for the synthesis of the first [(bis-NHC)Ni(C2H4)] and [(bis-NHC)nickelalactone] complexes.

Couplages de Negishi et Couplages Oxydants à Base de Complexes de Nickel

Ce projet de recherche porte sur la formation de nouvelles liaisons C-C et la production de produits chimiques valorisables grâce à l'utilisation de complexes de nickel chélatés.

La première partie de cette thèse est dédiée à des complexes bis-phosphines de nickel employés comme catalyseurs pour le couplage croisé de Negishi. Le précatalyseur [(dcpp)Ni(η 2 -toluene)] (dcpp = 1,3-bis(dicyclohexylphosphino)propane) permet de coupler efficacement des chloro-arènes avec des organozinciques en utilisant de faibles charges catalytiques (0,2 mol% -0,1 mol%) et des conditions douces (THF, 60 °C). Une étude mécanistique fondée sur des réactions stoechiométriques et des calculs DFT prouve la présence

1 State of the art 1 . 1

 11 Palladium catalyzed C sp2 -C sp2 Negishi cross couplings 1.1.1 Advantages of Negishi cross coupling reactions

Scheme 1 . 5 :

 15 Scheme 1.5: Highly efficient Pd-PEPPSI-IPr and Pd-PEPPSI-IPent catalyzed Negishi cross coupling

employing 5

 5 mol% of the Ni(0) precursor [Ni(PPh3)4] generated from commercially available [Ni(acac)2], PPh3 and DIBALH, Negishi managed to couple aryl halides with aryl zinc

Scheme 1 . 8 :

 18 Scheme 1.8: Nickel amido pincer complexes for the Negishi cross coupling between aryl chlorides

2 Scheme 1 . 9 :

 219 Scheme 1.9: General and efficient Negishi cross coupling methodology developed by Frech relying on

  Br] undergoes oxidative addition of an aryl bromide, producing the Ni(III) intermediate [(PEt3)2Ni III (Ar)(Br)2]. Subsequently, a ligand transmetallation between the arylnickel(III) complex and [(PEt3)2Ni II (Ar)(Br)] leads to a new bis-arylNi(III) intermediate [(PEt3)2Ni III (Ar)2(Br)]. Finally, reductive elimination releases the biaryl and regenerates the catalyst [(PEt3)3Ni I Br]

Scheme 1 . 15 :

 115 Scheme 1.15: [(dcpp)Ni] based Negishi catalysis between aryl halides and phenyl zinc chloride derivatives.

Scheme 1 . 16 :

 116 Scheme 1.16: Properties of the [(dcpp)Ni] system.

4 2. 1 . 1

 411 catalytic tests with nickel precursors in different oxidation states. The experimental results are supported by DFT calculations carried out by Dr. Nicolas Mézailles at the LHFA (Toulouse). Ni(0)/Ni(II) catalytic cycle Ni(I)/Ni(III) catalytic cycle 2. (Bis-phosphine)nickel-catalyzed Negishi cross coupling 2.1 Synthesis of precatalyst [(dcpp)nickel(toluene)] II-Synthesis of Ni(II) precursor [(dcpp)NiCl 2 ] II-3 The [(dcpp)NiCl2] complex II-3 is readily synthesized at RT by the stoichiometric reaction between [(DME)NiCl2] II-2 and the bis(dicyclohexylphosphino)propane ligand II-1 (dcpp),

Scheme 2 . 1 :

 21 Scheme 2.1: Synthesis of [(dcpp)NiCl 2 ] II-3.

Figure 2 . 1 :

 21 Figure 2.1: Molecular structure of [(dcpp)NiCl 2 ] II-3 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -P 1 2.196(3), Ni 1 -P 2 2.187(3), Ni 1 -Cl 1 2.240(4), Ni 1 -Cl 2 2.239(3), P 1 -Ni 1 -P 2 99.40(11), Cl 1 -Ni 1 -Cl 2 88.55(13), P 1 -Ni 1 -Cl 1 85.97(12), P 2 -Ni 1 -Cl 2 86.29(13).

Scheme 2 . 2 :

 22 Scheme 2.2: a) Jonas' synthesis of [(dcpe)-] and [(dcpp)Ni(C 6 H 6 )],[2] b) Pörschke's synthesis of [[(dtbpe)Ni 2 ](C 6 H 6 )].[3]

Scheme 2 . 3 :

 23 Scheme 2.3: Synthesis of precatalyst [(dcpp)Ni(toluene)] II-4.[1]

dihydride II- 5

 5 most likely originate from Csp2 or Csp3 activation of toluene in [(dcpp)Ni(toluene)] II-4. GC-MS analysis of the reaction mixture shows a peak at m/z = 182.26 corresponding to a C14H14 compound, which can be either 4,4'-dimethyl-biphenyl or 1,2diphenylethane. Upon formation of one of these organic compounds, H2 is released and trapped by the [(dcpp)Ni]-fragment. [(dcpp)Ni(toluene)] II-4 is characterized in 31 P{ 1 H} NMR by a singlet at δ = 15.4 ppm. Despite the coordination of the asymmetric toluene molecule, the two phosphorus atoms are in an apparent equivalent chemical environment. Low temperature NMR experiments were carried out down to -80 °C but the fast equilibration process could not be slowed down. In addition, the integrations in 1 H NMR suggests the coordination of only one toluene ligand per [(dcpp)Ni]fragment, ruling out a bridged structure, as the one observed by Pörschke for [[(dtbpe)Ni]2(C6H6)

[

  (dcpp)Ni(toluene)] II-4 solution at -25 °C. The structure confirms the monomeric nature of the complex as well as the η 2 coordination of the toluene ligand. The methyl group of toluene is found to occupy preferentially a meta position. The C1-C2 coordinating bond of toluene measures 1.442(2) Å and is only slightly elongated compared to free toluene (1.39 Å). This indicates a weak donation of the [(dcpp)Ni]-fragment in the C-C bond and a poor coordination of the ligand in good agreement with its high sensitivity and reactivity. The bite angle P1-Ni1-P2 measures 104.10(4) ° and is larger by 4.7 ° compared to the bite angle of Ni(II) complex [(dcpp)NiCl2] II-3, proving the flexibility of the dcpp ligand.

Figure 2 . 2 :

 22 Figure 2.2: Molecular structure of [(dcpp)Ni(toluene)] II-4 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -P 1 2.1465(11), Ni 1 -P 2 2.1558(12), Ni 1 -C 1 2.001(4), Ni 1 -C 2 1.993(4), C 1 -C 2 1.425(6), C 2 -C 3 1.413(6), C 3 -C 4 1.347(6), C 4 -C 5 1.422(6), C 5 -C 6 1.353(6), C 5 -C 7 1.519(6), P 1 -Ni 1 -P 2 104.10(4), C 1 -Ni 1 -C 2 41.80(16).

1

  Scheme 2.5: [(dcpp)Ni(toluene)] II-4 catalyzed Negishi cross coupling between 4-chlorotoluene II-7 and phenylzinc chloride II-8.

4 2 -

 2 Cl-3-NH2Py 4 90 In summary, the [(dcpp)Ni(toluene)] II-4 catalyzed Negishi cross couplings between aryl chlorides and phenyl zinc chloride derivatives allow the formation of biaryl compounds in good yields, under mild conditions and with low catalyst loading. While [(dcpp)Ni(toluene)] II-4

2. 3 . 1

 31 Stoichiometric reactions Mechanistic investigations are undertaken in order to determine whether the reaction with the strongly donating bidentate dcpp ligand is going through a Ni(0)/Ni(II) or a Ni(I)/Ni(III) catalytic cycle. Stoichiometric reactions are carried out to study the feasibility and the outcome of each elementary step of the reaction, namely oxidative addition, transmetallation and reductive elimination.

2. 3 . 1 . 1 6 .Scheme 2 . 6 :

 311626 Scheme 2.6: Oxidative addition of aryl chlorides to [(dcpp)Ni(toluene)] II-4.

Figure 2 . 3 :

 23 Figure 2.3: Molecular structure of [(dcpp)Ni(Ph)(Cl)] II-14 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -P 1 2.1565(4), Ni 1 -P 2 2.2528(4), Ni 1 -C 28 1.9292(15), Ni 1 -Cl 1 2.2170(4), P 1 -Ni 1 -P 2 98.086(16), C 28 -Ni 1 -Cl 1 86.42(4).

  (II) complex [(dcpp)Ni(Ph)(Cl)] II-14 and an aryl chloride was investigated. However, when [(dcpp)Ni(Ph)(Cl)] II-14 is heated for days at 80 °C with an excess of chlorobenzene II-11, no reaction is observed. The strong coordination of the bidentate phosphine prevents the reduction to a Ni(I) species and subsequent formation of homocoupled product II-10 and shuts down a Ni(I)/Ni(III) catalytic cycle.

Scheme 2 . 9 :

 29 Scheme 2.9: Reactivity between [(dcpp)NiCl 2 ] II-3 and a stoichiometric amount of PhZnCl II-8: generation of the Ni(I) dimer [(dcpp)NiCl] 2 II-16.

Figure 2 . 4 :

 24 Figure 2.4: Room Temperature X-Band EPR Spectrum of Ni(I) complex [(dcpp)NiCl] 2 II-16.

Figure 2 . 5 :

 25 Figure 2.5: Molecular structure of [(dcpp)NiCl] 2 II-16 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -P 1 2.1856(4),

Scheme 2 . 11 : 8 .

 2118 Scheme 2.11: Reaction between Ni(I) complex [(dcpp)NiCl] 2 II-16 and PhZnCl II-8.

Scheme 2 . 13 :

 213 Scheme 2.13: Proposed mechanism for the [(dcpp)Ni(toluene)] II-4 catalyzed Negishi cross coupling, suggested by the stoichiometric and catalytic investigations.

2. 4 . 1 Scheme 2 . 14 :

 41214 Scheme 2.14: Reaction pathways for the coordination of chlorobenzene II-11.

η 2 Scheme 2 . 15 :

 2215 Scheme 2.15: Reaction pathway for one catalytic cycle starting from the aryl chloride complex C.

  The Ni-C (2.04 Å) and the Zn-C (2.20 Å) bond lengths indicate that the phenyl is almost fully transferred to the nickel. The elimination of ZnCl2(THF)2 from F releases G, which lies 12.6 kcal.mol -1 lower in energy. The whole transmetallation process is exergonic with an energy gain from D to G of 9.0 kcal.mol -1 .

Figure 2 . 6 :

 26 Figure 2.6: Computed structures of E (left), TS EF (top) and F (right).

Finally, the productScheme 2 . 16 :

 216 Scheme 2.16: Reaction pathways for the product to reagent exchange: regeneration of η 2 aryl chloride complex C.

2. 6 . 3 . 2 4 [

 6324 Synthesis of [(dcpp)Ni(toluene)] II-(dcpp)NiCl2] II-3 (200 mg, 3.53×10 -4 mol, 1 eq.) and KC8 (100 mg, 7.42×10 -4 mol, 2.1 eq.) are suspended in 10 mL of toluene and the reaction mixture is stirred overnight at RT. [(dcpp)Ni(η 2 -toluene)] II-4 and [(dcpp)NiH]2 II-5 are extracted from precipitated KCl and graphite through successive centrifugations. The first fraction contains most of the [(dcpp)NiH]2 II-5 and is therefore put aside. At best only 7 % of [(dcpp)NiH]2 II-5 were produced. Yellow fractions 2 -5 containing [(dcpp)Ni(toluene)] II-4 are collected and titrated by31 P{ 1 H} NMR with PPh3 as internal standard. The complex is kept as a toluene solution at -25 °C. Single crystals for X-ray diffraction analysis could be grown from this solution.

  of [(dcpp)Ni(toluene)] II-4 (58.7 mg, 1.0×10 -4 mol, 1 eq.). After 4 min. at 40 °C the solution becomes light yellow. All volatiles are removed under reduced pressure and the remaining solid is washed with 2 × 5 mL of pentane. The oxidative addition product [(dcpp)Ni(Ph)(Cl)] II-14

4-methylbiphenyl II- 9 - 4 ( 5 .

 945 Following the general protocol 1 mol% of [(dcpp)Ni(toluene)] II-88 mg, 1.0×10 -5 mol) is dissolved in 2 mL of THF. 4-chlorotoluene II-7 (119 µL, 1.00×10 -3 mol, 1 eq.) and PhZnCl II-8 (1.1×10 -3 mol, 1,1 eq.) are added to the solution, which is reacted at 60 °C. After work up 4-methylbiphenyl II-9 is collected as a white solid in 93 % yield (156.5 mg). (eluent: ethylacetate/pentane = 10/90).

Scheme 3 . 5 : 3 . 2 . 2 3. 2 . 1

 3532221 Scheme 3.5: Dream reaction: metal catalyzed synthesis of acrylic acid from ethylene and CO 2 .

Scheme 3 . 6 .

 36 Scheme 3.6. [(dcpe)nickelalactone] and [(bipy)nickelalactone] are obtained in 50 % and 85 % yield and display a characteristic carboxylic absorption band in IR spectroscopy at 1620 cm -1

Scheme 3 . 6 :

 36 Scheme 3.6: First oxidative couplings between ethylene and CO 2 at Ni(0) complexes.[22]

Scheme 3 . 9 :

 39 Scheme 3.9: Catalytic cycle computed by Buntine for the production of acrylic acid from ethylene and CO 2 at [(DBU) 2 Ni] complexes.[46]

  3 kcal.mol -1 for various bidentate N-and P-type donor ligands. The oxygen atom of the carbonyl moiety attacks MeI, prior the insertion of iodide to the nickel center. The generated five coordinated nickel complex bear the weakly coordinated iodide in an apical position and can easily undergo isomerization at energy barriers lower than ΔG ≠ = 14.3 kcal.mol -1 . Iodide-oxygen interconversion facilitates β hydride elimination and the formation of a [(hydrido)(acrylate)Ni]complex [(L)2Ni(η 2 -CH2=CHC(O)OMe)(H)(I)]. After further isomerization, methyl acrylate is decoordinated and [(L)2Ni(H)(I)] formed alongside. However, the regeneration of the catalyst and the overall Gibb's free energy of the reaction ΔG = 19.1 kcal.mol -1 are unfavorable,

[

  (dtbpe)Ni(CH2CH2C(OMe)=O)] + I -. This reaction intermediate could be independently synthesized by adding acidic H(Et2O)2BArF to the methyl acrylate complex [(dtbpe)Ni(CH2=CHC(O)OMe)]. The nickel complex is in equilibrium with the analogous four membered cationic nickelalactone. The isomerization occurs through β agostic intermediates with an overall barrier of ΔG ≠ = 22.0 kcal.mol -1 , which is comparable to the experimentally measured ΔG ≠ = 21.5 kcal.mol -1 during kinetic investigations. In order to generate the π methyl acrylate nickel complex [(dtbpe)Ni(CH2=CHC(O)OMe)], the β agostic intermediate must be deprotonated, releasing HI.

[

  (dtbpe)nickelalactone] to sodium [(dtbpe)Ni(acrylate)]. It turns out that instead of having directly a β-hydride elimination on the nickelalactone, the base abstracts an acidic proton in α of the carbonyl group and the ligand rearranges itself to an acrylate. The sodium cation is of major importance as in its absence no reaction takes place and the regeneration of [(dtbpe)Ni(C2H4)] performs sluggishly. The setup of the catalytic process was intricate as it required for each cycle a CO2 rich regime to generate the nickelalactone followed by a CO2

Scheme 3 . 18 :

 318 Scheme 3.18: Reaction systems and TON recently described for the nickel catalyzed formation of

  Scheme 3.20: Reductive carboxylation of styrene using CO 2 and Et 2 Zn.[69]

  Scheme 3.21: Potential reactivity of nickelalactones with transmetallating reagents.

4. 1

 1 Synthesis of [(dcpp)Ni(C 2 H 4 )] IV-3 and [(dcpp)nickelalactone] IV-4 In order to investigate the reductive functionalization of CO2 at [(dcpp)nickel]-complexes, the metal-organic intermediates [(dcpp)Ni(C2H4)] IV-3 and [(dcpp)nickelalactone] IV-have been independently synthesized. 4.1.1 Synthesis of [(dcpp)Ni(C 2 H 4 )] IV-3 Starting from commercially available [Ni(COD)2] IV-2, [(dcpp)Ni(C2H4)] IV-3 is obtained through successive ligand substitutions, as shown in Scheme 4.1. The first 1,5-cyclooctadiene

Scheme 4 . 1 :

 41 Scheme 4.1: Synthesis of [(dcpp)Ni(C 2 H 4 )] IV-3 from [Ni(COD) 2 ] IV-2 through successive ligand substitutions.

Figure 4 . 1 :Scheme 4 . 2 :

 4142 Figure 4.1: Molecular structure of [(dcpp)Ni(C 2 H 4 )] IV-3 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: C 4 -C 5 1.407(15), P 1 -Ni 1 -P 2 104.50(10).

Figure 4 . 2 :

 42 Figure 4.2: Molecular structure of [(dcpp)nickelalactone] IV-4 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -O 1 1.9074(18), Ni 1 -C 3 1.968(3), C 1 -O 2 1.214(3), P 1 -Ni 1 -P 2 99.66(2).

4. 2 Scheme 4 . 3 : 4 .Scheme 4 . 4 :

 243444 Scheme 4.3: Equilibrium between [(dcpp)Ni(C 2 H 4 )] IV-3 and [(dcpp)nickelalactone] IV-4.

Figure 4 . 3 :

 43 Figure 4.3: Kinetic profile of the reaction in THF at 55 °C.

Figure 4 . 4 :

 44 Figure 4.4: ln([Nilactone] 0 /[Nilactone]) as a function of time for the reaction in THF at 55 °C.

4 .5 to 4 . 8 . 1 .

 4481 in THF ΔG ≠ = 27.21 kcal.mol -1 . This value seems coherent, since harsher conditions are required to push the reaction forward on either side. Afterwards, the experiment has been repeated at different temperatures ranging from 60 °C to 90 °C. The solvent has been switched from THF to toluene. The plots ln([Nilactone]0/[Nilactone]) = f(t) are represented in Figure The calculated rate constants kobs and the corresponding ΔG ≠ values can be found in Table 4.The average ΔG ≠ value is of 27.39 kcal.mol -1 .

Figure 4 . 5 :

 45 Figure 4.5: ln([Nilactone] 0 /[Nilactone]) over time for the reaction in toluene at 60° C.

Figure 4 . 6 :

 46 Figure 4.6: ln([Nilactone] 0 /[Nilactone]) over time for the reaction in toluene at 70 °C.

Figure 4 . 7 :

 47 Figure 4.7: ln([Nilactone] 0 /[Nilactone]) over time for the reaction in toluene at 80° C.

Figure 4 . 8 :

 48 Figure 4.8: ln([Nilactone] 0 /[Nilactone]) over time for the reaction in toluene at 90 °C.

Figure 4 . 9 : 3 .

 493 Figure 4.9: Eyring plot for the conversion of [(dcpp)nickelalactone] IV-4 into [(dcpp)Ni(C 2 H 4 )] IV-3 and CO 2 between 60 °C and 90 °C. Slope: -12673.609 K; intercept: 20.489.

Scheme 4 . 5 : 4 .Scheme 4 . 6 :- 8

 454468 Scheme 4.5: Reaction between [(dcpp)Ni(C 2 H 4 )] IV-3 and catecholborane in [d8]-THF at RT.

Scheme 4 . 7 :

 47 Scheme 4.7: Reaction between [(dcpp)nickelalactone] IV-4 and pinacolborane IV-8 in [d8]-THF at RT. After 10 min. formation of [(dcpp)NiH] 2 IV-9 is observed.

  C NMR at the end of the reaction. The spectrum is depicted in Figure 4.10. A doublet at δ = 10.3 ppm ( 1 JC, C = 137.7 Hz), a doublet of doublet at δ = 25.2 ppm ( 1 JC, C = 137.7 Hz, 1 JC, C = 151.5 Hz) and a doublet at δ = 66.7 ppm ( 1 JC, C = 151.5 Hz), each integrating for one carbon atom are assigned to the propyl chain of the organic product. Surprisingly, no signals are observed between 180 and 220 ppm indicating that the carbonyl function has been reduced. A small

Figure 4 . 10 :

 410 Figure 4.10: 13 C{ 1 H} NMR spectrum at 75 MHz of the reaction between 13 C[(dcpp)nickelalactone] IV-4( 13 C) and 3 eq. of pinacolborane IV-8 at RT. For more clarity solvent has been switched from [d8]-THF to C 6 D 6 .

Figure 4 . 11 :

 411 Figure 4.11: 1 H{ 13 C} 1D HSQC NMR spectrum at 500 MHz of the reaction between 13 C[(dcpp)nickelalactone] IV-4( 13 C) and 3 eq. of pinacolborane IV-8 at RT in [d8]-THF.

Scheme 4 . 10 :

 410 Scheme 4.10: Alternative synthesis of propoxypinacolborane IV-11.

1 H

 1 NMR spectra of the spiking experiment can be found in Figure4.12.

Figure 4 . 12 : 1 H

 4121 Figure 4.12: 1 H NMR spectra of the spiking experiment. a) Reaction between [(dcpp)nickelalactone] IV-4 and 3 eq. of pinacolborane IV-8 after 2 h. at RT. b) After addition of 5 eq. of propoxypinacolborane IV-11.

4. 4 8 . 8 Scheme 4 . 11 : 8 .Scheme 4 . 12 :

 4884118412 Scheme 4.11: First step of the mechanism of the reaction between [(dcpp)nickelalactone] IV-4 and pinacolborane IV-8.

Scheme 4 . 14 :

 414 Scheme 4.14: Metal hydride catalysts used for the reduction of CO 2 by pinacolborane to methanol derivatives.

  However, that doesn't rule out the presence of other catalytically active nickel hydride species. Additional experiments will be undertaken in order to quantify [(dcpp)NiH]2 IV-9 at the end of the reaction with an internal standard. The amount of [(dcpp)NiH]2 IV-9 should help discriminating between the proposed mechanisms. The reduction of propanoic acid pinacolborane ester IV-14 through a metal hydride would appear plausible if the yield of [(dcpp)NiH]2 IV-9 almost reaches 100 %. [(dcpp)NiH]2 IV-9 could arise from the recombination of less stable hydridic intermediates. Though, if the yield does not exceed 50 %, other mechanisms, especially involving Ni(I) intermediates (4.4.4), would seem more likely.4.4.3 Reduction of propanoic acid pinacolborane ester through oxidative additionAn alternative pathway for the reduction of propanoic acid pinacol ester IV-14 could be initiated by η 2 coordination of the C=O double bond to the naked Ni(0) fragment, followed by oxidative addition of nickel into the C=O bond. Reaction of the nickel-acyl complex with pinacolborane IV-8 would generate diboroxane pinBOBpin IV-12 and formaldehyde. The possible reaction scheme is depicted below.

Scheme 4 . 16 :

 416 Scheme 4.16: Reduction of the carbonyl through an oxidative addition mechanism.

  investigated Itami's results in depth by DFT calculations, trying to disclose the different mechanisms and to understand the origin of the various reactivities. It turns out that in any case the presence of an aromatic system is of utmost importance to achieve the oxidative addition of the Caryl-O and Cacyl-O bonds of an ester to the nickel center. The π electrons of the naphthyl moiety systematically assist the cleavage of C-O bonds.

Scheme 4 . 19 :

 419 Scheme 4.19:Houk and Garg's nickel catalyzed activation of C acyl -O bonds of methyl esters.[63]

14 .

 14 Scheme 4.20. Then, considering the strong B-O bond, oxidative addition can be performed only in the Cacyl-O bond CH3CH2C(O)-OBpin of the ester. However, the lack of π electrons in the molecule, which proved to be essential in all the processes presented above, probably disfavors the reaction.

Scheme 4 . 20 :

 420 Scheme 4.20: Additional planned experiment: reaction between the Ni(0) complex [(dcpp)Ni(toluene)] IV-17 and propanoic acid pinacolborane ester IV-14.

pinacolborane IV- 8

 8 to the [(dcpp)Ni(0)]-fragment, can then comproportionate in the presence of remaining Ni(0), leading to Ni(I) intermediates. This would explain the formation of [(dcpp)NiH]2 IV-9 and rationalize the presence of a Ni(I) boryl active catalyst [(dcpp)Ni(Bpin)], as shown in Scheme 4.21. A related mechanism supported by stoichiometric, kinetic and computational investigations has been suggested for the first time by Martin et. al.

  P{ 1 H} NMR spectroscopy. Further experiments involving EPR spectroscopy as well as the independent synthesis of the to date unknown [(dcpp)Ni(I)boryl] complex are now needed to prove this approach. Scheme 4.21 proposes a reaction pathway to the targeted compound. Dimeric Ni(I) complex [(dcpp)NiCl]2 IV-20 is obtained after one electron reduction of [(dcpp)NiCl2] IV-19 through KC8. In the next step, the dimer is broken down through the addition of bis(pinacolato)diborane B2pin2 IV-21, affording the Ni(I) boryl complex [(dcpp)Ni(Bpin)] IV-18 and B-chloropinacolborane pinBCl IV-22. The reaction will be driven by the formation of the strong B-Cl bond.

Scheme 4 . 22 :IV- 18 .

 42218 Scheme 4.22: Proposed reaction pathway for the synthesis of Ni(I) boryl complex [(dcpp)Ni(Bpin)] IV-18.

At 6 1 . 2 .

 612 bar of CO2 and 60 °C, [(dcpp)Ni(C2H4)] IV-3 can be oxidatively coupled in small amounts to [(dcpp)nickelalactone] IV-4, as discussed in paragraph 4.Catalysis would require higher CO2 pressures and moderate heating in an autoclave to achieve this reaction efficiently. During the next step [(dcpp)nickelalactone] IV-4 reacts quickly at RT with 3 eq. of pinacolborane IV-8 to generate the nickel dihydride complex [(dcpp)NiH]2 IV-9 together with propoxypinacolborane IV-11 and pinBOBpin IV-12. One equivalent of borane does the transmetallation and opens the nickelalactone, the other ones participate in the reduction of the ester to an alcohol derivative. Finally, the catalyst [(dcpp)Ni(C2H4)] IV-3 must be regenerated during the last step through ligand exchange. Diamagnetic [(dcpp)NiH]2 IV-9 is formally made of two paramagnetic Ni(I)

4. 23 .Scheme 4 . 23 :

 23423 Scheme 4.23: Proposed catalytic cycle for the reaction between ethylene, CO 2 and pinacolborane IV-8 at the [(dcpp)Ni]-center.

[Scheme 4 . 24 :

 424 Scheme 4.24: Reaction between [(dcpp)Ni(C 2 H 4 )] IV-3 and pinacolborane IV-8 under 1 bar of CO 2 .

Figure 4 . 13 :

 413 Figure 4.13: Molecular structure of [(dcpp)Ni(CO) 2 ] IV-23 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -P 1 2.2189(10), Ni 1 -P 2 2.2202(10), Ni 1 -C 1 1.768(4), Ni 1 -C 2 1.768(4), C 1 -O 1 1.151(5), C 2 O 2 1.144(4), P 1 -Ni 1 -P 2 101.83(4), C 1 -Ni 1 -C 2 111.62(17).

  (CO)2] IV-23 could arise from the reaction between [(dcpp)NiH]2 IV-9 and CO2 during the regeneration of [(dcpp)Ni(C2H4)] IV-3. Therefore, [(dcpp)NiH]2 IV-9 has been reacted with 1 bar of CO2. The reaction is much cleaner in toluene compared to THF. At RT the main product observed by 31 P{ 1 H} NMR is the known [(dcpp)Ni(CO2)] IV-24 complex, characterized by two doublets at δ = 7.55 ppm ( 2 JP, P = 28.2 Hz) and δ = 43.42 ppm

  chemistry of electron rich chelating bis-phosphine [(dcpp)nickel]-complexes was explored especially towards ethylene and CO2. The [(dcpp)Ni(C2H4)] IV-3 complex was synthesized from [Ni(COD)2] IV-2 in 64 % yield and [(dcpp)nickelalactone] IV-4 could be obtained from [(tmeda)nickelalactone] IV-5 in 68 % yield. These two complexes stand in equilibrium: the oxidative coupling between [(dcpp)Ni(C2H4)] IV-3 and CO2 is observed above 6 bar of CO2, while moderate temperatures reductively decouple [(dcpp)nickelalactone] IV-4. The activation parameters of the equilibrium were determined through a kinetic study. ΔG ≠ measures in average 27.4 kcal.mol -1 and the reaction is mainly enthalpy driven. In a second step, the cleavage of [(dcpp)nickelalactone] IV-4 by pinacolborane IV-8 was investigated. In the presence of 3 eq. of pinacolborane IV-8, the reaction generates [(dcpp)NiH]2 IV-9 and forms quantitatively propoxypinacolborane IV-11 along with diboroxane pinBOBpin IV-12. The first equivalent of pinacolborane IV-11 is involved in the transmetallation and opens [(dcpp)nickelalactone] IV-11 to afford a highly reactive [(dcpp)Ni(0)]-fragment and propanoic acid pinacolborane ester IV-14. The rest of the mechanism is not clear yet. The reduction of the ester into a propanol derivative could be achieved through three different pathways: through a nickel hydride catalyst, through oxidative addition of nickel in the C=O bond or through nickel(I) boryl intermediates. To date, none of them could be ruled out. Further experiments are planned to test each of these hypotheses and to get better insight into the mechanism. The catalytic production of propanote derivatives from ethylene and CO2 was considered at last. A catalytic cycle consisting in oxidative coupling between [(dcpp)Ni(C2H4)] IV-3 and CO2, transmetallation with pinacolborane IV-8, followed by the reduction of the intermediate ester into propoxypinacolborane IV-11 and ligand substitution from [(dcpp)NiH]2 IV-9 to regenerate [(dcpp)Ni(C2H4)] IV-3 could be built up stoichiometrically and run step by step. Oxidative addition requiring high CO2 pressures is presumably the rate determining step. However, the reaction performs poorly under catalytic conditions favoring the production of [(dcpp)Ni(CO)2] IV-23 over propoxypinacolborane IV-11. Even by varying the ethylene/CO2

1 H 2 A) ppm. 13 C{ 1

 12131 NMR (300 MHz, CD2Cl2): δ 0.39 (t,3 JH, H = 7.5 Hz, 2H, Ni-CH2), 1.78 (t,3 JH, H = 7.5 Hz, 2H, C(O)-CH2), 2.27 (s, 4H, N-CH2 αA + αB), 2.32 (s, 2H, N(CH3)2 B), 2.54 (s, 2H, N(CH3)H} NMR (75 MHz, CD2Cl2): δ -0.8 (Ni-CH2), 37.5 (C(O)-CH2), 47.1 (N(CH3)2 A), 49.2

13 C{ 1 3 [

 13 H} NMR (75 MHz, CD2Cl2): δ -0.8 (d, 1 JC, C = 33.8 Hz, Ni-CH2), 38.0 (m, C(O)-CH2), 47.0 (N(CH3)2 A), 49.2 (N(CH3)2 B), 56.5 (CH2 αA), 61.4 (CH2 αB), 189.5 (dd, 1 JC, C = 45.2 Hz, C=O) ppm. 4.8.3.2 Synthesis of [(dcpp)Ni(C 2 H 4 )] IV-Ni(COD)2] IV-2 (250 mg, 9.06×10 -4 mol, 1 eq.) is suspended in 10 mL of THF and reacted with the dcpp ligand IV-1 (C = 0.759 mol.L -1 in THF, 1.19 mL, 9.06×10 -4 mol, 1 eq.). The yellow reaction mixture, containing the [[(dcpp)Ni]2(COD)] dimer, is freeze-pump-thaw degassed and 1 bar of ethylene is applied to the Schlenk. The solution is stirred for 15 min. at RT and a new slightly brighter yellow complex falls out. The solvent is removed under reduced pressure, the product is taken up with pentane in the glovebox and transferred to a centrifugation vial. [(dcpp)Ni(C2H4)] IV-3 is washed 3 × with pentane and then transferred back to a Schlenk to dry the product under vaccum. [(dcpp)Ni(C2H4)] IV-3 is collected as a yellow powder in 64 % yield (301.6 mg). The complex has a very poor solubility in all common organic solvents. Single crystals for X-ray diffraction analysis were grown by heating a THF solution of [(dcpp)Ni(C2H4)] IV-3 to reflux and letting it slowly cool down to RT.

13 C{ 1

 1 H}{31P} NMR (125, [d8]-THF): δ 22.5, 27.3, 28.1, 29.6, 30.0 (Cy + CH2), 31.7 (CH2 ethylene) * , 37.7 ppm. 31 P{ 1 H} NMR (121.5 MHz, [d8]-THF): δ 25.2 (s) ppm.

  research turned towards softer aluminium compounds. The reactivity between [(dcpp)nickel]complexes and trialkyl aluminium derivatives has been explored. AlR3 transmetallating reagents are expected, as pinacolborane, to cleave the Ni-O bond of [(dcpp)nickelalactone] V-1 and induce lactone ring opening providing the possibility of a β-hydride elimination. 5.1 Stoichiometric synthesis of propanoic acid derivatives 5.1.1 Blank reactions A good candidate for transmetallation should be inert towards CO2 and [(dcpp)Ni(C2H4)] V-2.

Scheme 5 . 1 : 5 First

 515 Scheme 5.1: Different reactivity between AlEt 3 V-3 and CO 2 in aromatic solvents compared to THF.

Scheme 5 . 2 :

 52 Scheme 5.2: Reaction between13 C[(dcpp)nickelalactone] V-1( 13 C) and AlEt 3 V-3 at RT.

Scheme 5 . 3 :

 53 Scheme 5.3: Reaction between13 C[(dcpp)nickelalactone V-1( 13 C) and Al(nBu) 3 V-5 in [d8]-THF generating [(dcpp)Ni(1-butene)] V-6 and13 C labelled di(n-butyl)aluminium propanoate V-7( 13 C).

Figure 5 . 1 :

 51 Figure 5.1: 31 P{ 1 H} NMR spectrum of [(dcpp)Ni(1-butene)] V-6 in THF at RT.

[ 5 . 2 .

 52 dcpp)Ni(1-butene)] V-6 crystallizes in the unusual space group P1. The geometry around the nickel center is square planar as evidenced in Figure The bite angle P1Ni1P2 is comparable to the one of [(dcpp)Ni(C2H4)] V-2 (104.01(5) ° vs. 104.50(1) °). The C1C2 bond distance is clearly in the range of a double bond (C1C2 = 1.375(8) Å).

Figure 5 . 2 : 1 .

 521 Figure 5.2: Molecular structure of [(dcpp)Ni(1-butene)] V-6 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -C 1 1.956(5), Ni 1 -C 2 1.975(5), C 1 -C 2 1.375(8), C 2 -C 3 1.529(7), C 3 -C 4 1.524(7), P 1 -Ni 1 -P 2 104.01(5), C 1 -Ni 1 -C 2 40.9(2).

AlR

Scheme 5 . 4 : 3 .Scheme 5 . 5 :

 54355 Scheme 5.4: Proposed mechanism for the reaction between [(dcpp)nickelalactone] V-1 and AlR 3 derivatives leading to the formation of dialkylaluminium propanoates.

Scheme 5 . 6 :

 56 Scheme 5.6: Coordination of a second AlEt 3 molecule V-3 to the [(dcpp)nickelalactone-AlEt 3 ] adduct induces transmetallation and formation of a nickel propanoate complex.

When 13 C 5 . 3 .

 1353 [(dcpp)nickelalactone] V-1( 13 C) is reacted with only 1.5 eq. of Al(tBu)3 V-8 in [d8]-THF at 60 °C, a new compound is observed, giving rise to two multiplets in 31 P{ 1 H} NMR between δ = 12.9 ppm and δ = 13.6 ppm and between δ = 35.0 ppm and δ = 35.5 ppm. The 31 P{ 1 H} NMR spectrum of the reaction is presented in Figure By increasing the amount of

Figure 5 . 3 :

 53 Figure 5.3: 31 P{ 1 H} NMR spectrum of the reaction between 13 C[(dcpp)nickelalactone] V-1( 13 C) and 1.5 eq. of Al(tBu) 3 V-8 after 4 d. at 60 °C in [d8]-THF.

Figure 5 . 4 :

 54 Figure 5.4: Molecular structure of the 13 C[(dcpp)nickelalactone-Al(t-Bu) 3 ] adduct V-9( 13 C) determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°]: Ni 1 -C 1 1.966(3), Ni 1 -O 1 1.966(3), C 3 -O 2 1.246(4), O 2 -Al 1 1.831(2), P 1 -Ni 1 -P 2 98.97(3), C 1 -Ni 1 -O 1 83.95(12), C 3 -O2-Al 1 150.3(3).

Figure 5 . 4

 54 Figure 5.4 shows the crystal structure of 13 C[(dcpp)nickelalactone-Al(t-Bu)3] V-9( 13 C). The adduct crystallizes in the space group C2/c. The aluminium is coordinated to the metallalactone through the oxygen of the carbonyl. The geometry around the oxygen atom is slightly bent as evidenced by the C3-O2-Al1 angle that measures 150.3(3) °. The Al1-O2 bond is particularly elongated with 1.831(2) Å. In comparison the Al-O bond in Ogoshi's oxanickelacycle-AlMe2OTf complex obtained through the oxidative coupling between diphenylacetylene and tert-butanal at a PCy3 substituted nickel center measures 1.799(3) Å. In the opened intermediate [(PCy3)2Ni(benzaldehyde)-AlMe2OTf] adduct, the length of the Al-O bond reaches

5 . 5 .Scheme 5 . 10 :

 55510 Scheme 5.10: a) Reaction between [(dcpp)nickelalactone] V-1 and Al(tBu) 3 V-8 generating [(dcpp)Ni(2-methylpropene)] V-10. b) Reaction scheme of the independent synthesis of [(dcpp)Ni(2methylpropene)] V-10.

Figure 5 . 5 :

 55 Figure 5.5: 31 P{ 1 H} NMR spectrum of [(dcpp)Ni(2-methylpropene)] V-10 in THF at RT.

Scheme 5 . 1 and AlEt3 V- 3 Scheme 5 . 11 :

 513511 Scheme 5.11 shows the presumed mechanism of this reaction. During the first step [(dcpp)nickelalactone] V-1 is generated from the oxidative coupling between [(dcpp)Ni(C2H4)] V-2 and CO2. Afterwards, the reaction between [(dcpp)nickelalactone] V-1 and AlEt3 V-3 releases diethylaluminium propanoate V-4 and regenerates directly the catalytic intermediate [(dcpp)Ni(C2H4)] V-2, without any need of additional ethylene. THF will be the solvent of choice. Aromatic solvents must be avoided because they promote the direct reaction between AlEt3 V-3 and CO2. Besides CO2 has a greater solubility in THF compared to benzene or toluene, which facilitates the formation of [(dcpp)nickelalactone] V-1.

Scheme 6 . 2 :

 62 Scheme 6.2: Synthesis of [(bis-NHC)Ni(II)] complexes. The outcome of the reaction is controlled by the size of the R substituents and the length of the spacer.

Scheme 6 . 10 :

 610 Scheme 6.10: Synthesis of [(LtBu)Ni(C 2 H 4 )] VI-9 and [(LDipp)Ni(C 2 H 4 )] VI-10.

Figure 6 . 1 :

 61 Figure 6.1: Molecular structure of [(LtBu)Ni(C 2 H 4 )] VI-9 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -C 1 1.953(4), Ni 1 -C 2 1.947(3), Ni 1 -C 3 1.923(3), Ni 1 -C 4 1.903(3), C 1 -C 2 1.419(5), C 1 -Ni 1 -C 2 42.66(15), C 3 -Ni 1 -C 4 91.45(14).

[Scheme 6 . 11 :

 611 Scheme 6.11: Synthesis of [(LtBu)Ni(diphenylacetylene)] VI-13 and [(LtBu)Ni(phenylacetylene)] VI-14.

  C shift of the triple bond in [(LtBu)Ni(diphenylacetylene)] VI-13 is located at δ = 138.7 ppm. The acetylenic carbons in [(LtBu)Ni(phenylacetylene)] VI-14 are found at δ = 124.1 ppm and δ = 138.49 ppm and the corresponding proton appears at δ = 6.49 ppm. This points towards a strong π-back donation that highly weakens the triple bond of the alkyne. In addition, both complexes could be crystallized from THF solutions either at RT or at -30 °C. The geometric parameters determined by X-ray diffraction back up the NMR results. The Ni-Cacetylene bonds are about 4 Å shorter than the Ni-CNHC bonds due to the back bonding. The acetylenic C1-C2 bonds measure 1.280(8) Å and 1.292(6) Å respectively and are closer to a C-C double bond than to a triple bond. The C18-C1-C2 (147.0(7) °) and C24-C2-C1 (145.1(7) °) angles in [(LtBu)Ni(diphenylacetylene)] VI-13 and H1-C1-C2 (144.8 °) and C18-C2-C1 (140.9(4) °) in [(LtBu)Ni(phenylacetylene)] VI-14 show that the alkynes are bent. The complexes can therefore also be described as metallacyclopropenes.

Figure 6 . 2 : 2 40

 622 Figure 6.2: Molecular structure of [(LtBu)Ni(diphenylacetylene)] VI-13 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -C 1 1.859(6), Ni 1 -C 2 1.866(7), Ni 1 -C 3 1.896(7), Ni 1 -C 4 1.901(7), C 1 -C 2 1.280(8), C 1 -Ni 1 -C 2 40.2(3), C 3 -Ni 1 -C 4 90.1(3), C 18 -C 1 -C 2 147.0(7), C 24 -C 2 -C 1 145.1(7).

Figure 6 . 3 :

 63 Figure 6.3: Molecular structure of [(LtBu)Ni(phenylacetylene)] VI-14 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -C 1 1.868(5), Ni 1 -C 2 1.880(4), Ni 1 -C 3 1.917(4), Ni 1 -C 4 1.923(4), C 1 -C 2 1.292(6), C 1 -H 1 0.9500, C 1 -Ni 1 -C 2 40.32(19), C 3 -Ni 1 -C 4 90.54(18), H 1 -C 1 -C 2 144.8, C 18 -C 2 -C 1 140.9(4).

Figure 6 . 4 :

 64 Figure 6.4: Molecular structure of [(LtBu)Ni(CO) 2 ] VI-15 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -C 1 1.755(4), Ni 1 -C 2 1.755(4), Ni 1 -C 3 2.009(3), Ni 1 -C 5 1.996(3), C 1 -O 1 1.167(5), C 2 -O 2 1.159(4), C 1 -Ni 1 -C 2 110.89(17), C 3 -Ni 1 -C 5 95.39(14).

  C NMR confirms the coordination of the bis-carbene to the nickel center with two quaternary carbons at δ = 180.6 ppm and δ = 186.1 ppm as well as the presence of a C=O bond at δ = 189.6 ppm. The presence of the lactone is further evidenced by FTIR spectroscopy displaying an absorption band at 1619 cm -1 . Single crystals for X-ray diffraction analysis have been grown from an acetonitrile solution at RT. [(LtBu)nickelalactone] VI-18 crystalizes in the space group P21/n and the geometry around the nickel center is square planar. The bite angle C4Ni1C5 (86.89(17) °) is significantly smaller compared to [(dcpp)nickelalactone] IV-4 (99.66(2) °). The chelate ring adopts a boat shape conformation. The Ni1C1 (1.942(4) Å) and Ni1O1 (1.909(3) Å) bond lengths are otherwise comparable to [(dcpp)nickelalactone] IV-4 (Ni1-C3 1.968(3) Å and Ni1-O1 1.9074(18) Å). The particularly elongated Ni1O1 bond should ease the cleavage of the lactone.

Figure 6 . 5 :

 65 Figure 6.5: Molecular structure of [(LtBu)nickelalactone] VI-18 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -O 1 1.909(3), Ni 1 -C 1 1.942(4), C 3 -O 2 1.228(5), Ni 1 -C 4 1.860(4), Ni 1 -C 5 1.957(4), C 4 -Ni 1 -C 5 86.89(17), C 1 -Ni 1 -O 1 . 84.73(16).

Figure 6 . 6 :

 66 Figure 6.6: Molecular structure of [(LtBu) prop nickelalactone] VI-20 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -O 1 1.9237(15), Ni 1 -C 1 1.961(2), C 3 -O 2 1.239(3), Ni 1 -C 4 1.853(2), Ni 1 -C 5 1.966(2), C 4 -Ni 1 -C 5 98.46(8), C 1 -Ni 1 -O 1 84.19(8).

Scheme 6 . 16 :

 616 Scheme 6.16: Two step synthesis of [LtBu)nickela-2-oxacyclopenten-4,5-diphenyl-3-one] VI-22.

Figure 6 . 7 :Scheme 6 . 17 :

 67617 Figure 6.7: Molecular structure of [(LtBu)nickela-2-oxacyclopenten-4,5-diphenyl-3-one] VI-27 determined by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni 1 -O 1 1.8837(14), Ni 1 -C 3 1.9520(19), C 2 -C 3 1.357(3), C 1 -O 2 1.236(2), Ni 1 -C 4 1.8658(19), Ni 1 -C 5 1.954(2), C 4 -Ni 1 -C 5 86.31(8), C 3 -Ni 1 -O 1 84.87(7).

Figure 6 . 8

 68 Figure 6.8 shows comparatively the energy profiles of the oxidative coupling for the dcpp and the LMe ligands.

Figure 6 . 8 :

 68 Figure 6.8: Comparison between the energy profiles of the oxidative coupling for the dcpp and LMe ligands at the wB97XD(THF)/TZVP level of theory.

Scheme 6 . 19 :

 619 Scheme 6.19: Two step synthesis of13 C[(LtBu)nickelalactone] VI-18( 13 C).

  )] and [(bis-NHC)Ni(II)]-complexes in good yields. In this way, starting from [Ni(COD)2] VI-8 through successive ligand substitutions, the family of [(bis-NHC)Ni-(alkene)] complexes could be extended to so far unreported ethylene compounds [(LtBu)Ni(C2H4)] VI-9 and [(LDipp)Ni(C2H4)] VI-10. Furthermore, the first [(bis-NHC)Ni-(alkyne)] complexes [(LtBu)Ni(diphenylacetylene)] VI-13 and [(LtBu)Ni (phenylacetylene)] VI-14 were described. The unsaturated [(bis-NHC)Ni(0)]-complexes were then successfully converted into the [(LtBu)Ni(CO)2] VI-15 and [(LDipp)Ni(CO)2] VI-16 compounds upon addition of carbon monoxide. Despite their high sensitivity, [(bis-NHC)Ni(0)]-complexes could constitute a good entry point in catalytic cycles. It would be interesting to extend the scope of available [(bis-NHC)Ni(

  addition, the first [(bis-NHC)nickelalactones], [(LtBu)nickelalactone] VI-18, [(LDipp)nickelalactone] VI-19, [(LtBu) prop nickelalactone] VI-20 and [(LtBu)nickela-2oxacyclopenten-4,5-diphenyl-3-one] VI-22 have been synthesized in good yields through ligand substitution from [(tmeda)nickelalactone] VI-17. They are also easily accessible through oxidative coupling between unsaturated [(bis-NHC)Ni(0)]-complexes, [(LtBu)Ni(C2H4)] VI-9, [(Dipp)Ni(C2H4)] V-10 and [(LtBu)Ni(diphenylacetylene)] VI-13, and CO2 under mild conditions. However, more efforts are still required to convert them efficiently into valuable chemicals such as acrylates or alcoholates.6.6 Experimental part 6.6.1 General RemarksAll reactions were carried out under an atmosphere of dry argon using standard Schlenk techniques or in a nitrogen-filled MBraun LabStar glovebox. Acetonitrile, diethyl ether, THF and pentane were taken from a MBraun SPS-800 solvent purification system. Acetonitrile, methanol and water were degassed by bubbling argon and all the other solvents were degassed by using the freeze-pump-thaw procedure. [d8]-THF, CD3CN and MeOD were degassed and stored over 4 Å molecular sieves.

13 C{ 1 H}

 1 NMR (75 MHz, [d6]-DMSO): δ 29.0 (CCH3), 29.2 (N-CH2-CH2-CH2-N), 46.1 (N-CH2), 59.6 (CCH3), 120.3 (CH imi), 122.6 (CH imi), 134.9 (N=CH-N) ppm. 6.6.3 Synthesis of [(bis-NHC)Ni(0)-alkene] and [-alkyne] complexes 6.6.3.1 [(LtBu)Ni(C 2 H 4 )] VI-9Di-tert-butyl-methylene-imidazolium dibromide LtBuH2Br2 VI-5 (400.0 mg, 9.47×10 -4 mol, 1 eq.) and KOtBu (233.9 mg, 2.08×10 -3 mol, 2.2 eq.) are mixed together in a Schlenk flask and suspended in 40 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [Ni(COD)2] VI-8 (261.5 mg, 9.47×10 -4 mol, 1 eq.) is added to the yellow solution and 10 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing [Ni(COD)2] VI-8. The solution is freeze-pump-thaw degassed without any previous stirring and 1 bar of ethylene is applied to the Schlenk. The reaction turns bright orange and is stirred overnight at RT. The gas and the solvent are removed under reduced pressure. The residue is washed with 3 × 15 mL of pentane and dried overnight at 70 °C under vacuum. [(LtBu)Ni(C2H4)] VI-9 is gathered as a brown product in 75% yield (247.6 mg).

13 C{ 1 H}NMR

 1 (125 MHz, CD3CN): δ 8.2 (Ni-CH2), 31.9 (CCH3), 38.5 (O-C(O)-CH2), 49.2 (N-CH2-CH2-CH2-N), 58.5 (CCH3), 119.2 (CH imi), 119.4 (CH imi), 122.2 (CH imi), 123.3 (CH imi), 179.3 (N=C-N), 189.2 (C=O) ppm. IR (cm -1 ): ν (C=O) = 1615 (m) 6.6.5.5 [(tmeda)Ni(C(Ph)=C(Ph)COO-κC,κO)] VI-21 [(tmeda)Ni(C(Ph)=C(Ph)COO-κC,κO)] VI-21 was prepared according to a modified published

13 C{ 1 6 ( 5 (

 165 H} NMR (75 MHz, CD3CN): δ 30.6 (CCH3), 31.3 (CCH3), 58.5 (CCH3), 59.2 (CCH3), 64.3 (N-CH2-N), 119.3 (CH imi A), 120.5 (CH imi B), 121.2 (CH imi B), 122.3 (CH imi A), 123.9 (CH para), 125.2 (CH para), 127.3 (CH meta), 127.6 (CH meta), 127.7 (CH ortho), 131.CH ortho), 141.0 (Ni-C(Ph)), 141.6 (O-C(O)-C(Ph)), 142.0 (C ipso), 176.0 (N-C-N), 176.C=O), 179.5 (N=C-N) ppm. IR (cm -1 ): ν (C=O) = 1604 (m).

7 Appendices 7 . 1

 771 Crystallographic dataSelected crystallographic data are summarized in the following tables.

7. 2 Scheme 7 . 1 : 1 H 1 .Schéma 1 :

 271111 Scheme 7.1: 1 H NMR spectrum of the ethylenic protons of [(LtBu)Ni(C 2 H 4 )] IV-9 at 300 MHz.

Figure 1 : 4 Schéma 8 : 5 .

 1485 Figure 1: Structure moléculaire de [(dcpp)Ni(C 2 H 4 )] IV-3 determinée par diffraction de rayons X sur monocristal. Les atomes d'hydrogènes sont omis par souci de clarté. Longueurs de liaisons [Å] et angles [°] sélectionnés: C 4 -C 5 1,407(15), P 1 -Ni 1 -P 2 104,50(10).

Figure 2 :

 2 Figure 2: Structure moléculaire de [(dcpp)nickelalactone] IV-4 determinée par diffraction des rayons X sur monocristal. Les atomes d'hydrogène sont omis par souci de clarté. Longueurs de liaisons [Å] et angles [°] sélectionnés: Ni 1 -O 1 1,9074(18), Ni 1 -C 3 1,968(3), C 1 -O 2 1,214(3), P 1 -Ni 1 -P 2 99,66(2).

[ 3 et CO2. 7 . 3 . 2 . 4 4 4 .

 3732444 (dcpp)nickelalactone] IV-4 est stable à temperature ambiante sous atmosphère inerte pendant des mois, mais se dégrade complètement à 60 °C en l'espace de 3 j. en [(dcpp)Ni(C2H4)] IV-Etude cinétique de l'équilibre entre [(dcpp)Ni(C 2 H 4 )] IV-3 et [(dcpp)nickelalactone] IV-La synthèse et l'étude de la stabilité de [(dcpp)nickelalactone] IV-4 ont mis en évidence l'équilibre entre [(dcpp)Ni(C2H4)] IV-3 et [(dcpp)nickelalactone] IV-Des études cinétiques ont été menées afin de déterminer l'ordre de la réaction et les paramètres d'activation. Pour cela une quantité connue de [(dcpp)nickelalactone] IV-4 est chauffée à différentes températures comprises entre 55 °C et 90 °C avec PPh3 comme standard interne. La décroissance de la concentration de [(dcpp)nickelalactone] IV-4 est suivie par RMN 31 P{ 1 H} (Figure 3).

Figure 3 :

 3 Figure 3: Profil cinétique de la réaction à 55 °C.

Figure 4 :Figure 6 :

 46 Figure 4: ln([Nilactone] 0 /[Nilactone]) en fonction du temps pour la réaction dans le THF à 55 °C.

Figure 7 : 7 . 3 . 2 . 6 8 .

 773268 Figure 7: Spectre RMN 1 H NMR de l'expérience de spiking. a) Réaction entre [(dcpp)nickelalactone] IV-4 et 3 eq. de pinacolborane IV-8 après 2 h. à température ambiante. b) Après ajout de 5 eq. de propoxypinacolborane IV-11.

Figure 8 :Schéma 13 :

 813 Figure 8: Structure moléculaire de [(LtBu)Ni(C 2 H 4 )] VI-9 determinée par diffraction de rayons X sur monocristal. Les atomes d'hydrogène sont omis par souci de clarté. Longueurs de liaisons [Å] et angles [°] sélectionnés: Ni 1 -C 1 1,953(4), Ni 1 -C 2 1,947(3), Ni 1 -C 3 1,923(3), Ni 1 -C 4 1,903(3), C 1 -C 2 1,419(5), C 1 -Ni 1 -C 2 42,66(15), C 3 -Ni 1 -C 4 91,45(14).

Figure 9 :

 9 Figure 9: Structure moléculaire de [(LtBu)nickelalactone] VI-18 determinée par diffraction de rayons X sur monocristal. Les atomes d'hydrogènes sont omis par souci de clarté. Longueurs de liaisons [Å] et angles [°] sélectionnés: Ni 1 -O 1 1,909(3), Ni 1 -C 1 1,942(4), C 3 -O 2 1,228(5), Ni 1 -C 4 1,860(4), Ni 1 -C 5 1,957(4), C 4 -Ni 1 -C 5 86,89(17), C 1 -Ni 1 -O 1 . 84,73(16).

  d'intermédiaires Ni(0)/Ni(II) au cours de la catalyse et exclut tout passage par un mécanisme de type Ni(I)/Ni(III). La seconde partie de ce travail porte sur le couplage oxydant entre l'éthylène et le CO2 sur des complexes de nickel chélatés par des bis-phosphines ou des bis-NHC. Une étude cinétique de l'équilibre entre [(dcpp)Ni(C2H4)] et [(dcpp)nickelalactone] a été réalisée. Le complexe [(dcpp)nickelalactone] peut ensuite être réduit et fonctionnalisé en dérivé du propanol en présence de pinacolborane. Des études mécanistiques et catalytiques préliminaires ont eté menées. De plus, de nouvelles méthodologies de synthèses ont été développées afin d'obtenir les premiers complexes [(bis-NHC)Ni(C2H4)] et [(bis-NHC)nickelalactone].

  d'intermédiaires Ni(0)/Ni(II) au cours de la catalyse et exclut tout passage par un mécanisme de type Ni(I)/Ni(III). La seconde partie de ce travail porte sur le couplage oxydant entre l'éthylène et le CO2 sur des complexes de nickel chélatés par des bis-phosphines ou des bis-NHC. Une étude cinétique de l'équilibre entre [(dcpp)Ni(C2H4)] et [(dcpp)nickelalactone] a été réalisée. Le complexe [(dcpp)nickelalactone] peut ensuite être réduit et fonctionnalisé en dérivé du propanol en présence de pinacolborane. Des études mécanistiques et catalytiques préliminaires ont eté menées. De plus, de nouvelles méthodologies de synthèses ont été développées afin d'obtenir les premiers complexes [(bis-NHC)Ni(C2H4)] et [(bis-NHC)nickelalactone].

Table of Contents I Table of Contents

 ofIof

  When the amount of nickel complex is reduced to 0.2 mol%, this model reaction performs slower: 19 h. are required to complete the reaction. At 0.01 % catalyst loading, the yield reaches 21 % after 58 h. and 47 % after 138 h., while the turnover frequency (TOF) keeps constant at 35 h -1 . This proves that the catalyst remains active over long periods of time and does not undergo any decomposition. The results are summarized in Table2.1.

Table 2 . 1 :

 21 Optimization of the reaction conditions: varying the catalyst loading.

	Entry Catalyst loading (mol%) Reaction time (h) Conversion (%)	TOF (h -1 )
	1	1	6	97	17
	2	0.2	19	98	26
	3	0.01	58	21	36
	4	0.01	138	47	34
	2.2.2 Reaction scope

With these efficient conditions in hand, the scope of the reaction was explored. The following experiments were performed using 0.2 mol% of [(dcpp)Ni(toluene)] II-4.

First of all, the electronic nature of the para substituent of the aryl chloride is modulated. As expected, electron donating groups on the electrophile, such as methyl or methoxy moieties, slow down the reaction. At 0.2 mol% of [(dcpp)Ni(toluene)] II-4, the reaction is complete within 16 h. for chlorobenzene II-11, 19 h. for 4-chlorotoluene II-7 and 32 h. for 4chloroanisole (Entry 3 -5). On the other hand, electron withdrawing groups like, a methyl ester or a CF3 unit, considerably increase the reaction rate. Full conversion is reached within only 1 h. for 4-chlorotrifluorotoluene II-12 and 6 h. for methyl 4-chlorobenzoate (Entry 1 -2). All the results are collected in Table

2

.2.

Table 2 . 2 :

 22 Influence of the para substituent of the aryl chloride on the reaction time.Afterwards, the electronic properties of the zinc derivative are varied. As commonly observed, adding an electron rich methoxy substituent on the para position of the nucleophilic reagent results in faster coupling. Table2.3 compares the reaction times between cross couplings

	Entry para substituent Reaction time (h) Isolated yield (%)
	1	CF3	1	94
	2	CO2Me	6	92
	3	H	16	98
	4	CH3	19	93
	5	OMe	32	90

carried out with PhZnCl II-8 and 4-OMePhZnCl II-13. For example, 4-chlorotoluene II-7 is coupled within 19 h. with PhZnCl II-8 whereas only 8 h. are required with 4-OMePhZnCl II-13 (Entry 4). Similarly, the reaction time decreases from 6 h. to 1 h. 30 min. for methyl 4chlorobenzoate by switching from the electron neutral to the electron rich phenyl chloride (Entry 2).

Table 2 . 3

 23

	Entry	para	Reaction time (h)		Yield (%)
		substituent
			PhZnCl	4-OMe-PhZnCl	PhZnCl	4-OMe-PhZnCl
	1	CF3	1	0.5	94	91
	2	CO2Me	6	1.5	92	89
	3	H	16	4	98	90
	4	CH3	19	8	93	88

: Comparison between the reaction times of [(dcpp)Ni(toluene)] II-4 catalyzed Negishi cross coupling using either PhZnCl II-8 or OMe-PhZnCl II-13.

By increasing the catalyst loading to 1 mol% more challenging substrates could also be efficiently coupled within short reaction times. The reaction between the ortho disubstituted 2,6-dimethylphenyl chloride and PhZnCl II-8 affords the corresponding biphenyl in 75 % yield after 7 h. Moreover, several chloropyridines also proved to be good coupling partners. For example, 2chloropyridine is coupled with PhZnCl II-8 in 96 % isolated yield in only 2 h. The reaction does even tolerate functional amino groups. When 2-chloro-3-amino-pyridine undergoes Negishi cross coupling, 90 % of 2-phenyl-3-aminopyridine are isolated after 4 h. The results are summarized in Table

2

.4.

Table 2 . 4

 24

	Entry	R-PhCl	Product	Reaction time (h) Isolated yield (%)
	1	2,6-(Me)2PhCl		7	75
	2	2-ClPy		2	96
	3	2,6-Cl2Py		6	58

: [(dcpp)Ni(toluene)] 4 catalyzed Negishi cross coupling of challenging substrates.

II-14 further reacts with PhZnCl II-8 to give the homocoupled product Ph-Ph II-10, which

  is identified by GC-MS spectrometry. Unfortunately, the second compound at δ = 56.0 ppm II-17 could not be separated from the other products or crystallized and remains therefore unidentified.

	The reactivity of the dimeric Ni(I) complex [(dcpp)NiCl]2 II-16 has been investigated to further
	rule out the involvement of a Ni(I)/Ni(III) catalytic cycle.
	[(dcpp)NiCl]2 II-16 does not react with an excess of chlorobenzene II-11 in toluene even after
	days at RT. However, when [(dcpp)NiCl]2 II-16 is reacted with a stoichiometric amount of
	PhZnCl II-8, [(dcpp)Ni(Ph)(Cl)] II-14 and a new compound characterized by a singlet at
	δ = 56.0 ppm in 31 P{ 1 H} NMR (II-17) are formed at RT within minutes. This mixture rapidly dχ''/dB evolves as [(dcpp)Ni(Ph)(Cl)]
	2750	2850	2950	3050	3150	3250	3350	3450	3550
				B in G

1 -P 1 2.1856(4), Ni 1 -P 2 2.1918(4), Ni 1 -Cl 1 2.3779(4), Ni 1 -Cl 1A 2.3361(4), Ni 1A -Cl 1 2.3361(4) P 1 -Ni 1 -P 2 101.721(17), Cl 1 -Ni 1 -Cl 1A 90.688(15).

chlorotoluene II-7 and PhZnCl II-8 at 1 mol% of catalyst

  in THF at 60 °C is chosen as standard reaction for the comparison of the catalysts. The results can be found in

	different oxidation states is then tested to confirm the involvement of a Ni(0)/Ni(II) catalytic
	cycle. The Negishi coupling between 4-
	2.3.2 Catalytic activity of Ni(0), Ni(I) and Ni(II) precursors
	All catalytic reactions presented above in 2.2 are carried out using the highly sensitive Ni(0)
	precatalyst [(dcpp)Ni(toluene)] II-4. The catalytic activity of various nickel complexes in

Table 2.5. Scheme 2.12: Standard

  reaction conditions chosen for the nickel catalyzed Negishi cross coupling to compare various catalysts.

	It is first verified that more easily manipulated Ni(II) complexes give similar results to
	[(dcpp)Ni(toluene)] II-4. The precatalyst [(dcpp)NiCl2] II-3 and the isolated oxidative addition
	intermediate [(dcpp)Ni(Ph)(Cl)] II-14 produce respectively 97 % and 95 % GC yield of cross
	coupled product II-9 together with 3 % and 5 % homocoupled product II-10 within 6 h.,
	supporting a Ni(0)/Ni(II) active catalytic cycle.
	However, when the isolated Ni(I) complex [(dcpp)NiCl]2 II-16 is used instead, the reaction
	performs sluggishly, reaching 40 % conversion after 72 h. The coupling is also less selective
	affording only 7 % of the desired 4-

methylbiphenyl II-9 and 33 % of biphenyl II-10, the homocoupled product. This specific Ni(I) complex II-16 is not an active species of the catalysis

	and demonstrates that a Ni(I)/Ni(III) cycle is much less efficient for this transformation.

Table 2 .5:

 2 Comparison of the conversion of the Negishi cross coupling reactions performed with nickel precursors at different oxidation states.

	Entry Oxidation state	Catalyst	Time (h.)	% Conv.	% Conv.
					4-CH3Ph-Ph	Ph-Ph
	1	Ni(0)	[(dcpp)Ni(toluene)] II-4	6	97	3
	2	Ni(II)	[(dcpp)NiCl2] II-3	5	97	3
	3	Ni(II)	[(dcpp)Ni(Ph)(Cl)] II-14	6	95	5
	4	Ni(I)	[(dcppNiCl]2 II-16	72	7

II-18 or to the heterobimetallic adduct with PhZnCl II-8 [

	3 Proposed catalytic cycle
	In light of the performed stoichiometric and catalytic reactions, a catalytic cycle is proposed in
	Scheme 2.13 for the [(dcpp)Ni(toluene)] II-4 catalyzed Negishi cross coupling between aryl
	halides and phenyl zinc chloride derivatives. The reaction consists of three elementary steps:
	oxidative addition of the aryl chloride to [(dcpp)Ni(0)], transmetallation at [(dcpp)Ni(Ar)(Cl)]
	with Ar'ZnCl and reductive elimination from [(dcpp)Ni(Ar)(Ar')]. All these elementary steps
	are achievable under mild conditions and involve either Ni(0) or Ni(II) intermediates.
	In order to identify the resting state of the catalyst, the reaction of 4-chlorotrifluorotoluene II-
	12 with PhZnCl II-8 catalyzed by 1 mol% of [(dcpp)Ni(toluene)] II-4 is monitored at 60 °C by
	31 P{ 1 H} NMR spectroscopy. The only species observed throughout the reaction, at 30 %, 70 %
	and at ca. full conversion, is characterized by two doublets at δ = 5.9 ppm and δ = 19.7 ppm,
	with 2 JP, P = 56.0 Hz. This complex displays almost identical spectroscopic features to
	[(dcpp)Ni(4-CF3Ph)(Cl)] II-18, that is, two doublets at δ = 5.9 ppm and δ = 19.3 ppm, with
	2 JP, P = 53,0 Hz in C6D6. It might either correspond to the oxidative addition product
	[(dcpp)Ni(4-CF3Ph)(Cl)] (dcpp)Ni(4-
	CF3Ph)(µ-Cl)Zn(Cl)(Ph)(THF)], making the transmetallation the rate determining step of the
	reaction. In contrast to Hartwig's [(BINAP)Ni(η 2 -NC-Ph)] complex that evolves to
	[(BINAP)2Ni

  The transition state TS EF for the transmetallation itself could then be found 21.8 kcal.mol -1 above E. In TS EF the Ni-Cl and Zn-Cl bonds are respectively 0.10 Å longer and 0.16 Å shorter than in E, showing the chloride transfer from the nickel to the zinc atom. Though the Zn-Ph

	bond is only slightly elongated by 0.07 Å and the Ni-Ph bond is still rather long measuring
	2.58 Å. Furthermore, a very short Ni-Zn intermetallic distance of 2.59 Å is calculated. The
	distance is clearly shorter than the sum of the van der Waals radii (3.02 Å) and just a bit longer
	than the sum of the covalent radii (2.52 Å), suggesting a metallophilic interaction. Transition

  The dissociation of the η 2 coordinated biphenyl from H requires 22.9 kcal.mol -1 . Similarly to the ligand to substrate exchange, the reaction proceeds through the highly electron deficient intermediate B, that takes in an aryl chloride moiety to produce η 2 coordinated [(dcpp)Ni(PhCl)] C. The product to substrate exchange is exergonic by 1.8 kcal.mol -1 .

	The associative mechanism proceeds through an 18 electron transition state TS HB2 that bears
	two η 1 coordinated ligands. Dissociation of the biphenyl leads to intermediate B 2 , which
	features an η 1 coordinated PhCl, that rearranges itself into η 2 coordinated [(dcpp)Ni(PhCl)] C.

II-14 or its adduct with PhZnCl II-8 [

	arene complex [(dcpp)Ni(toluene)] II-4, achieved by the two electron
	reduction of the Ni(II) precursor [(dcpp)NiCl2] II-3 with KC8 in toluene, provides a highly
	reactive catalyst for the Negishi cross coupling. [(dcpp)Ni(toluene)] II-4 catalyzes efficiently
	the reaction between aryl chlorides and an equimolar amount of phenyl zinc chloride derivatives
	at low catalyst loadings down to 0.2 mol%, under mild conditions. The catalyst remains active
	over long periods of time and does not show any sign of deactivation. Electron poor and electron
	rich substrates could be coupled in excellent yields and even challenging ortho disubstituted
	aryl chlorides or chloropyridines afforded the cross coupled products in good yields.
	Remarkably, the reaction also tolerates the amino functional group. This is the first time that a
	strongly electron donating bis-phosphine supported catalyst proves to be excellent for this type
	of nickel catalyzed process.
	Stoichiometric experiments supported by DFT calculations demonstrate the involvement of a
	Ni(0)/Ni(II) catalytic cycle upon use of chelating bis-phosphine ligands. All three elementary
	steps, i.e. oxidative addition, transmetallation and reductive elimination, are facile processes at
	RT and generate exclusively Ni(0) and Ni(II) intermediates among which
	[(dcpp)Ni(Ph)(Cl)] II-14 could be isolated and characterized. The DFT calculations further
	confirm that the oxidative addition and reductive elimination have particularly low activation
	barriers whereas the transmetallation and product to reagent exchange constitutes the rate
	determining steps. The catalyst's resting state is found to be either the oxidative addition
	product [(dcpp)Ni(Ph)(Cl)] (dcpp)Ni(Ph)[µ-
	Cl]Zn(Cl)(Ph)(THF)].
	No redox processes leading to Ni(I) intermediates could be identified in the reaction and
	isolated Ni(I) complex [(dcpp)NiCl]2 II-16 preferentially generates diamagnetic Ni(II) and/or
	Ni(0) species in the presence of PhZnCl II-8. When Ni(I) compounds are involved in the
	Negishi cross coupling, they lead to poor conversions and selectivities, favoring the
	homocoupled product over the cross coupled product. The use of a strongly donating bidentate
	bis-phosphine ligand is therefore key to shut down a Ni(I)/Ni(III) catalytic cycle.

  EPR at RT: g = 2.154, Aiso( 31 P, n = 2) = 72.2 G.

Elt. Anal. Calcd for C54H100Cl2Ni2P4 (1058.49): C, 61.10; H, 9.49. Found: C, 61.01; H, 9.64. 2.6.4 Negishi cross coupling 2.6.4.1 Synthesis of PhZnCl.LiCl II-8

Table 4 . 1 :

 41 Rate constants and calculated ΔG ≠ values for different temperatures.

	Temperature (°C)	Solvent					kobs (s -1 )		ΔG ≠ (kcal.mol -1 )
	55	THF					5.005×10 -6	27.21
	60	toluene					7.365×10 -6	27.38
	70	toluene					2.691×10 -5	27.34
	80	toluene					7.735×10 -5	27.42
	90	toluene					1.861×10 -4	27.58
	mean value	-						-			27.39
	This temperature variation allows to further get access to the activation parameters ΔH ≠ and
	ΔS ≠ of the reaction. These can be directly read from an Eyring plot (Equation 4.2).
	Equation 4.2
	ln	= -	∆	R	×	1	+ ln	k h	+	∆ R

Table 4 .

 4 2 gives the activation parameters determined by the Eyring plot. The reaction is mainly enthalpy driven. The entropy contribution ΔS ≠ is small.

Table 4 . 2 :

 42 Activation parameters for the conversion of [(dcpp)nickelalactone] IV-4 into [(dcpp)Ni(C 2 H 4 )] IV-3 and CO 2 obtained by the Eyring plot.

	ΔH ≠ [kcal.mol -1 ]	25.17
	ΔS ≠ [cal.mol -1 .K -1 ]	-6.50
	ΔG ≠ [kcal.mol -1 ]	27.33 (for 333 K)
		27.46 (for 353 K)

Table 4 . 3 :

 43 Various catalytic conditions tested for the synthesis of propoxypinacolborane IV-11.

	Entry	Catalyst	P(C2H4)	P(CO2)	Solvent	Products
			(bar)	(bar)
	1	[(dcpp)nickelalactone]	5	30	THF	[(dcpp)Ni(CO)2]
	2	[(dcpp)nickelalactone]	9	20	THF	[(dcpp)Ni(CO)2]
	3	[Ni(COD)2] + dcpe	9	20	THF	[(dcpe)Ni(CO)2]
	4	[Ni(COD)2] + dcpm	9	20	THF	mixture

  The supernatant is filtered away. The product is washed 2 × with Et2O and 2 × with pentane and then dried overnight under vaccum.[(dcpp)nickelalactone] IV-4 is gathered as a yellow to light orange powder in 68 % yield (546,3 mg). Crystals for X-ray diffraction analysis are grown at RT by diffusing pentane in a THF solution of [(dcpp)nickelalactone] IV-4.

	1 H NMR (500 MHz, [d8]-THF): δ 0.68 (m, 2H, Ni-CH2) * , 1.16 -2.00 (m, Cy + CH2), 2.02 (m,
	2H, C(O)-CH2) * , 2.26 (m, Cy + CH2), 2.50 (m, Cy + CH2) ppm.
	13 C{ 1 H} NMR (125 MHz, [d8]-THF): δ 13.1 (Ni-CH2) * , 18.5, 22.7, 27.0, 27.1, 27.8, 28.1, 28.4,
	29.2, 29.9, 30.2, 32.1 (Cy + CH2), 34.6 (CH αP), 37.2 (CH αP), 37.4 (C(O)-CH2) * , 186.6 (C=O) *
	ppm.

mL). The reaction mixture is stirred for 3 h. at RT. Pentane is added (≈ 70 mL) and the solution is stirred to precipitate [(dcpp)nickelalactone] IV-4. 31 P{ 1 H} NMR (121,5 MHz, [d8-THF]): δ 10.4 (d, 2 JP, P = 32.8 Hz), 32.1 (d, 2 JP, P = 32.8 Hz) ppm.

  2, PCy 3, Me 2 AlOTf

								Ph	R
					C 6 D 6, RT	Ph Cy 3 P	O O Ni O S Al	O CF 3
	Cy 3 P H Ph	Ni	PCy 3 O	> 99 % C 6 D 6, RT, 5 min. Me 2 AlOTf	Cy 3 P H Ph	Ni	PCy 3 O AlMe 2 OTf	C 6 D 6, RT, 5 min. = 87 %

Table 6 . 1 :

 61 Optimized reaction conditions for the synthesis of the LtBuH 2 Br 2 VI-5, LDippH 2 Br 2 VI-6 and LtBu (prop) H 2 Br 2 VI-7 imidazolium salts.

	Product	Reaction time (h.)	Temperature (°C)	Yield (%)
	LtBuH2Br2 VI-5	120	130	94
	LDippH2Br2 VI-6	72	150	60
	LtBu (prop) H2Br2 VI-7	72	130	73

6.2.2 Synthesis of [(bis-NHC)Ni(alkene)] complexes

  The solution is freeze-pump-thaw degassed without any previous stirring and 1 bar of ethylene is applied to the Schlenk. The solution turns red and is stirred overnight at RT. The gas and the solvent are removed under reduced pressure. The reaction turns red and is stirred overnight at RT. The solvent is removed under reduced pressure and the remaining red residue is washed with 3 × 8 mL of pentane and dried overnight at 70 °C under vacuum. [(LtBu)Ni(Ph-C≡C-Ph)] VI-13 is gathered as a red product in 76 % yield (179.7 mg). Crystals for X-ray diffraction analysis were grown from a saturated THF solution at RT. 04×10 -3 mol, 2.2 eq.) are mixed together in a Schlenk flask and suspended in 20 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [Ni(COD)2] VI-8 (130.7 mg, 4.74×10 -4 mol, 1 eq.) and phenylacetylene VI-12 (52.0 µL, 4.74×10 -4 mol, 1 eq.) are added to the yellow solution and 5 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing [Ni(COD)2] VI-8. The reaction turns brown and is stirred overnight at RT. The solvent is removed under JH, H = 2.1 Hz, 1H, CH imi), 7.05(d, 3 JH, H = 2.1 Hz, 1H, CH imi), 7.10 (d,3 JH, H = 2.1 Hz, 1H, CH imi), 7.10 -7.14 (m, 2H, CH ortho) ppm. of the Schlenk and to help solubilizing the reagent. The reaction is stirred overnight at RT, a beige product falls out of the solution. The supernatant is removed through a cannula and the residue is washed with 1 × 15 mL of THF, 1 × 15 mL of Et2O and 3 × 15 mL of pentane.

	13 C{1H} NMR (75 MHz, [d8]-THF): δ 23.8 (CH3), 24.9 (CH3), 25.2 (H2C=CH2), 28.8
	(CH(CH3)2), 63.1 (N-CH2-N), 118.9 (CH imi A), 121.8 (CH imi B), 123.3 (CH meta), 128.8
	(CH para), 139.7 (C ipso), 146.3 (C ortho), 200.1 (N=C-N) ppm.
	1 H NMR (300 MHz, [d8]-THF): δ 1.06 + 1.20 (AA'BB', 3 JA, A' = 3 JB, B' = 10.4 Hz, 3 JA, B' = The product is dried overnight at 60 °C under vacuum. [(LtBu)Ni(CH2CH2COO-κC,κO)] VI-
	12.3 Hz, 2 JA, B = -2.9 Hz, 4H, H2C=CH2)*, 1.68 (s, 18H, CCH3), 5.63 (d, 2 JH, H = 12.3 Hz, 1H, N-CH2-N), 5.97 (d, 2 JH, H = 12.3 Hz, 1H, N-CH2-N), 6.94 (d, 3 JH, H = 2.1 Hz, 2H, CH imi B), 7.00 (d, 3 JH, H = 2.1 Hz, 2H, CH imi A) ppm. 13 C{1H} NMR (75 MHz, [d8]-THF): δ 27.7 (H2C=CH2), 31.0 (CCH3), 57.2 (CCH3), 62.9 (N-CH2-N), 115.8 (CH imi B), 118.0 (CH imi A), 203.7 (N=C-N) ppm. * The coupling constants of the AA'BB' system were determined through simulations performed with TOPSPIN ("module daisy") at 300 MHz and 500 MHz. The simulated spectra can be found in the appendices (7.2). 6.6.3.2 [(LDipp)Ni(C 2 H 4 )] VI-10 Di-isopropylphenyl-methylene-imidazolium dibromide LDippH2Br2 VI-6 (400 mg, 6.34×10 -4 mol, 1 eq.) and KOtBu (156.6 mg, 1.40×10 -3 mol, 2.2 eq) are mixed together in a Schlenk flask and suspended in 40 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [Ni(COD)2] VI-8 (175.1 mg, 6.34×10 -4 mol, 1 eq.) is added to the brown solution and 10 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing [Ni(COD)2] VI-8. The residue is washed with 3 × 15 mL of pentane and dried overnight at 70 °C under vacuum. [(LDipp)Ni(C2H4)] VI-10 is gathered as a red product in 71 % (250.7 mg) yield. 1 H NMR (300 MHz, [d8]-THF): δ 0.14 (s, 4H, H2C=CH2), 1.02 (d, 3 JH, H = 6.9 Hz, 12H, CH3), 1.08 (d, 3 JH, H = 6.9 Hz, 12H, CH3), 2.78 (sept, 3 JH, H = 6.9 Hz, 4H, CH(CH3)2), 5.90 (s, 2H, N-CH2-N), 6.89 (d, 3 JH, H = 1.8 Hz, 2H, CH imi B), 7.09 -7.11 (m, 4H, CH meta), 7.22 (d, 3 JH, H = 1.8 Hz, 2H, CH imi A), 7.21 -7.26 (m, 2H, CH para) ppm. 6.6.3.3 [(LtBu)Ni(Ph-C≡C-Ph)] VI-13 Di-tert-butyl-methylene-imidazolium dibromide LtBuH2Br2 VI-5 (200.0 mg, 4.74×10 -4 mol, 1 eq.) and KOtBu (117.0 mg, 1.04×10 -3 mol, 2.2 eq.) are mixed together in a Schlenk flask and suspended in 20 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [Ni(COD)2] VI-8 (130.7 mg, 4.74×10 -4 mol, 1 eq.) and diphenylacetylene VI-11 (84.4 mg, 4.74×10 -4 mol, 1 eq.) are added to the yellow solution and 5 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing [Ni(COD)2] VI-8. 1 H NMR (300 MHz, [d8]-THF): δ 1.60 (s, 18H, CCH3), 5.79 (d, 2 JH, H = 12.3 Hz, 1H, N-CH2-N), 6.39 (d, 2 JH, H = 12.3 Hz, 1H, N-CH2-N), 6.78 -6.84 (m, 2H, CH para), 6.96 -7.01 (m, 4H, CH meta), 7.01 (d, 3 JH, H = 1.8 Hz, 2H, CH imi B), 7.13 (d, 3 JH, H = 1.8 Hz, 2H, CH imi A), 7.17 -7.21 (m, 4H, CH ortho) ppm. 13 C{ 1 H} NMR (75 MHz, [d8]-THF): δ 31.1 (CCH3), 57.6 (CCH3), 62.9 (N-CH2-N), 117.0 (CH imi B), 118.9 (CH imi A), 122.9 (CH para), 127.5 (CH meta), 128.5 (CH ortho), 138.7 (C≡C), 140.7 (C-Ph), 200.9 (N=C-N) ppm. 6.6.3.4 [(LtBu)Ni(Ph-C≡C-H)] VI-14 Di-tert-butyl-methylene-imidazolium dibromide LtBuH2Br2 VI-5 (200.0 mg, 4.74×10 -4 mol, dried overnight at 70 °C under vacuum. [(LtBu)Ni(Ph-C≡C-H)] VI-14 is gathered as a brown product in 73 % yield (146.6 mg). Crystals for X-Ray diffraction analysis were grown from a THF solution at -30 °C. 1 H NMR (300 MHz, [d8]-THF): δ 1.57 (s, 9H, CCH3), 1.83 (s, 9H, CCH3), 5.70 (d, 2 JH, H = 12.3 Hz, 1H, N-CH2-N), 6.14 (d, 2 JH, H = 12.3 Hz, 1H, N-CH2-N), 6.49 (s, 1H, C≡C-H), 6.78 -6.84 (m, 1H, CH para), 6.94 -7.01(m, 2H, CH meta), 6.98 (d, 3 JH, H = 2.1 Hz, 1H, CH imi), 7.00 (d, 3 (N=C-N), 201.9 (N=C-N) ppm. 6.6.4.1 [(LtBu)Ni(CO) 2 ] VI-15 [(LtBu)Ni(C2H4)] VI-9 (100 mg, 3.04×10 -4 mol, 1 eq.) is dissolved in 4 mL of THF. The brown solution is freeze-pump-thaw degassed and 1 bar of CO is applied to the Schlenk. The reaction is stirred for 15 min. at RT. The solution turns yellow and a yellow to orange precipitate falls out. The gas and the solvent are removed under reduced pressure. [(LtBu)Ni(CO)2] VI-15 is gathered as an orange powder in 88 % yield (92.3 mg). Crystals for X-Ray diffraction analysis were grown from an Et2O solution at -30 °C. 23 is gathered as a beige product in 83 % yield (307.5 mg). It is nearly insoluble in THF, Et2O and hydrocarbons and has a limited solubility in CH2Cl2 and CH3CN. Crystals for X-Ray diffraction analysis were grown from an acetonitrile solution at RT. Note: In DMF, formation of [(LtBu)Ni(carbonate)] VI-24 and of the product giving rise to a singlet at δ = 1.88 ppm in 1 H NMR is observed. [(LtBu)Ni(carbonate)] VI-24 crystals were grown from the DMF solution at -30 °C. The crystallographic data are available in the appendices. 1 H NMR (300 MHz, CD3CN): δ 0.47 (m, 1H, Ni-CH2), 0.74 (m, 1H, Ni-CH2), 1.75 (s, 9H, CCH3), 1.85 (s, 9H, CCH3), 2.03 (m, 2H, O-C(O)-CH2), 5.96 (d, 2 JH, H = 12.9 Hz, 1H, N-CH2-N), 6.99 (d, 13 C{ 1 H} NMR (75 MHz, CD3CN): δ 9.6 (Ni-CH2), 31.2 (CCH3), 31.8 (CCH3), 37.8 (O-C(O)-CH2), 58.7 (CCH3), 58.75 (CCH3), 64.1 (N-CH2-N), 118.6 (CH imi), 119.6 (CH imi), 120.6 1 eq.) and KOtBu (117.0 mg, 1.reduced pressure and the remaining brown residue is washed with 3 × 8 mL of pentane and 6.6.4 Synthesis of [(bis-NHC)Ni(0)-carbonyl] complexes (CH imi), 121.3 (CH imi), 180.6 (N=C-N), 186.1 (N=C-N), 189.6 (C=O) ppm.

13 C{ 1 H} NMR (75 MHz, [d8]-THF): δ 31.0 (CCH3), 31.1 (CCH3), 57.6 (CCH3), 57.8 (CCH3), 62.8 (N-CH2-N), 116.3 (CH imi), 117.0 (CH imi), 118.1 (CH imi), 118.7 (CH imi), 122.9 (CH para), 124.1 (≡C-H), 127.5 (CH meta), 128.8 (CH ortho), 138.5 (Ph-C), 140.3 (C ipso), 200.

9

1 H NMR (300 MHz, [d8]-THF): δ 1.79 (s, 18H, CCH3), 5.64 (s, 2H, N-CH2-N), 7.12 (d,

3

JH, H = 1.8 Hz, 2H, CH imi), 7.14

(d, 3

JH, H = 1.8 Hz, 2H, CH imi) ppm. walls

3

JH, H = 2.1 Hz, 1H, CH imi), 7.04

(d, 3

JH, H = 2.1 Hz, 1H, CH imi), 7.12 (d,

2

JH, H = 12.9 Hz, 1H, N-CH2-N), 7.20

(d, 3

JH, H = 2.1 Hz, 1H, CH imi), 7.23

(d, 3

JH, H = 2.1 Hz, 1H, CH imi) ppm.

  JH, H = 12.9 Hz, 1H, N-CH2-N), 6.99(d, 3 JH, H = 2.1 Hz, 1H, CH imi), 7.04(d, 3 JH, H = 2.1 Hz, 1H, CH imi), 7.13(d, 2 JH, H = 12.9 Hz, 1H, N-CH2-N), 7.20(d, 3 JH, H = 2.1 Hz, 1H, CH imi), 7.23(d, 3 JH, H = 2.1 Hz, 1H, CH imi) ppm. JC, C = 51.6 Hz, 2 JC, C = 1.4 Hz, C=O) ppm.

	C[(tmeda)nickelalactone] VI-18( 13 C). Starting from 371.6 mg (8.80×10 -1 H NMR (300 MHz, CD3CN): δ 0.20 -0.30 + 0.63 -0.73 (m, 1H, Ni-13 CH2), 0.47 -0.58 + 0.86 -1.00 (m, 1H, Ni-13 CH2), 1.75 (s, 9H, CCH3), 1.85 (s, 9H, CCH3), 1.94 (m, 2H, O-C(O)-13 CH2), 5.96 (d, 2 13 C{ 1 H} NMR (75 MHz, CD3CN): δ 9.6 (dd, 1 JC, C = 32.2 Hz, 2 JC, C = 1.4 Hz, Ni-13 CH2), 31.3 (CCH3), 31.8 (CCH3), 37.8 (dd, 1 JC, C = 51.6 Hz, 1 JC, C = 32.2 Hz, O-C(O)-13 CH2), 58.7 (CCH3), 58.8 (CCH3), 64.1 (N-CH2-N), 118.6 (CH imi), 119.6 (CH imi), 120.6 (CH imi), 121.3 (CH imi), 187.2 (N=C-N), 187.9 (N=C-N), 189.6 (dd, 1 The product is dried overnight at 60 °C under vacuum. [(LDipp)Ni(CH2CH2COO-κC,κO)] VI-19 is gathered as a beige product in 86 % yield (325.0 mg). It has poor solubility in common organic solvents. 1 H NMR (300 MHz, MeOD): δ 0.17 (t, 3 JH, H = 7.2 Hz, 2H, Ni-CH2), 1.04 (d, 3 JH, H = 6.9 Hz, 6H, CH3), 1.10 (d, 3 JH, H = 6.9 Hz, 6H, CH3), 1.34 (d, 3 JH, H = 6.9 Hz, 6H, CH3), 1.37 (d, 3 JH, H = 6.9 Hz, 6H, CH3), 1.52 (t, 3 JH, H = 7.2 Hz, 2H, O-C(O)-CH2), 2.61 (sept, 3 JH, H = 6.9 Hz, 2H, 29.6 (CH(CH3)2), 29.7 (CH(CH3)2), 38.5 (O-C(O)-CH2), 63.0 (N-CH2-N), 120.0 (CH imi), 121.4 (CH imi), 124.1 (CH meta), 124.9 (CH meta), 126.8 (CH imi), 127.2 (CH imi), 130.3 (CH para), 130.6 (CH para), 137.8 (C ipso), 138.2 (C ipso), 146.4 (C ortho), 146.5 (C ortho), 176.7 (N=C-N), 186.0 (N=C-N), 194.3 (C=O) ppm. IR (cm -1 ): ν (C=O) = 1562 (m). 6.6.5.4 [(LtBu) prop Ni(CH 2 CH 2 COO-κC,κO)] VI-20 1 H NMR (500 MHz, CD3CN): δ 0.19 (m, 2H, Ni-CH2), 1.94 (s, 18H, CCH3), 1.96 (m, 2H, O-C(O)-CH2), 4.28 (m, 3H, N-CH2-CH2-CH2-N), 5.76 (m, 3H, N-CH2-CH2-CH2-N), 6.90 (br s, 2H, CH imi), 6.96 (d, 3 JH, H = 2,0 Hz, 2H, CH imi), 7.00 (d, 3 JH, H = 2.0 Hz, 2H, CH imi), 7.07 CH(CH3)2), 2.95 (sept, 3 JH, H = 6.9 Hz, 2H, CH(CH3)2), 6.35 (s, 2H, N-CH2-N), 7.02 (d, 3 JH, 13 C-NMR (75 MHz, MeOD): δ 7.7 (Ni-CH2), 23.7 (CH3), 23.8 (CH3), 25.6 (CH3), 25.9 (CH3), (br s, 2H, CHimi) ppm.

6.6.5.3 [(LDipp)Ni(CH 2 CH 2 COO-κC,κO)] VI-19

Di-isopropylphenyl-methylene-imidazolium dibromide LDippH2Br2 VI-6 (400 mg, 6.34×10

-4

mol, 1 eq.) and KOtBu (156.6 mg, 1.40×10 -3 mol, 2.2 eq.) are mixed together in a Schlenk flask and suspended in 40 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [(tmeda)nickelalactone] VI-17 (156.7 mg, 6.34×10 -4 mol, 1 eq.) is added to the brown solution and 10 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing the reagent. The reaction is stirred overnight at RT; a beige product falls out of the solution. The supernatant is removed through a cannula and the residue is washed with 1 × 15 mL of THF, 1 × 15 mL of Et2O and 3 × 15 mL of pentane. H = 1.8 Hz, 1H, CH imi), 7.05

(d, 3

JH,H = 2.1 Hz, 1H, CH imi), 7.18 -7.28 (m, 4H, CH meta), 7.31 -7.44 (m, 2H, CH para), 7.50

(d, 3

JH, H = 2.1 Hz, 1H, CH imi), 7.52

(d, 3

JH, H = 1.8 Hz, 1H, CH imi) ppm.

Di-tert-butyl-propylene-imidazolium dibromide LtBu prop H2Br2 VI-7 (200 mg, 4.44×10-4 mol, 1 eq.) and KOtBu (109.6 mg, 9.77×10 -4 mol, 2.2 eq.) are mixed together in a Schlenk flask and suspended in 20 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [(tmeda)nickelalactone] VI-17 (109.6 mg, 9.77×10 -4 mol ,1 eq.) is added to the colorless solution and 6 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing the reagent. The reaction is stirred overnight at RT; a beige product falls out of the solution. The supernatant is removed through a cannula and the residue is washed with 1 × 7 mL of THF, 1 × 7 mL of Et2O and 3 × 7 mL of pentane. The product is dried overnight at 60 °C under vacuum. [(LtBu) prop Ni(CH2CH2COO-κC,κO)] VI-20 is gathered as a beige product in 62 % yield (116.0 mg). It has poor solubility in common organic solvents. Crystals for X-Ray diffraction analysis were grown at RT from a THF solution first layered with Et2O, then with pentane.

  The vessel is pressurized with 5 bar of CO2 and reacted over the weekend at RT. The solution turns red and an orange precipitate falls out. The supernatant is filtered away and the remaining product is washed with 3 × 8 mL of Et2O and dried overnight under vacuum at RT. [(tmeda)Ni(C(Ph)=C(Ph)COO-κC,κO)] VI-21 is gathered as an orange product in 61 % yield (435.5 mg). Crystals for X-Ray diffraction analysis were grown at -30 °C from a CD2Cl2 solution.(188.1 mg, 4.74×10 -4 mol, 1 eq.) is added to the yellow solution and 6 mL of THF are used to rinse the walls of the Schlenk and to help solubilizing the reagent. The reaction is stirred overnight at RT; a yellow product falls out of the solution. The supernatant is removed through a cannula and the residue is washed with 2 × 8 mL of Et2O and 3 × 8 mL of pentane. The JH, H = 1.8 Hz, 1H, CH imi A), 7.25(d, 3 JH, H = 2.1 Hz, 1H, CH imi A), 7.35(d, 3 JH, H = 1.8 Hz, 1H, CH imi B), 7.45(d, 2 JH, H = 12.6 Hz, 1H,

	product is dried overnight at 60 °C under vacuum. [(LtBu)Ni(C(Ph)=C(Ph)COO-κC,κO)] VI-
	22 is gathered as a yellow product in 86 % yield (221.2 mg). It has poor solubility in common
	organic solvents. Crystals for X-ray diffraction analysis were grown from a saturated
	acetonitrile solution at RT.
	1 H NMR (300 MHz, CD3CN): δ 1.41 (s, 9H, CCH3), 1.84 (s, 9H, CCH3), 6.10 (d,
	2 JH, H = 12.6 Hz, 1H, N-CH2-N), 6.48 -6.51 (m, 2H, CH ortho), 6.73 (d, 3 JH, H = 2.1 Hz, 1H,
	CH imi B), 6.72 -6.78 (m, 1H, CH para), 6.82 -6.91 (m, 2H, CH ortho), 6.88 -6.97 (m, 1H,
	CH para), 6.93 -7.02 (m, 4H, CH meta), 7.12 (d, 3
	13 C{ 1 H} NMR (75 MHz, CD2Cl2): δ 47.4 (N(CH3)2 B), 48.7 (N(CH3)2 A), 56.8 (N-CH2 αB),
	63.5 (N-CH2 αA), 124.7 (CH para), 125.3 (CH para), 126.9 (CH meta), 127.6 (CH meta), 127.9
	(CH ortho), 130.2 (CH ortho), 139.7 (C ipso), 147.2 (Ni-C(Ph)=C), 157.5 (O-C(O)-
	C(Ph)), 180.0 (C=O) ppm.
	IR (cm -1 ): ν (C=O) = 1633 (m).
	6.6.5.6 [(LtBu)Ni(C(Ph)=C(Ph)COO-κC,κO)] VI-22
	Di-tert-butyl-methylene-imidazolium dibromide LtBuH2Br2 VI-5 (200 mg, 4.74×10 -4 mol,

1 H NMR (300 MHz, CD2Cl2): δ 1.96 (s, 6H, N(CH3)2 A), 2.24 (br s, 2H, N-CH2 αB), 2.26 (br s, 2H, N-CH2 αA), 2.64 (s, 6H, N(CH3)2 B), 6.86 -6.90 (m, 2H, CH ortho), 6.93 -7.05 (m, 4H, CH meta), 7.06 -7.09 (m, 4H, CH ortho + para) ppm. 1 eq.) and KOtBu (117.0 mg, 1.04×10 -3 mol, 2.2 eq.) are mixed together in a Schlenk flask and suspended in 20 mL of Et2O. The reaction mixture is stirred for 2 h. at RT before it is filtered in the glovebox to remove precipitated KBr. [(tmeda)nickela-4,5-diphenyl-lactone] VI-21 N-CH2-N) ppm.

  .1.1 [(dcpp)NiCl 2 ] II-3 LtBu)Ni(C(Ph)=C(Ph)C(O)O)-κC,κO] VI-22

	ID 7.1.2 [(dcpp)Ni(toluene)] II-4 ID 7.1.3 [(dcpp)Ni(Ph)(Cl)] II-14 Florian26_a Florian4 ID Florian9m 7.1.4 [(dcpp)NiCl] 2 II-16 7.1.5 [(dcpp)Ni(C 2 H 4 )] IV-3 7.1.6 [(dcpp)nickelalactone] IV-4 7.1.7 [(dcpp)Ni(CO) 2 ] IV-23 7.1.8 [(dcpp)Ni(1-butene)] V-6 7.1.9 [(dcpp)nickelalactone-Al(tBu) 3 ] V-9 7.1.10 [(LtBu)Ni(C 2 H 4 )] VI-9 7.1.11 [(LtBu]Ni(PhC≡CPh)] VI-13 ID Alexia35_a 7.1.12 [(LtBu)Ni(PhC≡CH)] VI-14 ID Alexia38_a 7.1.13 [(LtBu)Ni(CO) 2 ] VI-15 ID AO446 7.1.14 [(LtBu)nickelalactone] VI-18 ID Alexia26 7.1.15 [(LtBu) prop nickelalactone] VI-20 ID Alexia30_a 7.1.17 [(LtBu)Ni(CO 3 )] VI-28 7.1.16 [(ID Alexia37_a ID Alexia25_a
	Formula Formula Formula ID ID ID ID ID ID ID Formula Formula Formula Formula Formula Formula Formula	C 27 H 49 Cl 2 NiP 2 C 34 H 58 NiP 2 C 45 H 67 ClNiP 2 alexia1m mo_Alexia9_0m_a Alexia16 alexia14m_a alexia3m Alexia43Bis_a Alexia29_b C 31 H 38 N 4 NiO 0.50 C 27 H 38 N 4 NiO C 17 H 24 N 4 NiO 2 C 18 H 28 N 4 NiO 2 C 20 H 32 N 4 NiO 2 C 32 H 37 N 5 NiO 2 C 19 H 31 N 5 NiO 4
	Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Formula Formula Formula Formula Formula Formula Formula Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 )	565.21 587.45 764.08 C 54 H 100 Cl 2 Ni 2 P 4 C 29 H 54 NiP 2 C 30 H 54 NiO 2 P 2 , 0,5(C 5 H 12 ) C 33 H 58 NiO 3 P 2 C 31 H 58 NiP 2 C 44 H 85 AlNiO 2.50 P 2 C 17 H 28 N 4 Ni 533.36 493.32 375.09 391.15 419.20 582.37 452.20
	Crystal system Crystal system Crystal system Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Molecular weight (g.mol -1 ) Crystal system Crystal system Crystal system Crystal system Crystal system Crystal system Crystal system	monoclinic triclinic triclinic 1061.54 523.37 603.46 623.44 551.42 801.74 347.14 monoclinic monoclinic orthorhombic monoclinic monoclinic monoclinic monoclinic
	Space group Space group Space group Crystal system Crystal system Crystal system Crystal system Crystal system Crystal system Crystal system Space group Space group Space group Space group Space group Space group Space group	P n P -1 P -1 triclinic monoclinic orthorhombic monoclinic triclinic monoclinic orthorhombic P 2 1 /n C c P 2 1 2 1 2 1 P 2 1 /n P 2 1 /c P 2 1 P 2 1 /n
	a (Å) a (Å) a (Å) Space group Space group Space group Space group Space group Space group Space group a (Å) a (Å) a (Å) a (Å) a (Å) a (Å) a (Å)	9.455(4) 8.9535(15) 8.9005(4) P -1 P 2 1 /n P c a 2 1 P 2/c P 1 C 2/c P n a 2 1 11.833(3) 15.6787(19) 9.5206(8) 12.016(2) 13.8222(6) 7.857(3) 10.1966(4)
	b (Å) b (Å) b (Å) a (Å) a (Å) a (Å) a (Å) a (Å) a (Å) a (Å) b (Å) b (Å) b (Å) b (Å) b (Å) b (Å) b (Å)	8.841(4) 11.0168(16) 10.6035(4) 10.5873(8) 9.4676(8) 19.7359(5) 19.9301(9) 8.7517(8) 18.1565(15) 14.3232(6) 10.566(3) 15.2754(17) 12.9925(12) 11.0221(19) 10.3392(4) 22.6159(9) 15.8385(6)
	c (Å) c (Å) c (Å) b (Å) b (Å) b (Å) b (Å) b (Å) b (Å) b (Å) c (Å) c (Å) c (Å) c (Å) c (Å) c (Å) c (Å)	16.771(9) 17.829(3) 23.4174(10) 12.1027(9) 9.0227(8) 10.6777(3) 9.3949(4) 10.3122(8) 21.8169(16) 10.9530(4) 22.554(6) 22.621(3) 14.7824(13) 14.146(3) 15.0732(6) 8.4121(3) 13.8130(5)
	α (°) α (°) α (°) c (Å) c (Å) c (Å) c (Å) c (Å) c (Å) c (Å) α (°) α (°) α (°) α (°) α (°) α (°) α (°)	90 76.369(6) 77.5586(15) 12.1731(9) 16.3342(15) 16.6426(4) 18.7119(6) 10.5977(14) 25.187(2) 11.4879(6) 90 90 90 90 90 90 90
	β (°) β (°) β (°) α (°) α (°) α (°) α (°) α (°) α (°) α (°) β (°) β (°) β (°) β (°) β (°) β (°) β (°)	91.255(14) 85.353(6) 87.0367(16) 108.590(4) 90 90 90 115.549(7) 90 90 99.464(12) 103.238(5) 90 95.469(8) 104.136(2) 97.2912(13) 91.163(3)
	γ (°) γ (°) γ (°) β (°) β (°) β (°) β (°) β (°) β (°) β (°) γ (°) γ (°) γ (°) γ (°) γ (°) γ (°) γ (°)	90 70.631(6) 77.3667(15) 101.906(4) 91.063(3) 90 106.937(2) 97.376(7) 110.893(4) 90 90 90 90 90 90 90 90
	Volume (ų) Volume (ų) Volume (ų) γ (°) γ (°) γ (°) γ (°) γ (°) γ (°) γ (°) Volume (ų) Volume (ų) Volume (ų) Volume (ų) Volume (ų) Volume (ų) Volume (ų)	1401.6(12) 1612.4(5) 2105.88(15) 100.654(4) 90 90 90 110.062(5) 90 90 2781.5(14) 5273.8(11) 1828.5(3) 1865.0(6) 2088.89(15) 1482.70(10) 2230.32(15)
	Z Z Z Volume (ų) Volume (ų) Volume (ų) Volume (ų) Volume (ų) Volume (ų) Volume (ų) Z Z Z Z Z Z Z	2 2 1392.78(18) 2 1395.1(2) 3507.16(16) 3351.7(2) 766.25(14) 9321.1(13) 1802.24(14) 4 8 4 4 4 2 4
	Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Z Z Z Z Z Z Z Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 )	1.339 1.210 1.205 1 2 4 4 1 8 4 1.274 1.243 1.363 1.393 1.333 1.304 1.347
	Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Calculated density (g.cm -3 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 )	1.011 0.722 0.629 1.266 1.246 1.143 1.236 1.195 1.143 1.279 0.725 0.761 1.077 1.059 1.950 0.691 1.537
	F(000) F(000) F(000) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) Absorption coefficient (mm -1 ) F(000) F(000) F(000) F(000) F(000) F(000) F(000)	606 640 824 0.920 0.825 0.669 0.705 0.755 0.537 1.079 1136 2112 792 832 896 616 960
	Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) F(000) F(000) F(000) F(000) F(000) F(000) F(000) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³)	0.251 x 0.121 x 0.060 0.200 x 0.060 x 0.040 0.399 x 0.281 x 0.136 574 572 1316 1352 302 3520 744 0.340 x 0.040 x 0.020 0.180 x 0.160 x 0.030 0.10 x 0.08 x 0.01 0.150 x 0.040 x 0.030 0.140 x 0.060 x 0.050 0.360 x 0.220 x 0.180 0.120 x 0.080 x 0.060
	Temperature (K) Temperature (K) Temperature (K) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Crystal size (mm³) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K)	193(2) 193(2) 193(2) 0.20 x 0.14 x 0.10 0.200 x 0.100 x 0.050 0.18 x 0.18 x 0.02 0.200 x 0.180 x 0.060 0.25 x 0.11 x 0.06 0.180 x 0.140 x 0.080 0.060 x 0.030 x 0.010 193(2) 193(2) 193(2) 193(2) 193(2) 193(2) 193(2)
	Wavelength (Å) Wavelength (Å) Wavelength (Å) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å)	0.71073 0.71073 0.71073 193(2) 193(2) 193(2) 193(2) 193(2) 193(2) 193(2) 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073
	Reflections collected Reflections collected Reflections collected Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Wavelength (Å) Reflections collected Reflections collected Reflections collected Reflections collected Reflections collected Reflections collected Reflections collected	4941 21935 62114 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073 21431 40271 22860 30635 30730 69849 11429
	Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Reflections collected Reflections collected Reflections collected Reflections collected Reflections collected Reflections collected Reflections collected Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint)	2350 (0.0943) 6060 (0.1175) 12855 (0.0345) 41637 35460 101184 6769 10678 69673 30631 4529 (0.2699) 12173 (0.0477) 3883 (0.0655) 3280 (0.1930) 4761 (0.0551) 9058 (0.0411) 3751 (0.0875)
	Reflections used for refinement Reflections used for refinement Reflections used for Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Independent reflections (Rint) Reflections used for Reflections used for Reflections used for refinement Reflections used for refinement Reflections used for refinement Reflections used for Reflections used for refinement	2350 6060 12855 10582 (0.0441) 2849 (0.1601) 14014 (0.0417) 6769 (?) 5726 (0.0461) 9470 (0.0535) 3650 (0.0528) 4529 12173 3883 3280 4761 9058 3751
	Refined parameters Refined parameters refinement Reflections used for refinement Reflections used for refinement Reflections used for refinement Reflections used for refinement Reflections used for refinement Reflections used for refinement Reflections used for refinement refinement refinement Refined parameters Refined parameters Refined parameters refinement Refined parameters	299 341 10582 2849 14014 6769 5726 9470 3650 224 232 250 270
	GOF on F 2 GOF on F 2 Refined parameters Refined parameters Refined parameters Refined parameters Refined parameters Refined parameters Refined parameters Refined parameters Refined parameters Refined parameters GOF on F 2 GOF on F 2 GOF on F 2 Refined parameters GOF on F 2	1.179 0.976 552 290 299 412 376 309 597 205 408 700 1.081 1.011 1.021 395 1.008
	R 1 [I>2σ(I)] R 1 [I>2σ(I)] GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 GOF on F 2 R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] GOF on F 2 R 1 [I>2σ(I)]	0.0497 0.0577 1.067 1.035 1.276 1.170 1.180 1.020 1.021 1.022 0.976 0.984 0.0391 0.0523 0.0374 1.041 0.0568
	wR 2 [all data] wR 2 [all data] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] R 1 [I>2σ(I)] wR 2 [all data] wR 2 [all data] wR 2 [all data] R 1 [I>2σ(I)] wR 2 [all data]	0.1721 0.1170 0.0389 0.0389 0.0788 0.0444 0.0609 0.0500 0.0528 0.0279 0.0740 0.0424 0.0897 0.0997 0.0948 0.0276 0.1352
	wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data] wR 2 [all data]	0.0893 0.0951 0.1562 0.1565 0.1199 0.1205 0.1387 0.0608 0.1661 0.0934 0.0583

H 1D-HSQC experiments give access to the protons directly bound to13 C carbons. Together with classical

2D-HSQC analysis, the protons of the organic product could be unambiguously identified. Terminal 13 CH3 appears as a triplet at δ = 0.92 ppm ( 3 JH, H = 7.5 Hz); 13 CH2 as a

H NMR (300 MHz, CDCl3): δ 1.47 (s, 9H, CH3), 6.97 (s,

2H, HC=CH), 7.53 (s, 1H, N-CH-N) ppm.

mol, 1 eq.) of di-tert-butyl-methylene-imidazolium dibromide LtBuH2Br2 VI-5,13 C[(LtBu)Ni(CH2CH2COO-κC,κO)] VI-18( 13 C) was isolated as a beige product in 79 % yield

Scheme 7.2: 1 H NMR spectrum of the ethylenic protons of [(LtBu)Ni(C 2 H 4 )] IV-9 at 500 MHz.

) et l'élimination réductrice (ΔG ≠ = 13,2 kcal.mol -1 ) ont des énergies d'activation particulièrement basses alors que la transmétallation (ΔG ≠ =

21,8 kcal.mol -1 ) et l'échange de ligands produit/substrat (ΔG ≠ = 22,9 kcal.mol -1 ) constituent les étapes cinétiquement

Remerciements

Scheme 3.13: Hypothetic catalytic cycle for the production of methyl acrylate through the cleavage of nickelalactones by methyl iodide computed by Xu and Sautet. [51] In 2013, Limbach and Hofmann also investigated the nickel mediated formation of acrylates from ethylene, CO2 and methyl iodide by DFT calculations at the BP86/def2-SV(P) level of theory and supported their work by experimental results. [52] The study is carried out on the dtbpe chelating bis-phopshine and the discovered mechanism differs from the one previously suggested by the experimental investigations [47,48,49] and Xu and Sautet's calculations. [51] The formation of acrylates would be rather promoted by the assistance of an external base instead of the electrophilic methyl iodide. [52] The classically assumed pathway would merely be a side reaction leading to the same product. [52] The full catalytic cycle with possible side processes is depicted in Scheme 3.14. [52] 5.4.2.2 Synthesis of Al(tBu) 3. Et 2 O V-8 [5] AlCl3 (1.333 mg, 1.00×10 -2 mol, 1 eq.) is solubilized in 25 mL of Et2O and tBuMgCl (C = 2.0 mol.L -1 in Et2O, V = 15.0 mL, 3.00×10 -2 mol, 3 eq.) is slowly added at -78 °C. The solution is stirred for 1 h. at -78 °C and allowed to come back to RT over 2 h. 30 min. MgCl2 precipitates out of the solution. The volatiles are removed under vacuum and the salts are washed two times with pentane to extract the product. If the collected solution is not clear at this point, additional filtration is necessary. The volatiles are then removed under reduced pressure and Al(tBu)3.Et2O V-8 is gathered as a colorless oily liquid in 80 % yield (2.168 g).

1 H NMR (300 MHz, C6D6): δ 1.20 (s, 27H, C(CH3)3), 0.62 (t, 3 JH, H = 7.2 Hz, 6H, CH3 (Et2O)), 3.47 (q, 3 JH, H = 7.2 Hz, 4H, CH2 (Et2O)) ppm. 13 C NMR (75 MHz, C6D6): δ 15.6 (CH3 (Et2O)), 32.9 (C(CH3)3), 65.9 (CH2 (Et2O)), 74.1 (C(CH3)3) ppm. [1,2] AlEt3 V-3 25 wt% in toluene (21.3 µl, 3.97×10 -5 mol, 1 eq.) is dissolved in 0,6 ml of C6D6 in a J. Young tube. The tube is freeze-pump-thaw degassed and 1 bar of CO2 is applied to it for 5 min at RT. The tube is then heated for 1 h. at 50 °C, the solution remains colorless.

Synthesis of diethylaluminium propanoate V-4

1 H NMR (300 MHz, C6D6): δ 0.27 (q, 3 JH, H = 8.1 Hz, 4H, CH2 ethyl), 0.71 (t, 3 JH, H = 7.5 Hz, 3H, CH3), 1.33 (t, 3 JH, H = 8.1 Hz, 6H, CH3 ethyl), 1.91 (q, 3 JH, H = 7.5 Hz, 2H, CH2) ppm. 13 C NMR (75 MHz, C6D6): δ -1.0 (CH2 ethyl), 9.0 (CH3), 9.1 (CH3 ethyl), 30.5 (CH2), 184.4 (C=O) ppm.

1 H NMR (300 MHz, [d8]-THF): δ -0.16 (q, 3 JH, H = 8.1 Hz, 4H, CH2 ethyl), 0.96 (t, 3 JH, H = 8.1 Hz, 6H, CH3 ethyl), 1.14 (t, 3 JH, H = 7.5 Hz, 3H, CH3), 2.55 (q, 3 JH, H = 7.5 Hz, 2H, CH2) ppm. Besides, these two families of nickel(II) complexes, there are scarce examples of other [(bis-NHC)Ni(II)] compounds. Green described the synthesis of an ethyl bridged [(bis-NHC)nickel(II)-dimethyl] complex. The ligand substitution between [Ni(bipy)Me2] and the free carbene gave the targeted compound in 73 % yield. [24] In addition, Cao and Shi reported [(LDipp)Ni(carbonate)] complexes with varying linkers, which were active in the Kumada cross-coupling reaction. [25]

Chelating [(bis-NHC)Ni(0)] complexes

More recently, chelating [(bis-NHC)Ni(0)]-complexes have been reported. They were first obtained through ligand substitution reactions between [Ni(COD)2] and free bis-carbenes.

During his investigations on bent nickel(imido) complexes in 2013, Hillhouse isolated the square planar [3,3'-methylenebis(1-tert-butyl-4,5-dimethylimidazoliylidene)Ni(η 2 -COD)] complex in excellent yield. [26] Afterwards Hofmann managed to obtain analogous [(LtBu)Ni(COD)] and [(LDipp)Ni(COD)} complexes although in moderate yields. [27]

Scheme 6.4: Synthesis of [(bis-NHC)Ni(COD)] complexes through ligand substitution from

We can note that the reduction of [(bis-NHC)Ni(II)] complexes with two equivalents of reducing agents such as KC8 or K(BEt3H) also generates Ni(0) carbene complexes. Kubiak synthesized in 2014 the first Ni(0) tetracarbene from [(bis-NHC)2Ni] 2+ 2[Br] -by using this procedure. [28] Subsequently, Driess managed to isolate mixed chelating [silylene-carbeneNi(0)] complexes. [29] 6. The first approach consists in oxidatively coupling unsaturated chelating [(bis-NHC)Ni(0)] complexes with CO2 to afford chelating [(bis-NHC)nickelalactones]. The synthesis of (bis-NHC)nickel dihalide complexes with small spacers is still out of reach (6.1.2.1). Therefore [(bis-NHC)Ni(alkenes)] will be preferentially generated directly from Ni(0) precursors rather than from two electron reduction of nickel(II) complexes. Following Hillhouse's [26] and Hofmann's [27] The second more straightforward approach uses preformed [(tmeda)nickelalactone] VI-17 and relies on the ligand substitution of the diamine by stronger coordinating bis-NHCs, as depicted in Scheme 6.6. A study released by Walther in 2006 showed that a pyridine ligand of [(dipyridine)nickelalactone] could be easily displaced by a NHC to give the corresponding [(pyridine)(NHC)nickelalactone]. However, these complexes dimerize easily in DMF upon loss of the labile pyridine ligand. [30] Nevertheless, this result seemed encouraging enough to extend this methodology to chelating bis-carbenes. Scheme 6.6: Synthesis of [(bis-NHC)nickelalactones] through ligand substitution.

Synthesis of [(bis-NHC)Ni(alkene)] and [(bis-NHC)Ni (alkyne)] complexes

According to the first strategy presented in 6.1.3, the starting point of this research project was the synthesis of new unsaturated chelating [(bis-NHC)Ni]-complexes. For this study 1,1'-ditert-butyl-3,3'-methylenedimidazoline-2,2'-diylidene (LtBu), 1,1'-di-(2,6-diisopropylphenyl) butyl-3,3'-methylenedimidazoline-2,2'-diylidene (LDipp) and 1,1'-di-tert-butyl-3,3'propylenedimidazoline-2,2'-diylidene (LtBu) prop were chosen as ligands. Scheme 6.7: Ligands selected for the following investigations.

Synthesis of bis-NHC precursors

Prior to the coordination chemistry, substituted imidazoles and their corresponding imidazolium salts were generated. N-tert-butylimidazole VI-1 [31] and N-(2,6diisopropylphenyl)imidazole VI-2 [32] were synthesized according to published procedures. Glyoxal, formaldehyde, ammonia and primary amines are reacted in polar solvents to afford the substituted imidazoles VI-1 and VI-2. Scheme 6.8 presents the reaction conditions. Scheme 6.8: a) Streubel's procedure for the synthesis of N-tert-butylimidazole VI-1. [31] b) Zhang's procedure for the synthesis of N- (2,6-diisopropylphenyl) imidazole VI-2. [32] Afterwards, the bis-imidazolium salts LtBuH2Br2 VI-5, LDippH2Br2 VI-6 and LtBu (prop) H2Br2 VI-7 were synthesized through SN2 reactions performed under pressure between the imidazoles and either dibromomethane VI-3 or dibromopropane VI-4. [33] Table 6.1 gives the optimized conditions for each carbene precursor.

Unfortunately, a symmetric side product characterized in 1 H NMR by a singlet at δ = 1.88 ppm presumably for its tBu group is also produced during the reaction. The formation of the [LtBu-CO2]-adduct, [(LtBu)Ni(carbonate)] or [(LtBu)Ni(oxalate)] complexes has been envisaged. Shi recently reported the synthesis of various alkane bridged [(LDipp)Ni(carbonate)] complexes in moderate yields by the reactions between bis-NHC imidazolium salts, NiCl2 and K2CO3 in acetonitrile. [25] However, [(LtBu)Ni(carbonate)] VI-24 could not be synthesized by following VI-13 in THF, no reaction is observed. Though by increasing the pressure to 5 bar of CO2, [(LtBu)nickela-2-oxacyclopenten-4,5-diphenyl-3-one] VI-22 is formed. The nickelalactone as well as the side product with the tBu group at δ = 1.88 ppm were identified by 1 H NMR in CD3CN. The need of harsher conditions is in good agreement with the fact that [(bis-NHC)Ni(alkyne)] complexes are more stable than alkene complexes and require a higher activation energy to perform the same transformation. Le spectre HSQC 1D 1 H présente un signal supplémentaire, appartenant aux protons des groupements méthyls du pinacolborane IV-8, l'abondance naturelle de 13 C n'étant plus négligeable au vu des huit carbones équivalents présents dans la molécule. Enfin, la RMN 11