Laurent Maron

Blanca Martin Vaca

Yannick Coppel

Francis Lacassin

Christian Bijani Du Service Rmn

Alain Moreau

Isabelle Borget

Christian Kane

participé à la réalisation de ce travail, et plus particulièrement à: Laure Vendier du service de diffraction des

En premier lieu, je tiens à remercier mes encadrants de thèse, le Dr. Chiara Dinoi et le Prof. Michel Etienne. Je vous remercie de m'avoir donné l'opportunité d'étudier en France en tant que doctorante. Au cours de ces trois années d'études, à travers le travail expérimental et les nombreuses discussions scientifiques que nous avons partagé, j'ai pu acquérir beaucoup de connaissances scientifiques ainsi que maîtriser des nombreuses techniques expérimentales précieuses sur la chimie organométallique. Avec votre patience et vos conseils rigoureux sur mon sujet de recherche, je suis parvenu à surmonter les difficultés rencontrées lors de la réalisation des expériences et de la rédaction de la thèse, en obtenant le diplôme de doctorat.

En second lieu, je tiens à remercier le Dr. Chloé Thieuleux et le Prof. Robin Perutz pour avoir accepté de juger ce travail en tant que Rapporteurs et pour leur participation à mon jury de thèse.

A

Table of contents

In Chapter 2, we show that it is indeed the case. The niobium bound methyl group in [Tp Me2 NbCH 3 (c-C 3 H 5 )(MeCCMe)] (1) abstracts a E-H of the cyclopropyl group intramolecularly, generating A under very mild conditions. Degenerate exchange reactions of methane isotopomers ( 13 CH 4 and CD 4 ) with 1 are reported. A kinetic study of the degenerated exchange between 1 and CH 4 through spin saturation transfer experiments is provided.

Productive methane activation of methane giving 1 from the mesitylene complex [Tp Me2 Nb(CH 2 -3,5-C 6 H 3 Me 2 )(c-C 3 H 5 )(MeCCMe)] ( 3) is then realized together with a detailed kinetic analysis of the reaction. DFT studies of key intermediates and transition state for the methane CH bond elimination/activation are presented.

In Chapter 3 we show how complex A is able to selectively activate the CH bond of heteroaromatics like furan and thiophene, unsaturated hydrocarbons like phenylacetylene, 1cyclopentene, pentafluorobenzene and ferrocene (FcH) to afford [Tp Me2 NbX(c-C 3 H 5 )(MeCCMe)] (X =2-C 4 H 3 O (4a), 2-C 4 H 3 S (4b), PhC≡C(5), 1-C 5 H 7 (6) , Fc (7), and C 6 F 5 (8)) complexes under mild conditions. Selectivity issues based on thermodynamic properties of CH versus NbC bond strengths are discussed. Electrochemical studies of complexes 1, 5, 6, and 7 have been carried out to study the influence of the hydrocarbyl groups on the redox properties of the complexes. An unexpected product arising from cyclopropyl ring opening followed by alkyne coupling from 8 is also presented and DFT studies aimed at understanding the mechanism of this reaction are discussed.

The manuscript ends up with a general conclusion where some perspectives are put forward.

Chapter 1 Methane CH bond activation processes Chapter 1: Methane CH bond activation processes

Methane is an highly abundant energy and chemical source found as the main component of natural gas (80-90%) and shale gas. 1,2 Current uses of methane include its full oxidation to CO 2 and H 2 O to provide heat (and electricity) and the production of syngas (a mixture of CO and H 2 ) by steam reforming (CH 4 + H 2 O = CO + 3H 2 ), a high energy demanding endergonic process catalyzed by nickel-based catalysts at high temperatures (ca 700 -1000 °C) and moderate pressure (< 25 bar), which is used for the synthesis of a whole range of commodity chemicals.

Efficient catalytic functionalization of methane directly into easily transportable and high value-added chemicals in a potentially clean and sustainable way remains a long term goal on scientific, environmental, and economic grounds. [3][4][5][6] Methane is indeed highly inert, showing an ionization potential of ~12.5 eV, 7,8 with Pauling electronegativities χ C = 2.55 and χ H = 2.20, an estimated pK a value of ~50-51 and a C-H homolytic bond dissociation enthalpy of 440 kJ.mol -1 at 25 °C. 9 Therefore, it is very difficult to activate the strong CH bond of methane directly, in either a heterolytic or homolytic way.

Nature has provided enzymes, the so-called methane monoxygenases (MMO) , 10 which are able to oxidize the C-H bond of methane and other alkanes. Methane monoxygenases (MMO) include two sorts of forms: the soluble form (sMMO) and the particulate form (pMMO). The proposed catalytic cycle for the transformation of methane to methanol catalyzed by sMMO is shown in Scheme 1.1. 11 The diiron centers in the MMOH red form react with the O 2 to generate the peroxide intermediate P. P then converts to intermediate Q, which was proposed to consist of two antiferromagnetically coupled highspin Fe IV centers with a diamond core structure. It is this species that oxidizes CH 4 to CH 3 OH, regenerating the MMOHox form. The intermediate Q oxidizes methane to methanol through either a radical or a nonradical pathway.

Scheme 1.1. Transformation of methane into methanol by MMO.

At the industrial level, transformation of methane into high value-added products is realized through either partial oxidation or alkane metathesis mechanisms catalyzed by heterogeneous catalysts. The attractive oxidative coupling of methane to ethylene and water (2 CH 4 + O 2 = C 2 H 4 + 2 H 2 O) has not reached levels of industrial relevance due to the limiting global yield, the lack of stability and selectivity of the catalysts under the harsh reaction conditions (> 700 °C). 12 Methanol CH 3 OH, a highly desirable large scale intermediate for fuel and chemical industry, is currently made from syngas (see above). The alternative selective oxidation of methane to methanol (CH 4 + ½ O 2 = CH 3 OH) pauses the recurrent problem of having a product, CH 3 OH, more prone to oxidation than the reactant CH 4 itself (CH bond dissociation energies for methane 440 kJ.mol -1 ; for methanol 374 kJ.mol - 1 ) yielding first formaldehyde CH 2 O then formic acid HCO 2 H and ultimately carbon dioxide CO 2 . [3][4][5][6] Remarkable selectivities (> 80%), turnover numbers (TON > 3000) and turnover frequencies (TOF > 6000 h -1 ) have been obtained recently in various Cu or Fe doped zeolites under an international competition scheme, "the Methane Challenge" funded by the Dow Chemical Company. 13 A promising approach to catalytic methane conversion is realized under the general scheme of alkane metathesis, a reaction that occurs at early transition metal sites (e.g. welldefined hydrides TaH and WH supported on silica or alumina;surface organometallic chemistry). 14 The equilibrating cross metathesis of methane and propane yields ethane (CH 4 + C 3 H 8 = 2 C 2 H 6 ), a reaction that can be extended to higher alkanes. 15 A most remarkable reaction is the non-oxidative coupling of methane to give ethane and dihydrogen (2 CH 4 = C 2 H 6 + H 2 ). Despite an unfavorable Gibbs free energy thus low conversions, high selectivity (98%) could be obtained in continuous flow reactors at moderate pressure (P CH4 = 50 bar) and temperature (T = 250 °C). The mechanism is shown in Scheme 1.2. 16

Scheme 1.2. Proposed mechanism for the non-oxidative coupling reaction of methane catalyzed by the silica-supported tantalum hydride (≡SiO) 2 Ta-H

The reaction is speculated to begin with the CH bond activation of methane by the silica-supported tantalum hydride (≡SiO) 2 Ta-H to generate a surface tantalum-methyl species and molecular hydrogen. This tantalum-methyl, upon dehydrogenation, is transformed to a carbene-hydride and a carbyne species and dihydrogen. Both species are able to activate methane through a σ-bond metathesis mechanism, yielding a methylmethylidene key intermediate. Then, the methyl group in (≡SiO) 2 Ta(=CH 2 )CH 3 , migratory inserts onto the carbene ligand to yield the tantalum-ethyl bond. Finally, in the presence of large excess of methane, the ethyl ligand can be exchanged with methane via σ-bond metathesis, to generate ethane and return to a tantalum-methyl species.

With the hope of benefiting of the flexibility ascribed to homogeneous systems, catalysis with discrete transition metal complexes has emerged as an important strategy.

Furthermore it potentially allows precise tuning of various factors so that the molecular design of catalysts and the detailed understanding of the intimate mechanism of the reactions are possible. A summary of the different ways of cleaving a CH bond by transition metal complexes are shown in Scheme 1.3. Early and late transition metal complexes activate methane by different mechanistic pathways, but only a few of them have led to catalytic applications. In the following sections we will make a general overview of the CH bond activation schemes by which late and early transition metal complexes may activate methane in homogeneous conditions. Scheme 1.3. CH bond activation pathways at transition metal centers.

1.1-CH bond activation of methane by late transition metal complexes

Late transition metal complexes activate the CH bond of methane by different pathways such as the electrophilic substitution, oxidative addition, carbene insertion and metalloradical addition. In the following section the main features of each type of CH bond activation mechanism is discussed.

1.1.1-Electrophilic substitution

Late-or post-transition metals act as Lewis acids, reacting with methane by electrophilic substitution in polar medium (Equation 1.1).

Systems working through electrophilic substitution are described in Scheme 1.4. They are remarkable since they constitute rare examples of catalytic methane functionalization with homogeneous metal complexes, although with low TOF and TON. As developed by Catalytica (Periana), [17][18][19] routes C and D in Scheme 1. 4 show the synthesis of methanol (or some of its derivatives) by oxidation of methane avoiding the syngas route. These ways use Hg or Pt catalysts and operate under harsh acidic conditions (H 2 SO 4 ) at high temperatures (180 to 220°C) ([Pt] = [(bpym)PtCl 2 ], bpym = 2,2'-bipyrimidine). They give methylbisulphate in 70% yield albeit with a very modest TOF (10 -3 s -1 ). These studies all rely on the initial fundamental studies of Shilov conducted in the late 1960's (routes A and B). 20 The conversion of methane to acetic acid has also been achieved. It is performed either with V-based catalysts (complex radical mechanism, trifluoroacetic acid, peroxodisulfate as oxidant, 50% yield route E), 21 or with a Pd(II) catalysts under Catalytica-like conditions with a TON of 4 (route F). 22 Note that in these methane functionalization reactions, CH bond activation does not occur through a conventional CH oxidative addition but rather via an electrophilic mechanism where H + is expelled directly.

Scheme 1.4. Catalytic methane CH bond functionalization by electrophilic substitution.

1.1.2-Oxidative addition

The most common mechanism of oxidative addition of the CH bond of methane is a concerted 3-center addition in which the reducing equivalent from the metal must be high enough in energy to locate at the C-H V orbital and generate two new bonds (M-C and M-H). According to equation 1.2, therefore, the increasing of the metal oxidation state by two units is accompanied by the formation of two new metal-methyl and metal-hydride bonds. In the 1980s, Janowicz and Bergman 23 and Graham 24,25 discovered that electron-rich, low-oxidation-state cyclopentadienyliridium(I) transients activate the CH bond of methane.

Graham et al. found that [Cp*Ir(CO)] and [CpIr(CO)], photogenerated from [Cp*Ir(CO) 2 ] and [CpIr(CO) 2 ] added methane (20 bar) at room temperature in cyclohexane yielding the respective iridum(III)hydridomethyl complex in 20% yield (Scheme 1.5). 23 Scheme 1.5. Oxidative addition of methane by a photogenerated unsaturated Ir(I) complex.

Bergman reported that [Cp * Ir(PMe 3 )], produced by thermolysis (140-150°C for several hours) of [Cp * Ir(PMe 3 )(c-C 6 H 11 )H] via reductive elimination, was able to cleave the C-H bond of methane (20 bar) to give [Cp * Ir(PMe 3 )(CH 3 )H] in 58% yield (Scheme 1.6). 23,26 Scheme 1.6. Oxidative addition of methane by a thermolytically generated unsaturated Ir(I) complex.

The same authors discovered 10 years later that a cationic iridium(III) complex 27,28 was even more reactive toward methane. By exchange reaction in the presence of 13 In the presence of Re, Rh and Ir precursors, oxidative addition, coupled to borylation reactions, has shown great potential for the functionalization of alkanes and arenes toward the formation of extended C-C bonds. 33 However none of these transformations have been expanded to CH 4 , possibly due to difficulties in manipulating this gas as well as to potential competition between CH 4 and the solvent itself.

[Cp*Ir(PMe 3 )(CH 3 )(CH 2 Cl 2 )][BArF 4 ]

1.1.2.1-σ-CH 4 intermediates along the oxidative addition pathway

In the oxidative addition pathway, the cleavage of the methane CH bond by the transition metal complexes likely starts by the coordination of the CH bond of methane to the metal center. Four possible methane binding modes have been proposed for the σ-coordinated complexes, including the η -H, η 2 -H,H, η 2 -C,H, and η 3 -H,H,H interactions (Scheme 1.9). 34

Scheme 1.9. Possible coordination modes of methane to a transition metal center.

Evidence for the existence of these σ-complexes comes from sources such as matrix and gas phase IR and UV/vis photolysis and, more recently, NMR spectroscopy; however the actual coordination mode of methane is often unknown. Evidence of methane coordination was initially suggested through UV/vis photolysis experiments in which a ligand dissociated from the metal center generating a coordinately unsaturated complex, which was then trapped by methane (Equation 1.3).

Unsaturated d 6 metal complexes such as [M(CO) 5 ] (M = Cr, W) and [CpML 2 ] (M = Mn, Re) have been found to trap methane to form σ-methane complexes. In the classic work of Perutz and Turner, for instance, a methane complex was inferred by changes in the UV/vis photolysis experiments where [Cr(CO) 5 ] was photogenerated from [Cr(CO) 6 ] in methane matrix at 12 K. 35 The detection of σ-methane complexes has also been carried out by using time-resolved infrared spectroscopy studies (TRIR). George and coworkers have recently used TRIR to detect transient metal carbonyl σ-methane and σ-ethane complexes generated in the supercritical alkane at room temperature from their IR spectra in the region of the CO stretching frequencies. 36 In scCH [CpMn(CO) 2 (CH 4 )], and [CpRe(CO) 2 (CH 4 )], respectively. 37 Theoretical calculations have also established the plausibility of methane adducts especially as intermediates on the pathway to oxidative addition. Methane complexation to 14 and 16 electron fragments, sometimes derived from hypothetical complexes, has been studied by a variety of computational methods. Depending on the computed system, the authors proposed either a η 2 -C,H, 38 or η 1 -H, 39 or η 2 -H,H 40 or η 3 -H,H,H, 41 coordination mode, showing how all of the structures reported in Scheme 1.9 may be envisaged.

Evidence for σ-methane complex intermediates can be inferred from isotopic labeling experiments: specifically, i) the observation of deuterium exchange between hydride and methyl sites (such as between [L n M(CH 3 )D] and [L n M(CH 2 D)H]), and ii) the measurement of kinetic isotope effects (KIEs). A series of kinetic and thermodynamic deuterium isotope effects have been described in the literature by the groups of Norton, 42 Bercaw, 43 Green, 44 Parkin 45 and Jones 46 (Scheme 1.10).

Scheme 1.10. Interconversion of [L n M(CH 3 )D] and [L n M(CH 2 D)H].

In the interconversion of [W{(η- Other complexes that exhibit inverse KIE include [Cp* 2 W(CH 3 )H] (0.70), 43 [Cp 2 Re(CH 3 )H] + (0.8), 47 [(Me 3 tacn)Rh(PMe 3 )(CH 3 )H] + (0.74), 48 and [(tmeda)Pt(CH 3 )H(Cl)] (0.29). 49 Jones has analyzed this inverse isotope effect systematically. 50 As shown in Scheme 1.12, the proposed reaction steps are similar for all these examples and the formation of the methyl hydride product is reversible. The intermediacy of a σ-methane complex is often demonstrated by preparation of a methyl deuteride complex, which then reversibly scrambles the deuterium into the α-hydrogen of the methyl ligand, presumably via the unseen σ-methane complex. Dissociation of methane from the σ-complex, followed by irreversible trapping of the reactive fragment [L n M] with an external ligand, is typically observed as a slower process.

C 5 H 4 )CMe 2 (η-C 5 H 4 )}(CH 3 )D] to [W{(η- C 5 H 4 )CMe 2 (η-C 5 H 4 )}(CH 2 D)H]
The This ratio is identical to the equilibrium isotope effect (EIE) for interconversion of [L n M(CH 3 )D] and [L n M(CH 2 D)H] which would be predicted to be inverse because the zeropoint energy difference between M-H and M-D bonds in the methyl hydride complex is smaller than the zero-point energy difference between C-H and C-D bonds in the σ-methane complex. In most of the examples the inverse kinetic isotope effect is due to an inverse equilibrium isotope effect (i.e. K eq H/D < 1). Two possibilities have to be used to illustrate this phenomenon. In one case, shown in Scheme 1.13(a), besides the zero point energy difference in the reactant and product, there are also zero point energy differences in the transition state.

In this case there is an inverse kinetic isotope effect (k rc H /k rc D < 1) for the reduction coupling step and a normal kinetic isotope effect (k oc H /k oc D > 1) for the reverse oxidative cleavage step.

In the second case, shown in Scheme 1.13 (b), there is no zero point energy difference between C-H and C-D bonds in the transition state. Normal kinetic isotope effects are assumed on both directions. However, kinetic isotope effect in the oxidative cleavage step is much larger than kinetic isotope effect in the reductive coupling step, which results in an inverse isotope effect for the global reaction.

Scheme 1.13. Inverse isotope effect.

Two important points can be noted. First, both situations are consistent with an inverse equilibrium isotope effect separating the methyl hydride complex from the σ-methane complex, since the transition state does not affect the equilibrium. Second, the observation of an inverse kinetic isotope effect for the loss of methane from a methyl hydride/deuteride complex cannot, in itself, distinguish between these two cases. The examples reported until now for the D(H)/CH 3 (CH 2 D) scrambling [45][46] show a normal kinetic isotope effect for reductive coupling and oxidative cleavage, the latter being larger than the former. These systems, therefore, are best described by Scheme 1.13 (b), by which the inverse equilibrium isotope effect arises as a result of two normal isotope effects of different magnitudes that oppose each other. Scheme 1.13(a) has been never observed for the D(H)/CH 13 C NMR experiments.

Scheme 1.14. Synthesis of the [(PONOP)Rh(CH 4 )][BAr F 4 ] σ-methane complex

As shown in Figure 1.2, the 1 H and 13 C NMR spectra of the [(PONOP)Rh( 13 CH 4 )][BAr F 4 ] compound at -110 °C exhibit a doublet of doublets at G -0.86 ppm (J C-H = 124 Hz, J Rh-H = 6.3 Hz) and a quintet at G -41.7 ppm (J C-H = 124 Hz), respectively.

Figure 1.2. The 1 H and 13 C NMR spectra of [(PONOP)Rh( 13 CH 4 )][B(Ar F ) 4 ] at -110 °C

Even at -110 °C, methane rapidly tumbles in the coordination sphere of rhodium, exchanging free and bound hydrogens. Kinetic studies reveal a half-life of about 83 minutes at -87°C for the dissociation of methane. Remarkably, the σ-methane complex, [(PONOP)Rh(CH 4 )][BAr F 4 ] is shown to be more stable than the corresponding rhodium(III) methyl hydride complex. DFT calculations showed an unsymmetrical interaction between rhodium and the methane ligand, with only one C-H bond coordinated to the metal according to a η 2 -C,H coordination mode in Scheme 1.9.

1.1.2.2-σ-complex-assisted metathesis (σ-CAM) mechanism

Different from the familiar oxidative-reductive elimination mechanisms and σ-bond metathesis (see below), CH bond activation of methane could also happen without changing the oxidation state of the late transition metal in the σ-complex through the so-called σ-CAM mechanism. 57 Hartwig first has proposed that borylation of methane at [CpM(CO) n (boryl)] complexes (M = Fe, W) could occur through the σ-CAM mechanism via DFT calculation. 58 The mechanism is shown in Scheme 1.15. A σ-CH 4 complex A is first transformed to σborane B via a σ-CAM step. Then B undergoes σ-rotation to form σ-borane B'. B' is converted to σ-alkylborane C with B-C bond coupling via a second σ-CAM step.

[CpRhH{B(OR) 2 }] or [CpRh{B(OR) 2 } 2 ] complexes also follows this mechanism.

Scheme 1.15. Borylation of methane at [CpM(CO) n (boryl)] complexes (M = Fe, W) via σ-

CAM mechanism

Ng et al. has also reported that [TpRuH(MeCN)(PPh 3 )] catalyzes H/D exchange between methane and C 6 D 6 at 100 °C. The σ-CAM mechanism has been analyzed by DFT calculations. η 2 -H 2 , η 2 -CH 4 , and η 2 -benzene ligands are involved in this mechanism (Scheme 1.16). 59 The bonds between the nonmetal atoms connected to the metal center could be cleaved in the transition states of the σ-CAM step. ] and then to [M(CH 3 )] and dihydrogen via DFT calculations. 60 In contrast to the oxidative cleavage mechanism for Os, Fe and Ru complexes follow the σ-CAM mechanism.

1.1.3-Carbene insertion

A different approach to selective alkane CH activation and functionalization involves the insertion of electrophilic carbon into CH bonds. This strategy has been realized recently by the Perez, Etienne and Asensio groups via the catalytic insertion of a carbene into the C-H bond of methane. 61,62 By using a perfluorinated tris(indazolyl)borate ligand on Ag(I), high pressures of methane (160 atm), supercritical CO 2 conditions (250 atm), and ethyldiazoacetate (EDA) as the carbene source, methane has been converted into ethylpropionate by carbene :CHCO 2 Et insertion into a CH bond of methane (Scheme 1.17(a)). TON and TOF of 734 and 1.4 × 10 -2 s -1 , respectively, have been achieved, the latter being, to the best of our knowledge, the highest conversion rate reported to date for methane functionalization under homogenous conditions. A plausible mechanism for CH functionalization of methane (and other hydrocarbons) involves metal-catalyzed N 2 elimination from the diazo substrate followed by carbene transfer mediated by a highly electrophilic metal carbene intermediate (Scheme 1.17 (b)). In this chemistry, therefore, the metal complex does not break directly the CH bond of methane but rather a very electrophilic silver carbene complex polarizes the inert CH bond to induce insertion. Scheme 1.17. Catalytic functionalization of methane by carbene insertion.

1.1.4-Metalloradical addition

In the early 1990s Wayland et al. discovered that (tetramesitylporphyrin)Rh(II) compounds activated the CH bond of methane by a metalloradical pathway to form a Rh-H and a Rh-Me bond under mild conditions. [63][64][65] Kinetic studies showed that the reaction rate was second order in the (tetramesitylporphyrin)Rh(II) radical system and first order in CH 4 .

The rate determining step was proposed to be the CH cleavage of methane with the formation of a linear 4-center Rh…H…C…Rh transition state featuring a trigonal pyramidal methane (Scheme 1.18). When CD 4 was employed, a large, temperature dependent, KIE was measured (8

1.2-CH bond activation of methane by early transition metal complexes.

High oxidation state early transition metal complexes (and related electropositive metals such as lanthanides or actinides) have been also shown to activate the CH bond of methane (and more generally hydrocarbons). In these cases, the mechanistic pathways involved are σ-bond metathesis, 1,2-addition (often following α-H abstraction) and 1,3addition (often following β-H abstraction).

1.2.1-σ-Bond metathesis.

Electron poor d 0 and d 0 f n transition metal alkyl and hydride complexes are able to activate saturated C-H bonds by σ-bond metathesis. [70][71][72] The σ-bond metathesis mechanism involves the formation of a polar four-center, four-electron transition state in which the metalalkyl bond of the original complex is broken to form a new metal-alkyl bond and free alkane (Scheme 1. 19). The σ-bond metathesis differs from the other mechanisms discussed herein in that the CH activation is the rate-determining step.

Scheme 1.19. σ-bond metathesis of methane.

Watson found that yttrium and lutetium metallocenes methyl complexes activate the CH bond of methane through a four-center transition state, where the hydrogen atom being transferred is located at the midway point between the two methyl groups. 13 and CH 4 at 70°C (Scheme 1.20). 70 The decrease of the intensity of the 13 Bercaw's group has undertaken an in-depth investigation of the reaction of permethylscandocene hydride or alkyl derivatives for the activation of sp 3 , sp 2 and sp CH bonds of various hydrocarbons. 71 Maron and Eisenstein showed that the relative rates of reactions for Sc, Y, Lu are influenced by the ionic radius of the metal. 73 According to experimental and computational papers, [73][74][75][76] the reaction coordinate proceeds to the transition state (TS) with no intermediate. The TS for the CH bond cleavage has a distinctive kite shape with an almost linear CH 3 …H…CH 3 disposition and the C-H bond distance is much longer in the transition state (1.33 Å) than in free methane (1.09 Å). 74 The alternation of charge is reminiscent of a proton transfer between two negatively charged methyl groups (Scheme 1.19). Bond critical points (BCP) are located along the M-C and C-H coordinates, and ring critical points (RCP) are located inside the c-M-C-H-C, which suggests the four-center geometry of the σ-bond metathesis transition state. The fact that no BCP can be located along M-H coordinate suggests no direct interaction between the hydrogen and the metal center during H transfer. 77,78 Marks has found that the thoracyclobutane [Cp* 2 Th( c CH 2 C(CH 3 ) 2 CH 2 )] activates methane in a similar manner affording [Cp* 2 Th(CH 3 )(CH 2 C(CH 3 ) 3 )] in 50% yield (Scheme 1.21). 72

1.2.2-1,2-CH addition.

Addition of a CH bond across a metal-ligand multiple bond has been referred to as 1,2-CH bond addition. Although this process resembles σ-bond metathesis, since it involves a 4-center transition state, the outcome is significantly different. The 1,2-CH bond addition, indeed, involves the generation of a high energy, metal-ligand multiply bonded intermediate, which performs the CH cleavage step. A remarkable system based on four-coordinate group 4, 5 and 6 metals has been developed by Wolczanski and coworkers. The first example of methane activation (among many hydrocarbons) is exemplified by tris(amido)alkyl zirconium complexes of the type [( t Bu 3 SiNH) 3 ZrR]. 83,84 With R = Cy, cyclohexane is eliminated above 80°C by an intramolecular α-CH abstraction of an amido hydrogen to give the transient, unobserved, diamido imido complex [( t Bu 3 SiNH) 2 Zr=NSi t Bu 3 ] (Scheme 1.23). After methane coordination, this reactive unsaturated complex cleaves the C-H bond of methane to give the methyl complex [( t Bu 3 SiNH) 3 Zr(CH 3 )] via 1,2-CH addition. The experimental study of the microscopic reverse, methane elimination from [( t Bu 3 SiNH) 3 Zr(CH 3 )], reveals large intramolecular kinetic isotope effects and a negative entropy of activation pointing to an ordered transition state not dissimilar to that of the σ-bond metathesis with a characteristic alternation of charges as corroborated by computational studies. 85 Intricate kinetic and thermodynamic studies on this and related systems (i.e. [( t Bu 3 SiO) 2 Ti(NHSi t Bu 3 )R]) 86,87 indicate that an unobserved methane adduct precedes the CH bond activation process. The stability of the alkane adduct is central to the understanding of selectivity issues. Related amido imido vanadium 88 and tungsten 89 96 Also alkyl groups bearing β-hydrogens may undergo the competitive deleterious β-H elimination yielding an alkene hydride complex (microscopic reverse of the migratory insertion) (Scheme 1.25).

Scheme 1.25. β-H abstraction and β-H elimination in alkyl complexes.

The unique case of methane activation following this 1,3-CH bond addition pathway has been realized at a tungsten center in the group of Legzdins (Scheme 1.26).

1.3-Conclusion and perspectives

Scientifically methane functionalization by soluble transition metal complexes remains a major challenge because of the need to combine two independent steps in one reaction: i) the cleavage of the CH bond and ii) the functionalization of the activated methane. Among all pathways discussed in the present chapter, only three has led to catalytic methane functionalization: i) the electrophilic route which involves homogeneous electrophilic complexes mainly based on Hg, Pt, Pd and V; ii) the carbene insertion, which involves highly electron poor perfluorinated scorpionate silver complexes and iii) the σ-bond metathesis at metallocene-alkyl complexes of scandium, by which the catalytic hydromethylation of propene and dehydrocoupling of diphenylsilane with methane were developed. The other pathways, on the other hand, namely i) the oxidative addition and metalloradical addition (ideally at low valent late transition metal centers), ii) the 1,2-CH bond addition across a M=X bond (M = High valent early transition metals complexes; X = C, NR, etc) and iii) the 1,3-CH bond addition across a M-η 2 -alkene (M = high valent early transition metal complexes) have never been incorporated into a catalytic scheme for the functionalization of methane. Although these processes do not provide a catalytic functionalization, they realize the methane C-H bond cleavage, a central step toward the design of catalytic reactions.

In the following chapters, we will focus on the CH bond cleavage of methane and different unsaturated molecules by a transient η 2 -cyclopropene complex of niobium by using a methodology based on the synergy between synthesis, characterization and mechanistic studies.

Chapter 2

CH bond activation of methane by a transient η reactions with first order in A and first order in methane or benzene, respectively. At 323 K, the rate constant k 2 for the reaction of A with benzene is shown to be ca 3 times that for the reaction of A with methane (k -1 ) (Scheme 2.2). Therefore, A reacts ca 3 times faster with benzene than with methane itself at 323 K, which implies the possibility of the activation of methane by the intermediate A.

Scheme 2.2. Reaction of A with methane and benzene.

As shown by 1 H, 2 H, and 13

Scheme2.3. C-D bond activation of benzene-d 6 by A.

The

η 2 -cyclopropene species A, [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)
] is trapped by mixing 1 with an excess of pyridine, resulting in the air-sensitive η 2 -cyclopropene pyridine complex [Tp Me2 Nb(c-C 3 H 4 )(NC 5 H 5 )(MeCCMe)] (Scheme 2.4). 1 This species is a formally 18e -niobium(I) complex. It is significant that no C-H bond of heteroaromatic pyridine is activated under these conditions.

Scheme 2.4. Trapping of the η 2 -cyclopropene species

A, [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)],
with pyridine.

2.2-Objectives of this work

Inspired by the work cited above, we aimed to see whether 1 was capable to activate the inert CH bond of methane in a similar way as benzene. The mechanistic pathway was first detected by isotopic labelling experiments using 13

Scheme 2.5. CH bond activation of methane by a transient η 2 -cyclopropene Nb intermediate

A. 4 and CD 4

2.3-Degenerate reaction of 1 with 13 CH

2.3.1-Degenerate reaction of 1 with 13 CH 4

Initially a medium pressure J-Young NMR tube was charged with compound 1 (0.030 g, 0.060 mmol) in cyclohexane-d 12 (0.5 mL) and was pressurized with 13

Figure 2.2(a)

. 1 H NMR spectrum of the reaction of 1 with 13 CH 4 to give 1-13 CH 3 .

Figure 2.2(b)

. 13 C NMR spectrum of the reaction of 1 with 13 CH 4 to give 1-13 CH 3 . This preliminary experiment establishes the presence of an equilibrium between 1 and 1-13 CH 3 (Scheme 2.6), confirming that intermediate A readily reacts with methane.

Scheme 2.6. Reaction of 1 with 13 CH 4 to give 1-13 CH 3 .

For longer reaction times, however, decomposition of 1 and 1-13 CH 3 was observed from the decrease of the Nb 12 CH 3 and Nb 13 CH 3 signals. The pressure of 13 CH 4 is not high enough to prevent the decomposition of the intermediate A, 1 precluding a kinetic analysis of the reaction.

2.3.2-Degenerate reaction of 1 with CD 4 2.3.2.1-Characterization of [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] by 1 H ROESY NMR

In order to assess the stereochemical outcome of the formation of 1-d 4 (Scheme 2.

Figure 2.3(a)

. 1

Figure 2.3(b)

. 1 H ROESY NMR spectrum of 1(expansion plot).

2.3.2.2-C-D Bond Activation of CD 4 by 1.

A medium pressure J-Young NMR tube was charged with compound 1 (0.030 g, 0.060 mmol) in perfluorobenzene (0.5 mL) and was pressurized with CD 4 to ca 8 bar at 157 K (ethanol/liquid N 2 bath) for 3 mins. The tube was then placed in the 1 H NMR spectrometer with the probe set at 303 K. At this temperature the pressure of CD 4 in the tube was ca. 16

bar. The reaction was subsequently monitored by 1

2.4-CH bond activation of 12 CH 4 by 1 (SST)

In order to investigate how methane undergoes elimination/addition more quantitatively, we next focused on the reaction of 1 with methane ( 12 CH 4 ) by spin saturation transfer (SST) experiments.

2.4.1-Mechanism of spin saturation transfer (SST) experiments

A summary of the SST experiments including data acquisition and kinetic treatment could be found in the literature. 5 The two sites exchange kinetics for the reaction of The rate equations for the lower and upper spin-state populations are considered first in the absence of spin-lattice relaxations as given in equations 2.1 and 2.2.

(2.1)

[ *] (2.2)
Moreover, spin-lattice relaxation processes that keep spin-state populations in thermal equilibrium are considered in the absence of chemical exchange as expressed in equations 2.3 and 2.4.

(2.3) (2.4)
The rate equations for the net magnetization in the presence of both the chemical exchange and spin-lattice relaxation are given in equations 2.5 and 2.6.

(2.5)

(2.6)
When the peak of methane is saturated by a low power CW irradiation during a time τ sat ≥ 5T 1(CH4) making the upper and lower spin state populations of methane equal, that is (2.9)

M sat(CH4) = [CH 4 ] -[CH 4 *] = 0,
(2.10)

It must be noted here that this requires the assumption that C A is constant. This is fully reasonable since A is a very reactive intermediate implying that a steady state approximation can be applied.

The SST experiments were carried out at high and medium CH 4 pressure, on Bruker Avance 400 and 500 spectrometers, respectively.

2.4.2-SST experiments of 1 with medium pressure 12 CH 4

A medium pressure J-Young NMR tube was charged with compound 1 (0.030 g, 0.060 mmol) in C 6 F 6 (0.5 mL, with one drop of cyclohexane-d 12 for the lock of the spectrometer)

and was pressurized with 12 CH 4 to ca 6 bar at 213 K for 3 mins. The tube was then placed in the 1 H NMR spectrometer with the probe set at 341 K. The reaction was subsequently monitored by 1 H NMR in a 500 MHz NMR spectrometer. 4 equivalents of 12 CH 4 could be quantified in solution by integration of the 12 CH 4 signal at δ 0.14 compared to the Tp Me2 CH signal of complex 1. Saturation of the NbCH 3 resonance at δ 0.59 was followed by observation of the CH 4 resonance at δ 0.14 after the probe pulse. Integration of the CH 4 resonance before and after the saturation revealed a ca 10% decrease (Figure 2.5). However, a closer look at the 1 H NMR spectrum revealed significant decomposition of 1 at the temperature and CH 4 pressure used. Under these conditions, the decomposition process precluded the accurate measurement of the corresponding rate constant because decomposition of A is competitive with its reaction with CH 4 .

Figure 2.5. Irradiation of the Nb-CH 3 signal and decrease of the CH 4 peak intensity (10%)

after Spin Saturation Transfer.

2.4.3-SST experiments of 1 with high pressure of 12 CH 4

In order to favor the bimolecular reaction of methane with A with respect to its measurement of k 1 through the SST experiment. This can be ascribed to the difference in relaxation times T 1 for the protons of the niobium bound methyl group [T 1 (NbCH 3 ) =1.34 s] and those of methane [T 1 (CH 4 ) =4.95 s] with respect to the actual rate constant k 1 . From the activation parameters previously determined for the β-H intramolecular abstraction of methane from 1 (ΔH † = 99 ± 5 kJ/mol and ΔS † = -6 ±10 J/K•mol), 1 we can calculate a ΔG † value of 101 kJ•mol -1 at 351 K which corresponds to a k 1 value of 6.6×10 -3 s -1 . In that case, only a variation of less than 1 % of the NbCH 3 signal intensity would be expected upon saturation of the CH 4 signal. Considering equation 2.8 and introducing k 1 at 351 K previously determined for the β-H intramolecular abstraction of methane from 1 2 and T 1(NbMe), one calculates the expected M 0(NbMe) /M sat(NbMe) ratio, that is M 0(NbMe) /M sat(NbMe) = k 1(351) × T 1(NbMe) + 1 = (6.6× 10 -3 × 1.34) + 1 = 1.009. In other words, there is a factor of 4 in T 1 and another factor of 4 in k obs and since the magnesation effect on NbCH 3 is 12%, one can calculate the magnesation effect on CH 4 , which equals to We have nevertheless realized a productive activation of methane. Along the lines established by Jones [6][7][8][9] and Wolczanski, 10 we surmised that the reaction of a weak Nb- From the rate constants at different temperatures (Figure 2.9 and Table 2.1), the activation parameters can be obtained from the Arrhenius and Eyring equations. Similiarly, the Eyring equation has the form:

12% × × = 12% × × = 0.8% 2
The linear plot of with gives the slope of , while the intercept is (Figure 2.12). Therefore, the enthalpy and entropy of activation, ΔH † = 76 ± 5 kJ/mol and ΔS † = -84 ± 10 J/K.mol, respectively, were obtained. This kinetic analysis for the disappearance of 3 in the T = 290 -321 K range (Figure 2.9) revealed a first-order dependence in 3 and a zeroth order in methane which, together with a highly ordered transition state, are consistent with a rate-determining intramolecular loss of mesitylene forming A followed by fast reaction with CH 4 yielding 1 as expected (Scheme 2.9). The activation enthalpy for the elimination of mesitylene from 3 (ΔH † = 76 ± 5 kJ/mol) is smaller than that for the elimination of methane from 1 (ΔH † = 99 ± 5 kJ/mol) 2 because the Nb-mesityl bond is weaker than the Nb-methyl bond. The activation entropy for the elimination of mesitylene from 3 (ΔS † = -84 ± 10 J/K.mol) is significantly more negative than that for the elimination of methane from 1 (ΔS † = -6 ± 10 J/K.mol) 2 (Scheme 2.9). We suggest that this is the result of the elimination of the sterically demanding mesitylene from 3 as compared to that of methane from 1 which involves more substantial conformational changes to reach the ordered four-center transition state.

2.6-Discussion

2.6.1-β-H abstraction versus α-H abstraction

As detailed in chapter 1, the activation of methane by the σ-bond metathesis mechanism 11,12 is a bimolecular process. In the activation of methane by β-H abstraction/1,3addition and α-H abstraction/1,2-addition, 13 on the other hand, the rate determining step is the β-H abstraction and the α-H abstraction respectively, resulting in a first order rate constant for the overall process. While we cannot compare the rate constant of the σ-bond metathesis with the rate constants of the β-H abstraction/1,3-addition and the α-H abstraction/ 15 For the β-H abstraction of CH 4 and mesitylene from the cyclopropyl precursors 1 and 3, our group has obtained the rate constants 2.93 × 10 -5 s -1 and 2.03 × 10 -5 s -1 , respectively, 1 at 30°C.

Therefore, the rate constants corresponding to the CH bond activation of methane have the same order of magnitude for the β-H abstraction/1,3-addition and the α-H abstraction/1,2addition processes.

2.6.2-Thermodynamics of the reaction of 2 with CH 4 to form 1 and mesitylene.

During the reaction, a C-H bond and a M-C bond are broken and a C-H bond and a M-C bond are formed. We must therefore take into account these four factors to study the reaction. Jones et al. [6][7][8][9] and Wolczanski et al. 10 have shown that the correlation between the strengths of the M-C bonds in titanium and rhodium complexes, respectively, and the bond dissociation energy (BDE) of the C-H bond of the corresponding hydrocarbon lead to approximately linear slopes, which are larger than 1. This reflects the fact that the differences between the bond strengths of two M-C bonds is always larger than the differences between the bond strengths of the two C-H bonds of the corresponding hydrocarbons.

2.6.3-Involvement of a σ-CH 4 complex and transition state structure for the elimination/activation of CH 4

Whereas the generation of intermediate A and its ability to activate methane via a rare 1,3-CH bond addition pathway constitute a remarkable result, the intermediacy of a methane adduct along the reaction coordinate remained an open question. In several low-valent latetransition metal systems, methane complexes have been inferred from kinetic analyses, 16 directly observed by time-resolved spectroscopy 17 or matrix trapping experiments 18 and even characterized by NMR in solution more recently. 19 In high oxidation state early transition metal systems however, methane adducts have eluded direct characterization. 13 DFT modelling has yielded structures and energetic data suggesting little thermodynamic stability and high kinetic lability of these adducts (barely elongation of the CH bond length of the coordinated methane compared to free methane and small binding energy (4 kJ/mol) in the σ-CH 4 complexes). 13 These values are significantly smaller than those for a directly observed rhodium complex (significant elongation of the CH bond distance of the coordinated methane (1.14 Å) compared to free methane (1.09 Å) and large binding energy (61 kJ.mol -1 ) in the σ-CH 4 complexes). 19 In order to compare the Nb case with these systems, professor Michel NBO approach. 20 There is a slight excess of positive charge on the hydrogen being transferred between the two Nb bound carbons. The methyl carbon bears a strong negative charge;

whereas the negative charge on the incipient cyclopropene ligand is shared equally between the two carbon atoms. The NLMO describing the interaction in the transition state is mainly delocalized on the niobium and the β-CH but some tails on the α-C and the methyl carbon are also seen. Overall the bonding picture is that of a σ-bond metathesis [21][22][23] but with a distinctive involvement of both niobium-bound carbons of the η 2 -cyclopropene ligand engaged in the hydrogen transfer. This highlights the structural and electronic differences with the more common α-abstraction/1,2-CH addition mechanism. 13

2.7-Conclusion

We have shown that a transient unsaturated η 2 -cyclopropene complex of niobium

Experimental Section

All operations were performed with rigorous exclusion of air and moisture, using standard Schlenk, high-vacuum, and glovebox techniques under Ar (O 2 < 3 ppm, H 2 O < 1 ppm). Perfluorobenzene and cyclohexane-d 12 were dried over molecular sieves, filtered, degassed by several freeze-pump-thaw cycles and stored in sealed ampules in the glovebox. 1 H and 13 2) were prepared according to published procedures. 1,2 Medium pressure J-Young NMR tubes (5 mm diameter, thin wall precision, quick pressure valve, 7 inch length, max pressure ca 10 bar) were purchased from WilmadLabGlass.

For the high pressure (HP) NMR experiments, a single-crystal 10 mm sapphire (aluminulm oxide) NMR tube was used. The tube is quite chemically inert and it can withstand pressures up to 100 bar and temperatures up to 90 °C, which allows a sufficiently large sample volume (2 ml) for observation of nuclei such as 1 H, 13 C and all measurements were carried out under the limits of the tube. While the tube surface is not perfectly concentric, the loss of resolution that this entails was deemed to be less important. The high pressure measurements were carried out on a Bruker Avance 400 spectrometer equipped with a 10mm TBO probe.

Chart 2.2 shows a schematic view of the Ti-alloy valve connected to the sapphire NMR tube. 31 The valve is designed to put gases under pressure (6,

CH Bond Activation of 13 CH 4 by 1.

A medium pressure J-Young NMR tube was charged with compound 1 (0.030 g, 0.060 mmol) in cyclohexane-d 12 (0.5 mL) and was pressurized with 13

CD Bond Activation of CD 4 by 1.

A medium pressure J-Young NMR tube was charged with compound 1 (0.030 g, 0.060 mmol) in perfluorobenzene (0.5 mL) and was pressurized with CD 4 to ca 8 bar at 157 K (ethanol/liquid N 2 bath) for 3 mins. The tube was then placed in the 1 H NMR spectrometer with the probe set at 303 K. The reaction was subsequently monitored by 1 H NMR and 2 H{ 1 H} NMR for 4 hours at 303 K.

Spin saturation transfer (SST) experiments.

Samples preparation

Medium pressure NMR tube reaction: In the glovebox, a medium pressure J-Young

SST Acquisition procedure

The High Pressure or Medium Pressure NMR tube was set into the NMR spectrometer for 15 min to establish thermal equilibrium and the magnet was shimmed as necessary. The saturation pulses were carefully set up for obtaining a complete saturation of the irradiated signal with a minimum power value in order to limit as much as possible partial saturation of The spectra were processed with a special attention to phase and baseline corrections. The integration of the measured signals used for the rate constant calculations was then extracted from the reference and the saturation spectra.

T 1 measurements

The spin-lattice relaxation times T 1 were measured by the inversion recovery method.

On the 500 MHz spectrometer, at 341 K, the spin-lattice relaxation times for NbMe and CH 4

were T 1(NbMe) 1.00 s and T 1(CH4) 4.90 s, respectively. On the 400 MHz spectrometer, at 351 K,

SST by irradiation of the CH 4 peak

For the irradiation of the CH 4 peak, a similar procedure to that described above for the irradiation of the NbMe peak was followed. After finding the suitable saturation pulses for obtaining complete saturation of the irradiated CH 4 (δ 0.

Kinetic Experiments

Reaction of 3 with methane to give 1.

In a glove box, compound 3 (0.072 g, 0.12 mmol) was dissolved in a mixture of cyclohexane-d 12 (1 mL) and perfluorobenzene (1 mL) and then transferred into a high pressure (HP) single-crystal 10 mm sapphire NMR tube, which was then protected by a polycarbonate casing and equipped with a high-pressure manifold. The HP NMR tube was pressurized with 40 bars of methane and carefully shaken to ensure the dissolution of methane. To carry out a complete kinetic study, the course of the reaction was monitored at 290, 303, 314 and 321 K for more than 5 half-lives but for practical reasons was treated within 1.8, 2.3, 2.3 and 3.2 half-lives, respectively. Integration of the CH 4 signal compared to the Tp Me2 CH signal of 3 indicated that ca 12-33 equivalents of methane vs 3 were present in solution.

Computational studies

Calculations were performed at the DFT level with the different functionals as implemented in Gaussian09, revision D.01. 24 Nb was described using the Stuttgart/Dresden RECP (SDD) pseudo-potential and its associated basis set. 25 For N, C, B and H, the TZVP basis set was used. 26 Structure optimizations were carried out without symmetry constraints.

The nature of the stationary points was ascertained by a vibrational analysis within the harmonic approximation (1 atm and 298 K). Minima were identified by a full set of real frequencies and transitions states by the presence of a single imaginary frequency. Transitions states were relaxed on both reactant and product sides. Electronic energies are reported with zero-point energy correction as hartree in the text. Since reactions were carried out either in non-polar cyclohexane or in cyclohexane/hexafluorobenzene mixtures with similar results, solvent corrections were not included. NBO calculations were carried out with NBO 6.0 as implemented in Gaussian09. 27 Three types of functionals were tested in addition to the one used originally (M06/SDD/6-31G(d), CMD correction). 1 They either contain different types of dispersion corrections or no correction (wB97XD, PBE0-GD3BJ, PBE0). [28][29][30] While energies vary slightly, the geometries of stationary points do not vary significantly except for the methane adduct. In the absence of dispersion correction (ie PBE0), the interaction of methane with the η 2 -cyclopropene niobium complex [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)] appears much weaker with a long niobium…methane distance. ). [9][10][11][12] Although a number of early transition metal η 2 -alkene complexes (M = Ti, 13,14 Zr, 15 Hf, 16 Nb, Ta, 17 and W 18 ) could be generated from β-H abstraction of relevant alkyl compounds, only a few of them activate the CH bond of benzene 16 and terminal alkynes 14,15 via intermolecular 1,3-CH addition (Scheme 3.1 (c)).

Scheme 3.1. β-H abstraction/1,3-CH bond addition by different intermediates.

Previously our group has reported that a transient η 2 -cyclopropene niobium complex A generated by intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] ( 1) is able to activate the aromatic and benzylic

C-H bonds of alkyl aromatics under mild conditions. 6,19,20 For instance, A reacts with 1,3dimethylbenzene to give a 3:1 mixture of arene and benzylic CH activated products, translating to a 18:1 preference for aromatic versus benzylic CH bond activation on a per CH bond basis (Scheme 3.2). Computational studies have shown that the aryl complex is preferred over the benzylic complex by ca. 20 kJ•mol -1 .

Scheme 3.2. Aromatic and benzylic CH bond addition of 1,3-dimethylbenzene by A.

Different from other η 2 -alkene complexes, the cyclopropene niobium system is able to activate the CH bond of various hydrocarbons, including methane (Chapter 2), due to the unique interaction mode of cyclopropene with an early transition metal fragment. The strained (5) , PhC≡C(6), Fc(7), C 6 F 5 (8)) under mild conditions. These complexes were characterized by 1 H NMR, 13 C NMR, 19 F NMR spectroscopy, X-ray crystallography, elementary analysis. Moreover, thermolysis of 8 provided two unexpected isomeric products (9a,b) arising from cyclopropyl ring opening followed by alkyne coupling. The mechanism involved was investigated by computational analysis in collaboration with Dr. Abel Locati and Prof. Feliu Maseras (ICIQ, Tarragona, Spain). Electrochemical studies of 1, 5, 6, and 7

were carried out to compare the electronic effect of the different unsaturated substituents to the metal center. This research was carried out in collaboration with Dr. Pascal Oulié, a former post-doctoral associate in our group.

3.3-CH bond activation of furan and thiophene

3.3.1-Synthesis of [Tp Me2 Nb(c-C 3 H 5 )(2-C 4 H 3 X)(MeCCMe)] (4a, X = O; 4b, X = S)

[Tp Me2 NbMe(c-

3.3.2-1 H NMR and 13 C NMR spectroscopy characterization of 4a and 4b

Complexes 4a and 4b were characterized by 1 H NMR and 13 C NMR in benzene-d 6 at 298 K. The 1 H NMR spectrum of 4a confirms that the compound obtained is chiral (Figure

5-H-furyl 4-H-furyl 3-H-furyl HE E' HD Hβ

1 H NMR (benzene-d 6 , 298 K)

HE'

Hβ

The NMR characterization of 4b is similar to that described above for complex 4a. In In the 13 129.0 (4-C), and 126.0 (5-C) ppm. However, the resonance for Nb-C2 was so broadened that it could not be observed. This is due to the coupling between 13 C (I = 1/2) and the quadrupolar 93 Nb (I=9/2).

3.3.3-Structural description

Yellow single crystals of 4a suitable for X-ray characterization were obtained by slow evaporation of a pentane solution at -40 °C (Figure 3.3).The complex crystallizes in the space group P 21/n (monoclinic system).

5-H-thienyl 4-H-thienyl

3.4-C-H bond activation of 1-alkyne and alkene.

3.4.1-Synthesis and characterization of [Tp Me2 Nb(c-C 3 H 5 )(CCPh)-(MeCCMe)] (5) Scheme 3.5. C-H bond activation of phenylacetylene by 1

[Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] reacted with phenylacetylene (50 equivalents) to afford the complex [Tp Me2 Nb(c-C 3 H 5 )(CCPh)(MeCCMe)] (5) as orange crystals in 40% yield (Scheme 3.5), which is stable under Ar in benzene solution at 318 K.

In addition to the signals assigned to the Tp Me2 and alkyne ligands, the 1

3.4.2-Synthesis and characterization of [Tp Me2 Nb(c-C 3 H 5 )(c-1-C 5 H 7 )-(MeCCMe)] (6)

1 also activates the weaker non-conjugated Csp 2 -H bond of cyclopentene under similar conditions to give the cyclopentenyl complex [Tp Me2 Nb(c-C 3 H 5 )(c-1-C 5 H 7 )-(MeCCMe)] (6) in 66% yield, which can react with benzene solution at 318 K.

In the 1 H NMR spectrum, 6 exhibits a broad singlet at δ 6.

HE' C=CH

The η 1 -1-cyclopentenyl ligand [C8-C12] sits approximately in the same plane as that of the trans dimethylpyrazolyl ring (N3N4 based). The C8-C11 bond length is consistent with that of a carbon-carbon double bond and it is unremarkable as compared to those of other alkenyl niobium complexes 6,[26][27][28][29]

3.5-CH bond activation of ferrocene

We have been looking for convenient internal standards for kinetic studies on these and related reactions. Among other compounds, ferrocene (FcH), seemed a suitable candidate due to its ease of handling, its stability and inertness and its single 1 H NMR signal in a region devoid of reactant or product signals belonging to the [Tp Me2 NbR(c-C 3 H 5 )(MeCCMe)] family.

In a control experiment, we were surprised to see that 1 was very competent to activate a CH bond of FcH.

3.5.1-Synthesis and characterization of [Tp Me2 Nb(Fc)(c-C 3 H 5 )(MeCCMe)](7)

3.6-CH bond activation of pentafluorobenzene

3.6.1-Synthesis and characterization of [Tp Me2 Nb(C 6 F 5 )(c-C 3 H 5 )-(MeCCMe)] (8)

Given the ease with which different types of CH bonds were cleaved by intermediate A, we were interested to study whether it might be reactive also towards CF bonds. [31][32][33] Hexafluorobenzene showed no sign of reaction with 1 under similar conditions such as those described herein for other hydrocarbons; only decomposition of 1 occurred upon heating at higher temperatures. We then suggested that CF bond cleavage might be facilitated, at least entropically, after an initial CH bond cleavage in a mixed hydrofluoroarene. In this vein, we reacted 1 with pentafluorobenzene (Scheme 3.8). [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] reacted with C 6 F 5 H (24 equiv) affording, after 72h of stirring at room temperature, the yellow 8) complex in 60% yield.

[Tp Me2 Nb(C 6 F 5 )(c-C 3 H 5 )(MeCCMe)] (

Scheme 3.8. CH bond activation of pentafluorobenzene by 1.

The 1 H NMR spectrum of 8 is shown in Figure 3.10. One of the alkyne methyl groups shows through-space coupling with one fluorine appearing as a doublet (J HF = 5.6 Hz) at δ

3.6.3-Isomerization of 8. Synthesis and characterization of [Tp Me2 Nb(C 6 F 5 )[-

HC(CH 3 )-HC=C(CH 3 )-(CH 3 )C=Nb] (9a, 9b)

Upon gentle heating in benzene at 45°C, 8 underwent an intramolecular cyclopropyl ring opening and 2-butyne coupling to give an inseparable 88:12 mixture of two regioisomers 9a and 9b, respectively (Scheme 3.9). This ratio was consistently found when repeating the reaction under identical conditions. The interpretation of spectroscopic data and the identity of 9a and 9b was ascertained by a X-ray diffraction analysis on a single crystal (see below). This behavior differs from that of the phenyl derivative 2, which, in the presence of C 6 D 6 , undergoes reversible β-H abstraction of the cyclopropyl to form benzene and A, which In the 1 H and 13 C NMR spectra of 9a and 9b in benzene-d 6 , each set of the Tp Me2 CH hydrogens and carbons appears in a 1:1:1 ratio in accordance with the lack of a plane of symmetry in these niobacycles. In the 1 H NMR of 9a, the major isomer, (Figure 3 The 13 C NMR spectrum shows a deshielded NbCα with an alkylidene character at δ 260.2 ppm as well as the sequence of the three other carbons along the ring, the sp 2 Cβ (δ 120.0 ppm) and Cγ (δ 101.6 ppm dd, 1 J CH = 155 Hz) and the sp 3 Cδ (δ 87.3). The methyl groups attached to the quaternary carbons Cα and Cβ resonate at δ 24.4 and 22.9 ppm, respectively, whereas the methyl group attached to the tertiary carbon Cδ at δ 21.1. These data match perfectly those of the allyl-rearranged product [Tp Me2 NbCl(=CPh-CMe=CH-CHMe)]. 34 The 19 In order to see whether 9a could be converted to 9b through intramolecular hydrogen migration between the α and δ positions of 9a or 9b under heating, a variable temperature (VT) 1 H NMR study has been carried out in the 298 -343 K temperature range on the mixture of complexes 9a and 9b (ratio of 88:12) (Scheme 3.12). No broadening of the peaks of 9b was noted, especially the signal of the Hβ proton at δ 7.60 (Scheme 3.12), indicating that 9a and 9b do not interconvert through intramolecular hydrogen migration. Therefore, 9a and 9b must be formed from 8 by different pathways.

Scheme 3.12. Low field expansion of the VT 1 H NMR spectra of a mixture of 9a and 9b (ratio of 88:12).

Orange single crystals of 9a and 9b, suitable for an X-ray crystal structure determination, were obtained by slow evaporation of a pentane solution of 9a and b at -40 °C (Figure 3 5), C1-C2 1.412( 6), C1-C5 1.472( 6), Nb-C2 2.348( 4), C2-C3 1.421( 6), Nb-C3 = 2.347( 4), C2-C7a 1.493(2) C3-C7b 1.42( 4), Nb-C4 2.226(4) C3-C4 1.428 (7), C4-C6 1.501 (7), Nb-C11 2.344( 4), C1-C11 2.345( 6), C1C2C3C4 9.9°.

The noteworthy features are (i) the disordered η 4 -trimethylbutadienyl unit and (ii) the presence of pentafluorophenyl ring. The latter is perpendicular (88°) to the planar (C1-C2-C3-C4 10°) four-membered niobacycle and sits above the Cα (C2) and Cβ (C3) carbons and their hydrogen and methyl substituents, justifying the through-space couplings observed with the ortho fluorine atoms in the NMR spectra. The observed disorder was successfully modelled in a 88 (9a):12 (9b) ratio of two occupancies of methyl groups C7a on C2 (H3 on C3) and C7b on C3 (H2 on C2). The disorder does not involve any other atoms in the structure most particularly those of the metallacycle. The ratio observed in the solid state, therefore, matches that observed in solution. The five-membered niobacycle is folded around the C1-C4 axis (113°) so that all four carbons are at bonding distance of the niobium. There is a Nb-C double bond between Nb1 and C1 (Nb1-C1 = 2.002(5) Å) and a Nb-C single bond between Nb1 and C4 (Nb1-C4 = 2.226(4) Å). The C2 and C3 atoms interact symmetrically with the niobium center (Nb1-C2 2.348(4), Nb1-C3 2.347(4) Å). All C-C bonds in the fourcarbon chain are identical pointing to a fully delocalized picture (C1-C2 1.412(6), C2-C3 1.421(6), C3-C4 1.428(7) Å). These parameters are similar to those reported for the η 4butadienyl [Tp Me2 NbCl(=CPh-CMe=CH-CHMe)] complex 34 (Nb=Cα 1.993(4), Nb-Cβ 2.334(4), Nb-Cγ 2.370(5), Nb1-Cδ 2.277(5), Cα-Cβ 1.439 (7), Cβ-Cγ 1.418 (6), Cγ-Cδ 1.392 (7)) and are consistent with the limiting structures in Scheme 3.11.

Scheme 3.13. The 1 e -reduction and oxidation processes for complex 7

The complexes 1, 5 and 6 display a partially reversible one-electron wave at negative potentials, again attributed to the niobium-centered reduction. Table 3.1 shows the half wave reduction potentionals (E 1/2 /V) of compounds 1, 5, 6, and 7. The complexes are more easily reducible in the order 5 35 These complexes are formally pseudo-octahedral 16e species. Their molecular orbital diagram, shown in Figure 3.17, resembles that of the [TpNbCl 2 (MeCCMe)] compound. 36,37 In a first approximation we can consider the interaction diagram between the ML 5 fragment They also observed a linear correlation between D M-C and D C-H (BDE). [40][41][42][43] For the [Tp Me2 Rh{P(OMe) 3 }(R)(H)] compounds, two separate linear correlations, obtained by plotting the D Rh-C versus the D C-H (BDE), were found. The slope for α-substituted substrates and for unsubstituted hydrocarbons measured 1.45 and 1.55, respectively, the Rh-C bond energies for α-substituted substrates being higher by ~7 kcal.mol -1 than those for unsubstituted hydrocarbons. 42 Comparison of the analogous [Tp Me2 Rh(L)(R)(H)] (L = CNneopentyl, PMe 3 , P(OMe) 3 ) systems shows that the slopes of the correlations follow the order CNneopentyl < PMe 3 ≤ PMe 3 , indicating that the Rh-C bond strengths increase with the electron donating power of the ligand. Eisenstein and Perutz carried out DFT calculations on the metal-carbon bond strengths for both the [Tp′Rh(CNneopentyl)(R)(H)] and

complex a E 1/2 / V b ΔE p /mV c 1 -2.85 83
5 -2.41 92
6 -2.75 103
7 -2.
(E 1/2 = -2.41 V) > 7 (E 1/2 = - 2.67 V) > 6 (E 1/2 = -2.75 V) > 1 (E 1/2 = -2.85 V),
[Ti(R)(silox) 2 (NHSi t Bu 3 )] (silox = OSi t Bu 3 ) systems. 44 The calculated slopes of D M-C versus D C-H (BDE) are in excellent agreement with experiment. Generally, the higher the C-H bond strengths of the activated hydrocarbon, the greater the metal-carbon bond strengths formed.

The trend of the metal-carbon bond strengths is as follows: alkynyl > alkenyl ≈ cyclopropyl > arenyl ≈ benzyl > alkyl.

In order to get information on the Nb-R bond strengths in the [Tp Me2 Nb(R)( with benzene.

At room temperature or at mild heating 8) were dissolved in benzene and kept in solution for 1 day. These complexes however were so stable that they did Scheme 3.14. CH bond dissociation enthalpies, BDE (kcal/mol, 298 K). 40,43,46,48 Scheme 3.15. pK a values of hydrocarbons (DMSO, 298 K). [45][46][47]49 3.8.2-Positional disorder and implication for the reaction outcome and mechanism.

(318 K), [Tp Me2 Nb(c-C 3 H 5 )(2- C 4 H 3 O)(MeCCMe)] (4a), [Tp Me2 Nb(c-C 3 H 5 )(2-C 4 H 3 S)(MeCCMe)] (4b), [Tp Me2 Nb(c- C 3 H 5 )(CCPh)(MeCCMe)] (5) and [Tp Me2 Nb(c-C 3 H 5 )(C 6 F 5 )(MeCCMe)] (
: [Tp Me2 Nb(c-C 3 H 5 )(CCPh)(MeCCMe)] > [Tp Me2 Nb(c-C 3 H 5 )(2-C 4 H 3 O)(MeCCMe)] ≈ [Tp Me2 Nb(c-C 3 H 5 )(2-C 4 H 3 S)(MeCCMe)] ≈ [Tp Me2 Nb(c-C 3 H 5 )(C 6 F 5 )(MeCCMe)] > [Tp Me2 Nb- (Ph)(c-C 3 H 5 )(MeC≡CMe)] > [Tp Me2 Nb-(c-C 3 H 5 )(2-C 5 H 7 )(MeCCMe)] > [Tp Me2 Nb(c-
A remarkable feature of the disorder observed in the X-ray structure of 9a and 9b is that it involves only the Cβ and Cγ carbons and the hydrogen and methyl group directly attached to them. No other atoms in the structure and most strikingly none of the other atoms of the butadienyl ligand are involved in the disorder. This can be judged from the welldefined, small thermal ellipsoids of C1, C4, C5 and C6 for example. Consequently, Nb-C1

(NbCα) and Nb-C4 (NbCδ) bear well-defined double and single bond character, respectively.

Thus the disorder cannot result from a statistical distribution of H4 between C1 and C4. A mechanism occurring through the coupling of the 2-butyne ligand with a rearranged cyclopropyl group in 8 leading to 9a would result in C5, C1, C2, C7a and C3, C4, C6 belonging to the alkyne and cyclopropyl ligands in 8, respectively. Looking at 9b, one realizes that a similar route would necessitate C-C bond cleavage in the alkyne ligand, a potentially disfavored event. Alternatively, C6, C4, C3, C7b and C5, C1, C2 might originate from the formerly alkyne and cyclopropyl ligands, respectively, a much more reasonable assumption.

However in that case, the mechanism would imply the migration of at least one more hydrogen, namely H4. Recall that in solution, the same 88:12 ratio between 9a and 9b is observed and that there is no evidence that 9a and 9b interconvert thereby confirming that a fast migration of hydrogen between Cα and Cδ is most probably not at the origin of the observed chemistry. Also, this isomerism has never been observed is the previous examples of η 4 -butadienyl complexes.

Dr. Abel Locati and Prof. Feliu Maseras (ICIQ, Tarragona, Spain) have performed some theoretical calculations to give some insights on the mechanism of this rearrangement.

Although the whole pathway is still not fully satisfactory some ideas are presented here. The key idea was to account of the formation of two isomers from a common intermediate without invoking the apparently simpler HD migration between so-called Cα and Cδ. The M06 functional, as implemented in the Gaussian 09 suite of program, was used throughout this study where the SDD pseudopotential and its associated basis set was used to described Nb and a split valence 6-31G(d) basis et was used for all other atoms

The formation of the common intermediate split_int from which two reaction pathways split to is shown in Scheme 3.16. The origin of energy is the starting cyclopropyl complex 8. The first step is the migratory insertion of the cyclopropyl group on to the alkyne to give a high energy intermediate mig_int that undergoes cyclopropyl ring opening. This ring opening can be described an intramolecular CC bond activation similar to an oxidative addition. The rather stable intermediate open_mig then transfers a E-H to the D-C of the alkyne, now singly bound to Nb to give a more stable intermediate split_int. Interestingly, no true hydride complex was computed at this stage.

Scheme 3.16. Computed Gibbs energy profile for the formation of the common intermediate split_int

From split_int, two pathways independently lead to the products (Scheme 3.17).

Through a lower energy transition state alphaH_TS, the hydrogen on the alkyne can migrate back to the other end of the carbon chain giving the main product 9a_opt. A slightly higher energy pathway, now proceeding through a true hydride intermediate, eliminates another hydrogen from the formerly cyclopropyl ligand before reinserting it at the end the carbon chain to give 9b_opt.

The mechanism proposed here locates 9a as the most stable regioisomer. The apparent flexibility of the carbon backbone after opening of the cyclopropyl ring that was noted during the computational modeling might indicate that some other pathways could also be taken into account to rationalize perfectly the selectivity. Definitely more work is needed here.

Scheme 3.17. Computed Gibbs energy profile showing the pathway leading to either 9a or 9b

from a common intermediate split_int.

3.9-Conclusion

We have investigated the reactivity of complex 1 towards heteroaromatics, unsaturated hydrocarbons, ferrocene (FcH) and pentafluorobenzene, the mechanism involved passing through a β-H abstraction/1,3-CH bond activation pathway. Compound 1 is able to selectively

Experimental Section

All experiments were carried out under a dry argon atmosphere using either Schlenck tube or glove box techniques. Diethylether were obtained after refluxing purple solutions of Na/benzophenone under argon. Benzene, toluene, pentane, cyclohexane and dichloromethane were dried by refluxing over CaH 2 under argon. Deuterated NMR solvents were dried over molecular sieves, degassed by freeze-pump-thaw cycles and stored under argon. 1 H and 13 C NMR spectra were obtained on Bruker Avance 300 ( 1 H, 300.1 MHz ; 13

Synthesis of Tp Me2 Nb(c-C 3 H 5 )(2-C 4 H 3 O)(MeCCMe) (4a).

[Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] (0.66 g, 1.32 mmol) was dissolved in pentane (50 mL) and excess furan (0.44 mL, 0.420 g, 6.17 mmol) was added. After 2 days at room temperature, the solvent was evaporated to dryness. Pentane (10 mL) was added and stripped off. The yellow powder was extracted with pentane (50 mL), filtered through a pad of Celite.

The solvent was stripped off to leave [Tp Me2 Nb(c-

C 3 H 5 )(2-C 4 H 3 O)(MeCCMe)] 2a

Synthesis of Tp Me2 Nb(c-C 3 H 5 )(c-C 5 H 7 )(MeC≡CMe) (6).

[Tp Me2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (0.55 g, 1.10 mmol) was dissolved in pentane ( 50mL) and about 5 equivalents of cyclopentene (0.50 mL, 0.380 g, 5.50 mmol) were added.

After 2 days at room temperature, the yellow brown solution was evaporated to dryness, 10 mL of pentane was added and stripped off. The yellow brown powder is dissolved in 50 mL of pentane and filtrated trough a pad of Celite. The solvent is stripped off, the powder is washed with small among of cold pentane up to leave 6 as a yellow powder (0.120 g, 0.22 mmol, 20 %).

[Tp Me2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (0.30 g, 0.60 mmol) was dissolved in cyclopentene (2.5ml). After 22 h at 35 °C when the yellow solution turned to yellow brown, the solvent was evaporated and an oily yellow brown residue was obtained. The residue was dissolved in 20 mL of pentane, filtered through a pad of celite, and washed with pentane (5 mL) × 3 times. The solution was concentrated until a little microcrystalline came out and then filtered to a vial, which was stored at - 13 C NMR of 4b in benzene-d 6 at 298 K Figure 3.24. 13 C NMR of 5 in benzene-d 6 at 298 K 13 C NMR (benzene-d 6 , 298 K) 13 C NMR (benzene-d 6 , 298 K)

Figure 3.25. 13 C NMR of 6 in benzene-d 6 at 298 K Figure 3.26. 13 C NMR of 7 in benzene-d 6 at 287 K 13 C NMR (benzene-d 6 , 298 K) 13 C NMR (benzene-d 6 , 287 K)

.27. 13 C NMR of 8 in CD 2 Cl 2 at 233 K Figure 3.28. 13 C NMR of a mixture of 9a and 9b (molar ratio 88:12) in benzene-d 6 at 298 K 13 C NMR (CD 2 Cl 2 , 233 K) 13 C NMR (benzene-d 6 , 298 K)

Conclusion and perspectives

In the first part of this manuscript, we have proved that the transient K Reaction pathways, including the structure of the intermediates and transition states, will be unveiled by DFT calculations. Attempts at observing these intermediates

L n M(K 2 -c-C 3 H 4 )
directly in the gas phase is planned through the use of intermediates by UV-PES. This spectroscopy allows the determination of ionization potentials of the molecules, real electronic "fingerprints", which can be directly correlated with the molecular orbitals energy levels. Coupled with flash vacuum thermolysis, UV-PES would allow to monitor on-line the thermal degradation of the cyclopropyl complexes.

Finally, coupling these CH bond activation reactions with a reaction that would functionalize methane, ideally catalytically, is the ultimate goal. We must admit that currently, there is no such reaction.

1.1.3-Insertion d'un carbene

1.2.1-Métathèse des liaisons σ

1.2.2-Addition CH 1,2.

1.2.3-Addition

1.3-Conclusions et perspectives

Schéma 2.2. Reaction de A avec le méthane et le benzène.

Comme le montre la spectroscopie RMN du 1 H, 2 H et 13

2.3.2.2-Activation de la liaison C-D du CD 4 par le complexe 1.

Un tube RMN de J Young à moyenne pression a été chargé avec le composé 1 (0,030 g, 0,060 mmol) dans du perfluorobenzène (0,5 ml) et pressurisé avec CD Il faut noter ici que l'écriture de cette équation nécessite l'assomption de C A constant.

Ceci est parfaitement raisonnable puisque A est un intermédiaire très réactif, ce qui justifie la validité de l'approximation de l'état stationnaire.

Les expériences SST ont été effectuées à haute et moyenne pression de CH 4 sur des spectromètres Bruker Avance de 400 et 500 MHz, respectivement.

Concernant les expériences à moyenne pression, une décomposition significative du composé 1 a été observée, ce qui nous a empêché la mesure précise de la constante de vitesse correspondante. Par la suite, donc, seulement les expériences à haute pression de CH 4 seront illustrées.

2.4.2-Etude SST sur la réaction de 1 avec le 12 CH 4 à haute pression.

1 Chapter 1 : 8 1. 1 . 1 - 8 1. 1 . 2 - 9 1. 1 . 3 -B 3 . 4 -

 1181181291334 ..................................................................................................................... A List of abbreviations .................................................................................................................. C General introduction .................................................................................................................. Methane CH bond activation processes .................................................................. 3 1.1-CH bond activation of methane by late transition metal complexes .............................. Electrophilic substitution .......................................................................................... Oxidative addition ................................................................................................... Carbene insertion ................................................................................................... 1.1.4-Metalloradical addition .......................................................................................... 1.2-CH bond activation of methane by early transition metal complexes ......................... 1.2.1-σ-Bond metathesis .................................................................................................. 1.2.2-1,2-CH addition ...................................................................................................... 1.2.3-1,3-CH addition ...................................................................................................... 1.3-Conclusion and perspectives ....................................................................................... References ........................................................................................................................... Chapter 2: CH bond activation of methane by a transient η 2 -cyclopropene niobium complex ... .................................................................................................................................................. 2.1-Introduction ................................................................................................................... 2.2-Objectives of this work ................................................................................................. 2.3-Degenerate reaction of 1 with 13 CH 4 and CD 4 .............................................................. 2.4-CH bond activation of 12 CH 4 by 1 (SST) ...................................................................... 2.5-Kinetic study of the reaction between 3 and CH 4 ......................................................... 2.6-Discussion .................................................................................................................... 2.7 Conclusion ..................................................................................................................... Experimental Section ........................................................................................................... References ............................................................................................................................ Chapter 3: CH bond activation of unsaturated hydrocarbons by a niobium methyl cyclopropyl precursor .................................................................................................................................. 3.1-Introduction .................................................................................................................. 3.2-Objectives of this work ................................................................................................. 3.3-CH bond activation of furan and thiophene ................................................................. CH bond activation of 1-alkyne and alkene ................................................................. 3.5-CH bond activation of ferrocene .................................................................................. 3.6-CH bond activation of pentafluorobenzene .................................................................. 3.7-Electrochemistry ......................................................................................................... 3.8-Discussion .................................................................................................................. 3.9-Conclusion .................................................................................................................. Experimental Section .........................................................................................................

C 3 H 4 )

 34 (MeCCMe)] (A) could be a good candidate for the activation of CH 4 via a 1,3-addition pathway.

by 2 H

 2 NMR (Figure1.1), for instance, the disappearance of the deuteride signal is associated to the appearance and growth of the new CH 2 D signal .

Figure 1 . 1 .Scheme 1 . 11 .

 11111 Figure 1.1. The 2 H NMR spectra of [W{(η-C 5 H 4 )CMe 2 (η-C 5 H 4 )}(CH 3 )D] in benzene as a function of time (high field extension).

Scheme 1 . 12 .

 112 Scheme 1.12. Deuterium scrambling in the interconversion of [L n M(CH 3 )(D)] with [L n M(CH 2 D)(H)].

Scheme 1 .

 1 Scheme 1.16. H/D exchange between methane and C 6 D 6 at 100 °C through σ-CAM mechanism.

  CH 4 reacts with [Cp* 2 M(CH 3 )] (M = Lu, Y) through a degenerate exchange process to form [Cp* 2 M( 13 CH 3 )]

Figure 1 . 3 . 1 H

 131 Figure 1.3. 1 H NMR spectra of reaction of [Cp* 2 Lu(CH 3 )] with 13 CH 4 as a function of time at 70 °C.

  C NMR spectroscopy, reaction of 1 with C 6 D 6 gives two diastereomers with a selective insertion of a single deuterium at either of the two Cβ positions on the same enantioface of the cyclopropyl ring as niobium, in a ca 2:1 ratio. This conclusively establishes that A is an unsaturated η 2 -cyclopropene complex generated from 1 by a β-H or 1,3-abstraction of CH 4 . A CH/CD bond of C 6 H 6 /C 6 D 6 then rapidly adds the Nb-C bond of A in a stereospecific 1,3-fashion (Scheme 2.3) to give two diastereomers in 2:1 ratio. Competition reaction of 1 with an excess of a 1:1 molar mixture of benzene and benzene-d 6 at 310 K to form 2 and 2-d 6 affords an isotope effect (k H /k D ) of 4.0, reflecting a C-H/C-D bond cleavage event, with significant weakening of the bonds in the transition state from A to 2.

  CH 4 and CD 4 to see whether 1 could react with13 CH 4 and CD 4 to form 1-13 CH 3 and 1-d 4 , respectively (Scheme 2.5). A kinetic study of the degenerate exchange between 1 and 12 CH 4 through spin saturation transfer experiments (SST) was carried out to study the elementary steps. Kinetic studies of the reaction between [Tp Me2 Nb(CH 2 -3,5-Me 2 C 6 H 3 )(c-C 3 H 5 )(MeCCMe)] (3) and 12 CH 4 revealed that the β-H abstraction is the rate-determining step (RDS), providing rate constants and activation parameters which reflect the structure of the transition state in the RDS.DFT calculations, carried out by professor Michel Etienne, were used to compute the energy of key intermediates and transition states for the CH bond elimination/activation of methane. The mechanism was compared with the σ-bond metathesis and the α-H abstraction/1,2-addition mechanisms.

  CH 4 to ca 3 bar at 173 K (ethanol/liquid N 2 bath) for 1 min. The tube was then placed in the 1 H NMR spectrometer with the probe set at 313 K. 1.2 equivalents of 13 CH 4 could be quantified in solution by integration of the 13 CH 4 signal (δ 0.19 (d, 1 J CH = 125.0 Hz)) compared to the Tp Me2 CH signal of complex 1. The reaction was subsequently monitored by1 H NMR over 19 hours, displaying the activation of13 CH 4 by 1 to form [Tp Me2 Nb 13 CH 3 (c-C 3 H 5 )(MeCCMe)] 1-13 CH 3 . Within 5 hours the 1 H NMR spectrum showed the appearance and the growth of the Nb 13 CH 3 resonance as a doublet centered at δ 0.766 ppm with 1 J CH = 119.6 Hz (Figure 2.2(a)). This signal was superimposed with the singlet at δ 0.779 ppm corresponding to the Nb 12 CH 3 group in 1 (Figure 2.2(a)). The 13 C NMR spectrum showed the appearance and the growth of the Nb 13 CH 3 resonance at δ 48.90 ppm (Figure 2.2(b)).

Chart 2 . 1 .

 21 5), we carried out a 1 H ROESY NMR experiment of 1, allowing the assignment of all diastereotopic protons of the cyclopropyl group in 1 (Chart 2.1). The interpretation of the 1 H ROESY NMR spectrum of 1 follows that of [Tp Me2 NbPh(c-C 3 H 5 )(MeC≡CMe)], 2 where the 2butyne stays in the pseudo-symmetry plane of the complex, virtually parallel to the trans 3,5dimethylpyrazolyl ring, a structural characteristic of all [Tp Me2 NbXX'(MeC≡CMe)] complexes. The methyl protons of the 2-butyne ligand that are distal to Tp Me2 are deshielded (δ 2.92), whereas those proximal to Tp Me2 are shielded (δ 2.24). At 283 K, they exchange very slowly in cyclohexane-d 12 (Chart 2.1, Figure 2.3 (a)). In the ROESY spectrum, correlation between the distal methyl protons and H 7 , one of β-hydrogen of the cyclopropyl group, is clearly observed and serves to build the other ROESY correlations (Chart 2.1, Figure 2.3(b)). Diastereotopic protons of the cyclopropyl group in 1.

  H NMR and 2 H{ 1 H} NMR for 4 hours at 303 K in a 600 MHz NMR spectrometer equipped with a cryoprobe. Comparison of 1 H and 2 H{ 1 H} NMR spectra showed the formation of two diastereomers of [Tp Me2 NbCD 3 (c-C 3 H 4 D)(MeCCMe)] 1-d 4 in a 3:1 ratio, displaying, in addition to a Nb-CD 3 group, the D atom either in position 7 or 6 of the cyclopropyl ring, respectively. Interestingly, the βpositions D6 and D7 are on the same enantioface of the cyclopropyl group as the niobium (Figure 2.4). No deuterated solvent (i.e. cyclohexane-d 12 ) was added to the sample to lock the spectrometer, in order to avoid (i) overlap of the cyclopropyl deuterated signals by the deuterated solvent, (ii) saturating the 2 H NMR spectrum or (iii) reaction with adventitious CH/D bonds (ie benzene-d 6 ). Therefore the chemical shifts of the signals in Figure 2.4 had no formal reference and the CD 4 resonance was arbitrarily set at δ -0.

Figure 2 . 4 .

 24 Figure 2.4. Superimposed 1 H and 2 H{ 1 H} NMR spectra for the reaction of 1 with CD 4 to form 1-d 4 .

Scheme 2 .

 2 Scheme 2.7(a) shows the degenerate reaction of 1 with methane, whereas Scheme 2.7(b) shows the elementary steps. The forward reaction (rate constant k 1 ) is the unimolecular methane elimination that yields A and methane as shown previously. The reverse reaction (rate constant k-1 ) is the bimolecular reaction of A with methane that yields 1, the CH bond activation of methane.

Scheme 2 . 7 .

 27 Scheme 2.7. Degenerate reaction of 1 with methane (a) and its elementary steps(b).

1 Scheme 2 . 8 .

 128 Scheme 2.8. The two sites exchange kinetics for the reaction of 1 with methane.

8 )

 8 the saturated spin population is transferred to the methyl group of 1 via chemical exchange, resulting in partial loss of the net magnetization of the methyl group of 1 from its thermal equilibrium value, M 0(NbMe) , observed in the absence of saturation. The exchange between the saturated signal of CH 4 and the methyl group of 1 is therefore characterized by a decrease of the signal of the methyl group of 1. If CH 4 is irradiated in such a way that M sat(CH4) = 0 and that a steady state is reached for M sat(NbMe) (dM sat(NbMe) /dt = 0) , applying these conditions to equation 2.5 gives equation 2.7, from which the value of k 1 can be derived from equation 2.8:Vice versa, when the peak of the methyl group of 1 is saturated by a low power CW irradiation during a time τ sat ≥ 5T 1(NbMe) making the upper and lower spin state populations of the methyl group of 1 equal, that is M sat(NbMe) = [NbMe] -[NbMe*] = 0, the saturated spin population is transferred to methane via chemical exchange, resulting in partial loss of the net magnetization of methane from its thermal equilibrium value, M 0(CH4) , observed in the absence of saturation. Once again, if NbMe is irradiated in such a way that M sat(NbMe) = 0 and that a steady state is reached for M sat(CH4) (dM sat(CH4) /dt = 0) applying these conditions to equation 2.6 gives equation 2.9, from which the value of k 1 can be derived from equation 2.10:

14 (

 14 decomposition and to get quantitative results, we resorted to high pressure HP NMR techniques (400 MHz instrument). 1 (0.100 g, 0.200 mmol) was charged in a 1 cm (outside diameter) sapphire HP NMR tube and dissolved in a 1:1 mixture of C 6 F 6 /cyclohexane-d 12 (2 mL). The HP NMR tube was then pressurized with methane at ca 60 bar at room temperature and carefully shaken to ensure homogeneous methane dissolution. The SST study was carried out in a 400 MHz NMR spectrometer at 351 K with a [CH 4 ]:[1] ratio of 29. Saturation of the NbCH 3 resonance at δ 0.59 was followed by observation of the CH 4 resonance at δ 0.Figure 2.6). Integration of the CH 4 resonance before and after the saturation revealed a 12% decrease which translated to a rate constant k -1obs = k -1 C A = (2.67 ± 0.23) × 10 -2 s -1 , using equation 2.10.

Figure 2 . 6 .

 26 Figure 2.6. Irradiation of the Nb-CH 3 signal and decrease of the CH 4 peak intensity (11%), after Spin Saturation Transfer.

Figure 2 . 7 .

 27 Figure 2.7. Irradiation of the CH 4 (δ 0.18 ppm) signal and barely decrease of the Nb-CH 3 peak intensity after Spin Saturation Transfer.

. 5 -C 6 H 3

 563 Kinetic study of the reaction between [Tp Me2 Nb(CH 2 -3,5-Me 2 )(c-C 3 H 5 )(MeCCMe)] (3) and CH 4

  The Arrhenius equation has the form:The plot of lnk with is linear with the slope representing (Figure2.10) which yields activation energy: E a = 78 ± 5 kJ/mol.

Figure 2 . 10 .

 210 Figure 2.10. Arrhenius plot of the β-H abstraction of mesitylene in 3.

Figure 2 . 11 .

 211 Figure 2.11. Eyring plot of the β-H abstraction of mesitylene in 3.

Figure 2 .Figure 2 . 12 .

 2212 Figure 2.12. Relative Ti-C bond strengths (kcal/mol) in [(silox) 2 ( t Bu 3 SiNH)TiR] versus the C-H bond strength of the corresponding hydrocarbon. D(TiR) rel = D(TiR) -D(TiBz). The line (slope =1.1, r=0.95) is a least-squares fit to all points.

  Etienne carried out a series of DFT calculations performed to explore the origin of the elimination/activation of CH 4 . The computed electronic and free energy surface for the conversion of A + CH 4 → A-CH 4 → 1-TS→1 is shown in Figure2.13.

Figure 2 . 13 . 4 ((

 2134 Figure 2.13. (a) Potential energy surface (wB97XD functional) for the activation of methane by [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)](A) (electronic energy with ZPE, top; free energy, below italicized) (b) A-CH 4 and (c) 1-TS (distances in Å). Color code: Nb, gray, N; blue; C, large white; H, small white. Only atoms of ancillary ligands bound to Nb are shown for clarity.

Figure 2 .

 2 Figure 2.14 presents an analysis of the electron density of 1-TS following the NPA

Figure 2 . 14 .

 214 Figure 2.14. (a) Natural Charges and (b) NLMO for 1-TS.

  (A) was able to cleave a CH bond of methane via a 1,3-CH bond addition pathway under mild conditions. Degenerate reaction of 1 with 13 CH 4 formed the isotopologue product 1-13 CH 3 . In the presence of CD 4 , the formation of two diastereoisomers in a 3:1 ratio indicates the involvement of a stereospecific 1,3-addition process to the transient η 2 -cyclopropene intermediate A. Spin saturation transfer experiments demonstrate that [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] (1) activates the CH bond of methane by a reversible β-H abstraction/1,3addition pathway. Kinetic study on the reaction of [Tp Me2 Nb(Mesityl)(c-C 3 H 5 )(MeCCMe)] (3) with high pressure CH 4 shows that the reaction is 1st order in 3 and zeroth order in CH 4 , indicating that the formation of the transient η 2 -cyclopropene intermediate A is ratedetermining. A very negative activation entropy, finally, suggests an ordered transition state in the β-H abstraction step. The optimized transition state for the methane CH bond elimination/activation is in a four-center σ-bond metathesis mode with a distinctive involvement of both carbons of the η 2 -cyclopropene ligand engaged in the hydrogen transfer.

2. 2 )Chart 2 . 2 .

 222 at room temperature. The assembly is protected by a polymethylmethacrylate safety shield which allows the lowering of the tube into the probe avoiding the direct exposure of the operator to the pressurized tube. The pneumatic air system has to be controlled manually to allow the lower of the tube into the probe safely. The tube valve assembly has a light weight, which allows the tube to spin symmetrically using only a slight increase of the normal spinner nitrogen pressure usually employed. Schematic drawing of the Ti-alloy valve sapphire tube assembly. Numbered details are 1, valve body; 2, handle of the stem drive of the valve; 3, stem drive and packing gland; 4, nonrotating stem; 5, packing assembly; 6, gas inlet port; 7, assembly screw (total of 4); 8, Viton O-ring seal, 9, flange to which the tube is mounted; 10, epoxy sealant; 11, spinner turbine; 12, sapphire tube.

13 CH 4

 134 CH 4 to ca 3 bar at 173 K (ethanol/liquid N 2 bath) for 1 min. The tube was then placed in the 1 H NMR spectrometer with the probe set at 313 K. 1.2 equivalents of13 CH 4 could be quantified by integration of the signal [δ 0.19 (d, 1 J CH = 125.0 Hz)] compared to the Tp Me2 CH signal of complex 1. The reaction was subsequently monitored by 1 HNMR over 19 hours at 313 K.

  NMR tube was charged with compound 1 (0.030 g, 0.06 mmol), C 6 F 6 (0.5 ml), a drop of cyclohexane-d 12 (0.01ml) as the lock solvent and adamantane (7.6 mg, 0.0565 mmol) as internal standard. The tube was then frozen at 213 K, charged with CH 4 (6 bar) for 3 minutes, and sealed. It was then placed in a Bruker Avance 500 NMR spectrometer with the probe set at 341 K. 4 equivalents of CH 4 could be quantified by integration of the CH 4 signal (δ 0.14) compared to the Tp Me2 CH signal of complex 1.High pressure NMR tube reaction: In the glovebox, compound 1 (0.100 g, 0.200 mmol) and adamantane (8 mg, 0.06 mmol) as internal standard were dissolved in a mixture of cyclohexane-d 12 (1 mL) and C 6 F 6 (1 mL) and then transferred into a single-crystal 10 mm sapphire HP NMR tube. The tube was then protected by a polycarbonate casing and equipped with a high-pressure manifold. It was then pressurized with 60 bar of methane at RT and carefully shaken to ensure the dissolution of methane. The tube was then placed in a Bruker Avance 400 NMR spectrometer with the probe set at 351 K. 29 equivalents of CH 4 in solution could be quantified by integration of the CH 4 signal (δ 0.14) compared to the Tp Me2 CH signal of complex 1.

  side signals. A reference experiment was run with exactly the same parameters than the saturation experiment but with a saturation offset frequency set to a part of the NMR spectrum devoid of signals. The reference offset frequency (O ref ) was chosen such that its difference with the frequency of the measured signal (O mes ) was exactly the same as the difference between the measured and the irradiated (O irr ) frequencies: O ref -O mes = O mes -O irr . The saturation and reference experiments were repeated at least five times for error evaluations.

  the spin-lattice relaxation times for NbMe and CH 4 were T 1(NbMe) 1.34 s and T 1(CH4) 4.95 s, respectively. The corresponding T1 curves are shown below (Figure 2.15).

Figure 2 . 15 .

 215 Figure 2.15. Plots of the area of the NMR signal vs. delay time, after the inversion recovery sequence, allowing determination of the spin-lattice relaxation times for CH 4 and NbMe on the 500 MHz spectrometer at 341 K (a) and b) and on the 400 MHz spectrometer at 351 K (c) and d)).

  14) signal, the reference experiment was run by setting the saturation off-set frequency not to δ 1.04 (ca. 0.45 ppm downfield of the NbMe peak at δ 0.59) as usually done for a symmetrical reference, but to δ -0.44 (ca. 1.03 ppm upfield from the NbMe peak at δ 0.59 to avoid irradiation of the β c-C 3 H 5 peak at δ1.15.The irradiation of the CH 4 (δ 0.14) signal gave rise to a poorly reproducible, very small decrease of the NbMe peak intensity (between 0 and 5%), compared to the reference NbMe value. This small variation of the NbMe signal is likely due to the low T 1(NbMe) value. The saturation delays (t sat ) were chosen so that t sat ≥ 5T 1 of the irradiated signal (60 seconds for high CH 4 pressures (400 MHz spectrometer)).

Chapter 3 : CH bond activation of heteraromatic and unsaturated hydrocarbons by a niobium methyl cyclopropyl precursor 3 . 1 -

 331 IntroductionBesides σ-bond metathesis and 1,2-CH bond activation (often following α-H abstraction), 1,3-CH bond activation (often following β-H abstraction) has been reported to be an important mechanism for the activation of C-H bonds of simple hydrocarbons at early transition metals. To date, a number of transient early transition metal complexes with benzyne, alkyne, allene, diene and alkene ligands are competent to cleave the strong and inert C-H bonds of hydrocarbons via this mechanism. Erker first reported that [Cp 2 Zr(η 2 -aryne)], generated by reversible intramolecular β-H abstraction of diaryl zirconocenes, could cleave the CH bond of benzene by 1,3-addition.1 The reactions of other early transition metal η 2benzyne (Mo, 2,3 W, 2 V, 4,5 Nb 6 ) and η 2 -alkyne (W7,8 ) complexes are now well-documented (Scheme 3.1(a)). Early transition metal η 2 -allene and η 2 -diene intermediates generated from alkyl allyl complexes of the type [Cp*M(NO)(R)(η 3 -CH 2 CHCHR)] (M = Mo, W) also activate the CH bond of hydrocarbons via 1,3-addition mechanism (Scheme 3.1(b)

C 3

 3 ring, indeed, changes the properties of the alkene-based orbitals. Parallel computational study shows that the energy for β-H abstraction of methane from [Tp Me2 Nb(Me)(Et)(MeCCMe)] to form [Tp Me2 Nb(η 2 -C 2 H 4 )(MeCCMe)] (A et ) (ΔG = -28 kJ•mol -1 ) is more exergonic than β-H abstraction of methane from 1 to form A (ΔG = -1 kJ•mol -1

Scheme 3 . 4 .

 34 Scheme 3.4. CH bond activation of furan and thiophene by 1.

3. 1 )

 1 . Like complex 1,6 the resonances of the CH and CH 3 protons of the pyrazoles show three singlets, integrating for one proton each, and six singlets, integrating for three protons each, respectively. The methyl protons of the alkyne resonate as singlets at δ 3.16 and 2.27 ppm.The five protons of the cyclopropyl group resonate as five multiplets. Interestingly, the 4-H (δ 6.21 ppm) resonates as a doublet of doublets and couples with both 5-H (δ 7.35 ppm) and 3-H (δ 6.50 ppm) (J H-H = 1.5 Hz and 3.0 Hz), indicating that CH bond activation occurs selectively at the α position (Scheme 3.4). The 3-C at δ 108.2 ppm (ddd, J CH 169, 14, 6 Hz), 4-C at δ 120.5 ppm (dt, J CH 168, 6 Hz) and 5-C at δ 142.3 ppm (ddd, J CH 195, 11, 7 Hz, 5-Cfuryl) give characteristic signals by 13 C NMR. A strongly niobium-broadened C2 resonance appears at δ 210.4 ppm. According to the 2D ROESY spectrum of [Tp Me2 NbPh(c-C 3 H 5 )(MeCCMe)], the protons at the β'-position (Hβ'), on the same enantioface of the cyclopropyl group as the niobium, are more deshielded than those at the β-position (Hβ) on the same face as HD. Based on this assignment, we could distinguish in complex 4a the Hβ and Hβ' in the cyclopropyl group at δ 1.36, 0.76 and δ 2.27, 1.51 ppm, respectively. The α proton HD resonates at δ 2.03 ppm as a triplet of triplets. In the 13 C NMR spectra, the signals of the Cα, Cβ and Cβ carbons of the cyclopropyl group are observed at δ 72.2, 22.3, and 10.3 ppm.

Figure 3 . 1 . 1 H

 311 Figure 3.1. 1 H NMR of 4a in benzene-d 6 at 298 K.

the 1 H

 1 NMR, three thienyl signals at δ 7.57, 7.01, 6.85 ppm correspond to the protons 5-H, 4-H, and 3-H, respectively. The multiplets characteristic of the cyclopropyl group resonate at δ 2.52 (c-C 3 H 5 β'), 2.25 (c-C 3 H 5 α), 1.73(c-C 3 H 5 β'), 1.50 (c-C 3 H 5 β) and 0.96 (c-C 3 H 5 β) ppm.

Figure 3 . 2 . 1 H

 321 Figure 3.2. 1 H NMR of 4b in benzene-d 6 at 298 K.

Figure 3 . 3 .

 33 Figure 3.3. ORTEP plot of the X-Ray structure of complex 4a. Selected bond distances (Å) and angles (°):Nb-C1 2.179(4), Nb-C8 2.214(4), C1-C2 1.513(5), C1-C3 1.528(5), C2-C3 1.467(6), C8-C9 1.343(5), C9-C10 1.438(7), C10-C11 1.319(7), O1-C8 1.417(4), O1-C11 1.355(5), Nb-C8-O1 118.0

  H NMR spectrum showed the three characteristic signals of the phenyl alkynyl ligand at δ 7.47 (o-C 6 H 5 ), 7.02 (m-C 6 H 5 ) and 6.94 (p-C 6 H 5 ) ppm in a 2:2:1 ratio. The cyclopropyl group displays five multiplets at δ 2.25 (c-C 3 H 5 β'), 2.18 (c-C 3 H 5 α), 1.37 (c-C 3 H 5 β'), 1.29 (c-C 3 H 5 β) and 0.71 (c-C 3 H 5 β) ppm (Figure 3.4). In the 13 C spectrum, the signals corresponding to the Cα, Cβ and Cβ carbons of the cyclopropyl group are observed at δ 77.9 ( 1 J CH 146 Hz), 23.2 ( 1 J CH 159, 163 Hz) and 8.4 ( 1 J CH 159, 163 Hz) ppm, respectively. Moreover, the characteristic resonances of the NbCαCPh and NbCCβPh carbon atoms are observed at δ 149.2 and 125.9 ppm, respectively. No sign of aryl activation was seen.

Figure 3 . 4 .

 34 Figure 3.4. 1 H NMR of [Tp Me2 Nb(CCPh)(c-C 3 H 5 )(MeCCMe)] (5) in benzene-d 6 at 298 K.

Figure 3 . 5 .

 35 Figure 3.5. ORTEP plot of the X-Ray structure of complex 5. Selected bond distances (Å) and angles (°): Nb-C1 2.173(4), Nb-C8 2.197(4), C1-C2 1.490(6), C1-C3 1.528(7), C2-C3 1.479(8), C8-C9 1.206(5), Nb1-C8-C9 172.5(3), C8-C9-C10 179.0(4)°

Figure 3 . 6 . 1 H

 361 Figure 3.6. 1 H NMR of [Tp Me2 Nb(c-C 3 H 5 )(c-1-C 5 H 7 )(MeCCMe)] (6) in benzene-d 6 at 298 K.

Figure 3 . 7 .

 37 Figure 3.7. ORTEP plot of the X-Ray structure of complex 6. Selected bond distances (Å) and angles (°): Nb-C1 2.177(2), Nb-C8 2.230(2), C1-C2 1.501(3), C1-C3 1.537(3), C2-C3 1.480(4), C8-C9 1.509(3), C8-C11 1.351(3), Nb1-C8-C11 126.61(16), Nb1-C8-C9 126.02(15)°

Scheme 3 . 6 . 1 Complex 7

 3617 Scheme 3.6. CH bond activation of ferrocene by 1

Figure 3 . 8 . 1 HFigure 3 . 9 .longer

 38139 Figure 3.8. 1 H NMR of 7 in benzene-d 6 at 287 K.

2 .

 2 88 ppm. The resonances for the diastereotopic protons of the cyclopropyl group appear as multiplets at δ 2.72(c-C 3 H 5 α), 2.47(c-C 3 H 5 β'), 1.65(c-C 3 H 5 β'), 1.53 (c-C 3 H 5 β) and 0.24 (c-C 3 H 5 β) ppm (Figure 3.11). Again, the niobium-bound carbon of C 6 F 5 is not observed in the 13 C{ 1 H} NMR spectrum of 8. The Cα, Cβ and Cβ carbons of the cyclopropyl group resonate at 86.6, 24.9, and 2.1 ppm, respectively. One of the niobium-bound alkyne carbon δ 23.2 ppm (d, J CF = 10.7 Hz) and one of the cyclopropyl Cβ at δ 2.1 ppm (d, J CF = 10.2 Hz) shows through-space coupling to one fluorine. Five fluorine signals in the δ -100.2 --163.2 ppm range attributable to the pentafluorophenyl NbC 6 F 5 group are observed in the 19 F{ 1 H} NMR spectrum at room temperature consistent with slow rotation around the Nb-C 6 F 5 bond in 8.

Figure 3 .

 3 Figure 3.10. 1 H NMR of 8 in benzene-d 6 at 243 K.

  activates the C-D bond of C 6 D 6 to form 2-d 6 .6

Scheme 3 . 9 .

 39 Scheme 3.9. Isomerization of 8 to 9a and 9b

  .11), the characteristic Hγ proton (m, δ 6.09 ppm is in a trans-disposition to the Hδ proton (dd, δ 1.05 ppm) The Hδ proton couples with the Hγ proton ( 3 J HH = 12 Hz) but also to the methyl group (δ 2.40 ppm) bound to the same Cδ atom ( 3 J HH = 6 Hz). The CH 3 attached to Cα and Cβ resonate at δ 2.13 and 2.65 ppm, respectively.

Figure 3 .

 3 Figure 3.11. 1 H NMR of 9a in benzene-d 6 at 298 K

F

  NMR spectrum of 9a in benzene-d 6 shows the characteristic signals of the C 6 F 5 ring: two ortho-fluorines at δ -115.7 (dt) and -109.5 (br d) ppm, two meta-fluorines at δ -163.0 (m) and -164.1 (m) ppm and one para-fluorine at δ -158.3 (t) ppm (Figure 3.12). The minor peaks belong to the fluorine signals of the C 6 F 5 ring of 9b: two ortho-fluorines at δ -104.7 (dt) and -115.7 (dt) ppm, two meta-fluorines at δ -163.4 (m) and -164.3 (m) ppm and one para-fluorine at δ -158.4 (t) ppm.

Figure 3 .Figure 3 .

 33 Figure3.12.19 F NMR of a mixture of 9a and 9b (molar ratio 88:12) in benzene-d 6 at 298 K.

HOESY 1 H- 19 F 19 F

 11919 HOESY spectra. Given the low concentration of 9b, the gated 13 C{19 F} NMR spectrum did not yield reliable 1 J CH but all the carbons of the niobiacycle were observed.

Figure 3 .

 3 Figure 3.14. 1 H-19 F HOESY of 9a in benzene-d 6 at 298 K

Figure 3 . 15 .

 315 Figure 3.15. ORTEP plot of the X-Ray structure of complex 9a,b. Selected bond distances (Å) and angles (°): Nb-C1 2.002(5), C1-C2 1.412(6), C1-C5 1.472(6), Nb-C2 2.348(4), C2-C3

Scheme 3 . 11 .

 311 Scheme 3.11. Limiting structures for 9a,b

Figure 3 . 16 . 1 - 1

 31611 Figure 3.16. Room-temperature cyclic voltammogram of [Tp Me2 Nb(C 10 H 9 Fe)(c-C 3 H 5 )-MeCCMe)](7)(1×10 -3 M) with ferrocene as reference (THF, [n-Bu 4 N][PF 6 ], Pt electrode, scan rate 200 mV s -1

67 98 a

 98 In THF containing 0.1M [n-Bu 4 N]PF 6 , at a Pt-bead working electrode, except 7 with 0.08 M [n-Bu 4 N]PF 6 . The potential have been measured vs Fc/Fc + . b Defined as the average of the cathodic and anodic peak potentials (±0.02 V). c Defined as the separation of the cathodic and anodic peak potentials. d Ratio between the anodic and cathodic peak current.

[

  Tp Me2 NbR(c-C 3 H 5 )] with the alkyne. The fragment [Tp Me2 NbR(c-C 3 H 5 )] has no π-acceptor ligands and display d xz , d yz , d x2-y2 and d z2 type frontier molecular orbitals, according to an increasing energy order. The remaining d xy orbital forms strong σ-bond interactions with two pyrazolyl nitrogens, cyclopropyl and the R orbital and is consequently not shown. The d z2 orbital, involving some niobium p z character, points toward the alkyne group and displays the correct symmetry to interact with the alkyne-based π ║ orbital. The other occupied alkynebased π ┴ orbital interacts with the d yz orbital, affording a linear combination slightly higher in energy than that with the d z2 orbital. A further π-interaction occurs between the antibonding alkyne-based π ║ * orbital and the d xz orbital, providing as a linear combination the HOMO of the molecular orbital diagram. The LUMO of the system is the d x2-y2 orbital which is essentially a nonbonding orbital. The more S-accepting character of the R group, the lower the energy of the LUMO orbital and therefore the easier the reduction of the Nb metal center.

Figure 3 . 17 .

 317 Figure 3.17. Molecular orbital diagram for the interaction between the [TpNbCl 2 ] fragment and the alkyne.

3. 8 -Discussion 3 . 8 . 1 -

 8381 Relative Nb-carbon bond strengths The chemical selectivity in the formation of metal-carbon bonds is governed by kinetic and thermodynamic factors. The greater the metal-carbon bond strengths, the more readily can this bond be formed kinetically and/or thermodynamically. Marks et al. used calorimetry to measure various Zr-C and Th-C bond strengths, showing linear correlations between D M-C and D C-H (BDE). 38 Wolczanski et al. used exchange reactions to calculate relative Ti-C bond strengths for a series of [( t Bu 3 SiNH) 3 Ti(R)] complexes, formed by the 1,2-addition of RH to the [( t Bu 3 SiNH) 2 Ti=NSi t Bu 3 ] compound, reporting again a linear correlation between D Ti-C and D C-H (BDE).39 Jones et al. used the same method to calculate relative Rh-C bond strength for the systems [Tp Me2 Rh(L)(R)(H)] (L = CNneopentyl, PMe 3 , P(OMe) 3 ), formed by oxidative addition of RH to the unsaturated Tp Me2 Rh(L) complexes.

C 3 H 5 )Scheme 3 . 14 .

 35314 Scheme 3.14. Thermodynamic exchange reactions of [Tp Me2 Nb(R)(c-C 3 H 5 )(MeC≡CMe)]

  not react with benzene to give the exchange products [Tp Me2 Nb(c-C 3 H 5 )Ph(MeCCMe)] (2) with the corresponding hydrocarbons (RH), indicating that i) they are thermodynamicallymore stable than [Tp Me2 Nb(Ph)(c-C 3 H 5 )(MeC≡CMe)] (2) and ii) the Nb-R bond strengths of 4a, 4b, 5 and 8 are bigger than that of the Nb-Ph bond in [Tp Me2 Nb(Ph)(c-C 3 H 5 )(MeC≡CMe)] (2). However, [Tp Me2 Nb(c-C 3 H 5 )(2-C 5 H 7 )(MeCCMe)] (6) reacted with benzene at 318 K to form [Tp Me2 Nb(Ph)(c-C 3 H 5 )(MeC≡CMe)] and cyclopentene. The complex [Tp Me2 Nb(c-C 3 H 5 )(Fc)(MeCCMe)] (7) also reacted with benzene-d 6 to form [Tp Me2 Nb(C 6 D 5 )(c-C 3 H 5 )(MeC≡CMe)] and FcH even at 287 K. Therefore, the formation of the [Tp Me2 Nb(R)(c-C 3 H 5 )(MeC≡CMe)] species follows the following trend of thermodynamic stability

C 3 H 5 )

 35 (Fc)(MeCCMe)] > [Tp Me2 Nb(c-C 3 H 5 )-(Me)(MeCCMe)]. This translates to the following Nb-R bond strengths: Nb-alkynyl > Nb-heteroaromatic > Nb-phenyl > Nb-vinyl > Nb-Fc > Nb-Me . In accordance with the literature discussed above, if we plot the Nb-R bond strengths vs. the C-H bond strengths we expect an approximate linear correlation displaying the following hydrocarbon activation trend: phenylacetylene > furan ≈ thiophene ≈pentafluorobenzene > benzene > cyclopentene > methane (Scheme 3.14). In addition we are able to predict that the C-H bond of FcH should be stronger than that of methane and equal or weaker than that of benzene. Therefore, it can be concluded that the higher the BDE of the hydrocarbon CH bond , the more favorable its activation. Moreover, since the C-H bond activation of an hydrocarbon by the K 2 -cyclopropene Nb intermediate involves a proton transfer in a polar four center transition state , it can be predicted that the C-H bond of hydrocarbons with lower pK a values are easier to be cleaved by the K 2 -cyclopropene Nb intermediate. According to the pK a value reported in Scheme 3.15,[45][46][47][48][49] the C-H bond activation of the corresponding hydrocarbons follows the following thermodynamic preference order: phenylacetylene > furan ≈ thiophene ≈ pentafluorobenzene > benzene > cyclopentene > methane. The C-H bond activation of ferrocene should be thermodynamically more favorable than that of benzene according to their pK a value (Scheme 3.15).[45][46][47] However, the Nb-Fc bond reacts with benzene to form Nb-Ph and FcH, indicating that the Nb-Ph bond is stronger than the Nb-Fc one. A possible reason might be related to the steric congestion around the Nb center in the presence of the 2-butyne, cyclopropyl and the large Fc group. It is this steric hindrance around the metal which may drive the Nb-Fc to react with benzene to form a less steric demanding Nb-Ph bond. The regioselectivity for the CH bond activation of electron-deficient arenes is also controlled by the pK a values when polar transition states are involved. Furan and thiophene are activated at 2-position rather than 3position because the pK a values of the CH bond at 2-positions are smaller than those at 3positions (Scheme 3.15).

Figure 3 .

 3 19-21 show the The cyclic voltammogram (CV) of 5, 6, and 1.

Figure 3 . 19 .Figure 3 . 20 .

 319320 Figure 3.19. Room-temperature cyclic voltammogram of [Tp Me2 Nb(c-C 3 H 5 )(CCPh) (MeCCMe)] (5) (1×10 -3 M) in THF containing 0.1 M [n-Bu 4 N]PF 6 (platinum-bead electrode scan rate of 200 mV s -1 )

Figure 3 . 21 .

 321 Figure 3.21. Room-temperature cyclic voltammogram of [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] (1) (1×10 -3 M) in THF containing 0.1 M [n-Bu 4 N][PF 6 ](platinum-bead electrode, scan rate of 200 mV s -1 )

F 2 Final

 2 R indices [I>2s(I)] R1 = 0.0457, wR2 = 0.1066 R indices (all data) R1 = 0.0701, wR2 = 0.1186 G.O.F (S) 1.071 Nb of the reflections used 5980 (I>2s(I)]) Nb of the parameters used 351 'Umin/'Umax(e. Å -3 ) 1.296/ -0.953

Figure 3 .

 3 Figure 3.23.13 C NMR of 4b in benzene-d 6 at 298 K

5 ( 8 )

 58 2 -cyclopropene complex[Tp Me2 Nb(K 2 -c-C 3 H 4 )(MeCCMe)] (A), generated by an intramolecular abstraction of a E-H of the cyclopropyl group to form methane from [Tp Me2 NbCH 3 (c-C 3 H 5 )(MeCCMe)] (1), could activate the CH bond of methane via a 1,3-addition mechanism under mild conditions. Degenerate reaction of 1 with13 CH 4 formed the isotopologue product 1-13 CH 3 . The fact that the degenerate reaction of 1 with CD 4 yields two diastereoisomers in a 3:1 ratio conclusively indicates that two elementary steps are involved in the activation of methane: (i) the generation of the transient K 2 -cyclopropene intermediate A and, (ii) the activation of the CH bond of methane by A in a stereospecific 1,3-addition process. Spin saturation transfer experiments demonstrate that 1 activates the CH bond of methane by E-H abstraction/1,3addition pathway, a dissociative mechanism. Kinetic study on the reaction of [Tp Me2 Nb(Mesityl)(c-C 3 H 5 )(MeCCMe)] (3) with CH 4 shows that the reaction is 1st order in 3 and zeroth order in CH 4 , indicating that the formation of the transient K 2 -cyclopropene intermediate A is rate-determining. The E-H abstraction step has a very negative activation entropy due to a highly ordered transition state. The rate constants for the CH bond activation of methane by the E-H abstraction/1,3-addition mechanism have the same order of magnitude as those for D-H abstraction/1,2-addition processes in related complexes. DFT studies reveal that the intermediate A forms labile V-adduct complex with methane (V-CH 4 ) via a loose van der Waals interaction before the activation of the CH bond of methane. The optimized transition state for the methane CH bond elimination/activation is in a four-center V-bond metathesis mode with a distinctive involvement of both carbons of the K 2 -cyclopropene ligand engaged in the hydrogen transfer.In the second part of this manuscript, we have shown that the CH bond of heteroaromatics, unsaturated hydrocarbons, pentafluorobenzene and ferrocene (FcH) could be activated by A via the same 1,3-CH bond addition mechanism to form [Tp Me2 NbX(c-C 3 H 5 )(MeCCMe)] [X =2-C 4 H 3 O (4a), 2-C 4 H 3 S (4b), PhC≡C (5), 1-C 5 H 7(6) , Fc(7) and C 6 F ] under mild conditions. Electrochemical studies of complexes 1, 5, 6, and 7 shows oneelectron reversible reduction of the niobium center. The reducibility of complexes follows the order, 1 (E 1/2 = -2.85 V) < 6 (E 1/2 = -2.75 V) < 7 (E 1/2 = -2.67 V) < 5 (E 1/2 = -2.41 V), which reflects the LUMO energy of the complexes should in the order 1 > 6 > 7 > 5 influenced by the hybridization of the niobium-bound carbon sp 3 (Me) < sp 2 (c-C 5 H 7 ) < sp 2 (Fc) < sp (CCPh). The exchange reactions between 4a, 5, 6, 7 with benzene indicate that the regioselectivity issues are influenced by the thermodynamic preferences for the CH bond activation of hydrocarbons, which is determined by the intrinsic Nb-C bond strengths of the complexes. Besides, the regioselectivity is also affected by the pK a values of the C-H bond, and steric effects of both the complexes and the hydrocarbons.A first perspective of this work could be focused on synthesizing other cyclopropyl organometallic precursors of the general formula L n MR(c-C 3 H 5 ), (L n = monoanionic multidentate ligand; R = alkyl group), that would form L n M(K 2 -c-C 3 H 4 ) intermediates able to activate the strong CH bond of CH 4 and other hydrocarbons. The metals would be those of groups 3 to 6. Various ligands L n (Cp, Tp, etc.) forcing cis-arrangement needed for RH elimination would provide scaffolds whose steric and electronic properties can be tuned conveniently. In connection, a second perspective would be to achieve a full understanding of the whole E-H abstraction/1,3-CH bond addition mechanism in which the C-H bond of methane is cleaved by these L n M(K 2 -c-C 3 H 4 ) intermediates by isotopic labelling using CD 4 and 13 CH 4 , kinetic studies including SST and numerical simulation of concentration profiles.

Schéma 1 . 3 .Schéma 1 . 4 .

 1314 Addition oxydante Le mécanisme le plus commun pour l'addition oxydante d'une liaison C-H d'un alcane est l'addition concertée à 3-centre dans laquelle les équivalents de réduction du métal doivent être suffisamment hauts en énergie pour occuper l'orbitale V du C-H et former deux nouvelles liaisons (M-C et M-H). D'après l'équation 1.2, par conséquent, l'augmentation de deux unités de l'état d'oxydation du métal est accompagnée par la formation de deux nouvelles liaisons métal-hydrure et métal-méthyle. Dans les années 1980, Janowicz et Bergman et Graham ont découvert que des espèces transitoires de cyclopentadienyliridium (I), riches en électrons et à bas degré d'oxydation, pouvaient activer la liaison C-H du méthane (Schéma 1.3 et Schéma 1.4). Addition oxydante du méthane par un complexe insaturé d'Ir(I) photo généré. Addition oxydante du méthane par un complexe insaturé d'Ir(I) thermo généré. Les mêmes auteurs ont découvert 10 ans plus tard qu'un complexe cationique d'iridium (III), [Cp*Ir(PMe 3 )(CH 3 )(CH 2 Cl 2 )][BArF 4 ] est encore plus réactifs vis-à-vis du méthane. Hall solvant.

Scheme 1 . 7 .

 17 Scheme 1.7. Fonctionnalisation catalytique du méthane par insertion d'un carbene.

1. 2 -

 2 Activation de la liaison CH du méthane par des complexes de la gauche du tableau périodique Des complexes des métaux de transition de la gauche du tableau périodique (et de métaux électropositifs connexes tels que des lanthanides ou des actinides) sont également capables d'activer la liaison C-H du méthane (et plus généralement des hydrocarbures). Dans ce cas, les voies mécanistiques concernées sont la métathèse de liaison σ, l' addition 1,2 (souvent à la suite d'une abstraction d'un H en α) et l'addition 1,3 (souvent à la suite d'une abstraction d'un H en β).

Scheme 1 . 8 .

 18 Scheme 1.8. Réactions dégénérées entre le complexe [Cp 2 Lu(CH 3 )] et le 13 CH 4 /CD 4 .

Figure 1 . 1 .

 11 Figure 1.1. Spectre RMN du 1 H pour la réaction du [Cp* 2 Lu(CH 3 )] avec 13 CH 4 en fonction du temps à 70 °C.

L

  'addition d'une liaison C-H dans une liaison multiple métal-ligand a été désignée comme une addition 1,2 d'une liaison C-H. Bien que ce processus ressemble à la métathèse des liaisons σ, car elle implique un état de transition à 4-centres, le résultat est sensiblement différente. L'addition 1,2 d'une liaison C-H, en effet, implique la formation d'un intermédiaire à haute énergie, contenant une liaison multiple métal-ligand, qui exécute l'étape de clivage de la liaison C-H. Un système remarquable basée sur des métaux du groupe 4, 5 et 6 a été développé par Wolczanski et ses collègues. Le premier exemple décrivant l'activation du méthane (parmi beaucoup d'hydrocarbures) a été décrit avec des complexes alkyles tris (amido) de zirconium du type [( t Bu 3 SiNH) 3 ZrR]. Avec R = Cy, le cyclohexane est éliminé audessus de 80 °C par une abstraction intramoléculaire CH en α pour donner le complexe transitoire diamidoimido [( t Bu 3 SiNH) 2 Zr=NSi t Bu 3 ] non observé, (Schéma 1.11).

CH 1, 3 Schéma 1 . 13 .Schéma 1 . 14 .Schéma 1 . 15 .

 3113114115 Par rapport aux nombreux exemples d'abstraction CH réversible en position α / d'addition CH en position 1,2, il existe peu d'exemples d'abstraction CH réversible en position β / d'addition CH en position 1,3, à partir de complexes de dialkyle. Un seul exemple a été décrit pour l'activation du méthane. Erkerhas en premier a démontré qu'un intermédiaire relativement stable de [Cp 2 Zr(η 2 -benzyne)] (le benzyne agit comme une bonne base π), généré par une abstraction intramoléculaire réversible d'un H en position β des diarylzirconocenes, est capable de cliver la liaison CH du benzène par une addition 1,3. Dans la plupart des cas, cependant, le complexe η 2_ alcène, à la suite d'une perte rapide d'alcène, n'est souvent pas assez stable pour permettre l'activation de la liaison CH dans l'étape suivante (Schéma 1.13). Abstraction d'un H β et élimination d'un H β dans un complexe alkyle. Le seul cas d'activation du méthane suivant cette voie d'addition CH en position 1,3 a été réalisé sur un centre de tungstène dans le groupe de Legzdins (Schéma 1.14). Le complexe insaturé (non observé mais piégé) [Cp*W(NO)(η 2 -trans-1,3-butadiene)] est généré via une élimination du CMe 4 à partir du précurseur néopentyl-allyle [Cp*W(NO)(CH 2 CMe 3 )(η 3 -CH 2 CHCHMe)]. Ce complexe peut cliver la liaison C-H du méthane (70 bars), de l'éthane (27 bars) et du n-pentane à température ambiante pour donner les complexes alkyl-allyle correspondants. Activation de la liaison CH du méthane et du pentane-d 12 par l'intermédiaire [Cp*W(NO)(η 2 -butadiene)]. Résumé 157 Notre groupe a également proposé une voie similaire d'addition en position 1,3-pour l'activation de la liaison CH du benzène et de différents alkylaromatiques. Cela implique un intermédiaire transitoire de niobium η 2 -cyclopropène, [Tp Me2 Nb(c-C 3 H 4 )(MeCCMe)] (A), généré par une abstraction d'un H en position β à partir du complexe méthyle cyclopropyle de Nb [Tp Me2 NbCH 3 (c-C 3 H 5 )(MeCCMe)] (1) (Schéma 1.15). La discussion de cette étude ainsi que nos efforts pour activer la liaison CH du méthane par cette voie et cet intermédiaire seront discutés dans le prochain chapitre. Activation de la liaison CH du benzène et de différents alkylaromatiques par l'intermédiaire [Tp Me2 Nb(c-C 3 H 4 )(MeCCMe)] (A).

  Scientifiquement, la fonctionnalisation du méthane reste un défi majeur en raison de la nécessité de combiner deux étapes indépendantes en une seule réaction: i) le clivage de la liaison CH et ii) la fonctionnalisation du méthane activé. Parmi toutes les voies examinées dans le présent chapitre, trois seulement ont conduit à la fonctionnalisation catalytique du méthane: i) les routes électrophiles, impliquant des complexes électrophiles homogènes basées principalement sur du Hg, Pt, Pd et V; ii) l'insertion d'un carbène, impliquant des complexes scorpionate perfluorés d'argent très pauvres en électrons et iii) la métathèse des liaisons σ dans des complexes métallocène-alkyles de scandium, par laquelle la hydrométhylation catalytique du propène et la déshydrogénation du diphénylsilane avec du méthane ont été développées. Les autres voies, d'autre part, à savoir i) l'addition oxydante et l'addition radicalaire (idéalement sur des métaux de transition à faible valence), ii) l'addition de la liaison CH en position 1,2-sur une liaison multiple M = X (M = complexes des métaux de transition à haute valence, X = C, NR, etc) et iii) l'addition d'une liaison CH en position 1,3 sur un système M-η 2 -alcène (M = complexes de métaux de transition à haute valence) n'a jamais été incorporé dans une fonctionnalisation catalytique du méthane. Bien que ces procédés ne fournissent pas une fonctionnalisation catalytique, ils réalisent le clivage de la liaison CH du méthane, une étape importante vers la conception des réactions catalytiques. Dans les chapitres suivants, nous nous concentrerons donc sur le clivage de la liaison CH du méthane et de différentes molécules insaturées par un intermédiaire η 2 -cyclopropène de niobium en utilisant une méthodologie basée sur la synergie entre la synthèse, la caractérisation et études mécanistiques.

Chapitre 2 :Schéma 2 . 1 .

 221 Activation de la liaison CH du méthane par un complexe transitoire cyclopropeneη 2 de niobium 2.1-Introduction Des études réalisées auparavant dans notre équipe indiquent que le complexe cyclopropyle de niobium [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] (1) est capable d'activer la liaison CH du benzène par l'abstraction d'un H en β suivi par une addition en position 1,3 (Schéma 2.1). Le complexe 1 réagit avec du benzène à température ambiante (t 1/2 ca. 7 h) pour donner le composé [Tp Me2 NbPh(c-C 3 H 5 )(MeCCMe)] (2) et du méthane. Des études cinétiques ont montré que la réaction de conversion de 1 en 2 est du premier ordre par rapport au composé 1 et d'ordre zéro par rapport au benzène avec une constante de vitesse k obs de 2.93 ± 0.05 × 10 -5 s -1 à 303 K. Les études cinétiques effectués à différentes températures (303-323 K) conduisent aux paramètres d'activation ΔH † = 99 ± 5 kJ/mol and ΔS † =-6 ± 10 J/Kmol. La réaction de 1 avec C 6 D 6 au lieu de C 6 H 6 montre un effet isotopique négligeable avec un rapport k H /k D de 1.0 à 303 K. Ces données confirment que l'étape cinétiquement déterminante est l'abstraction intramoléculaire d'un H β du groupe cyclopropyle pour former du méthane, ce qui donne l'intermédiaire transitoire insaturé cyclopropène-η 2 /metallacyclobutane [Tp Me2 Nb(η 2 -c-C 3 H 4 )-(MeCCMe)] A. Ceci est suivi par l'addition de la liaison CH du benzène en position 1,3 pour donner le composé 2. Des études DFT confirment que la formation de A passe par un état de transition à quatre centres (1-TS) présentant une disposition presque linéaire du système C... H ... C (angles CHC > 166°) avec l'hydrogène positionné symétriquement entre les deux carbones (Schéma 2.1). Ce réarrangement structural est très similaire à ceux décrits par Legzdins et rappelle le processus de métathèse des liaisons σ. Activation de la liaison CH du benzène par un complexe transitoire cyclopropene-η 2 de niobium [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)] (A). Des modélisations cinétiques effectuées par simulation numérique montrent que les réactions d'activation 1,3 de la liaison CH du méthane et du benzène par l'intermédiaire A sont des réactions biomoléculaires, et donc du premier ordre par rapport à A et du premier ordre par rapport au méthane et au benzène, respectivement. A 323 K, la constante de vitesse (k 2 ) pour la réaction de A avec le benzène est environ 3 fois plus importante que celle pour la réaction de A avec le méthane (k -1 ) (Schéma 2.2). Par conséquent, A réagit ca 3 fois plus vite avec le benzène qu'avec le méthane lui-même à 323 K.

  C, la réaction du composé 1 avec le C 6 D 6 donne deux diastéréoisomères avec l'insertion sélective d'un seul deutérium dans un des deux carbones β sur la même enantioface du cycle cyclopropyle que le niobium, selon un rapport de ca 2:1. Ceci établit de façon claire que l'intermédiaire A est un complexe cyclopropène-η 2 insaturé, obtenu à partir de 1 par une abstraction d'un H β pour former du CH 4 . La liaison CH/CD du C 6 H 6 /C 6 D 6 s'additionne successivement sur la liaison Nb-C de A selon une disposition 1,3 stéréospécifique (Schéma 2.3) pour donner deux diastéréoisomères dans un rapport de 2:1.

Figure 2 . 1 . 3 (Schéma 2 . 5 .

 21325 Figure 2.1. Spectre RMN du 1 H pour la réaction de 1 avec le 13 CH 4 pour former 1-13 CH 3 .

  Pour de plus longues durées de réaction, toutefois, une décomposition de 1 et de 1-13 CH 3 a été observée à partir de la diminution des signaux du Nb 12 CH 3 et Nb 13 CH 3 . La pression de 13 CH 4 n'est pas suffisamment élevée pour empêcher la décomposition de l'intermédiaire A, ce qui empêche l'analyse cinétique de la réaction.

2. 3 . 2 - 4 2. 3 . 2 . 1 -

 324321 Réaction dégénérée de 1 avec le CD Caractérisation du [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] par RMN ROESY du1 H Afin d'évaluer le résultat stéréochimique de la formation de 1-d 4 (Schéma 2.4), nous avons réalisé une expérience RMN ROESY du 1 H du composé 1, permettant l'attribution de tous les protons diastéréotopiques du groupe cyclopropyle dans ce complexe 1 (Tableau 2.1). L'interprétation du spectre RMN ROESY du 1 H de 1 suit celle de [Tp Me2 NbPh(c-C 3 H 5 )(MeC≡CMe)], préalablement décrite. Les protons méthyliques du ligand 2-butyne qui sont lointain du groupe Tp Me2 sont déblindés (δ 2.92), alors que ceux qui sont proches du groupe Tp Me2 sont blindés (δ 2.24). À 283 K, ils échangent très lentement dans le cyclohexane-d 12 (Tableau 2.1, Figure 2.2 (a)). Dans le spectre ROESY, on observe clairement une corrélation entre les protons méthyliques lointains et H7, un des hydrogènes β du groupe cyclopropyle. Ceci sert à construire les autres corrélations ROESY (Tableau 2.1, Figure 2.2 (b)).

Tableau 2 . 1 .

 21 Protons diastéréotopiques du groupe cyclopropyle dans le compose 1.

Figure 2 . 2 (

 22 Figure 2.2(a). Spectre RMN ROESY du composé 1, montrant la corrélation ROESY pour les protons méthyliques du ligand 2-butyne.

Figure 2 .

 2 Figure 2.2(b).Spectre RMN ROESY du 1 H du composé 1 (expansion).

  4 jusqu'à environ 8 bar à 157 K pendant 3 minutes. Le tube a ensuite été placé dans le spectromètre RMN équipé d'une sonde cryogénique fixée à 303 K. Le mélange réactionnel a ensuite été étudié par RMN du 1 H et du 2 H{ 1 H} pendant 4 heures à 303 K. La comparaison des spectres du 1 H et du 2 H{ 1 H} a montré la formation de deux diastéréoisomères de [Tp Me2 NbCD 3 (c-C 3 H 4 D)(MeCCMe)] 1-d 4 dans un rapport de 3:1. En plus d'un groupe Nb-CD 3 , la présence d'un atome de D soit en position 7 soit en position 6 du cycle cyclopropyle a été observée, respectivement. Fait intéressant, les positions β D6 et D7 sont sur la même enantioface du groupe cyclopropyle que le niobium (Figure 2.3). Ces résultats confirment que A est générée par une étape cinétiquement déterminante à voir l'abstraction intramoléculaire d'un H β du groupe cyclopropyle par le groupe méthyle lié au niobium pour former du méthane à partir de 1. Ce processus est suivie par son inverse microscopique, à voir l'addition stéréospécifique en position 1,3 de la liaison CH/D du CH 4 /CD 4 sur la liaison Nb-C du composé Nb(η 2 -c-C 3 H 4 ) (Figure 2.3).

Figure 2 . 3 .

 23 Figure 2.3. Spectres RMN du 1 H et du 2 H{ 1 H} superposés pour la réaction de 1 avec le CD 4 pour former 1-d 4 .

T 1 [Schéma 2 . 7 .

 127 NbMe] et T 1[CH4] sont les temps de relaxation de spin pour le groupe méthyle de 1 et pour le méthane, respectivement. k 1 est la constante de vitesse du premier ordre pour la transformation de 1 en A et du méthane, en supposant un mécanisme dissociatif. k -1 est la constante de vitesse du second ordre pour la réaction bimoléculaire inverse. Cinétiques d'échange des deux sites pour la réaction de 1 avec le methane. Si CH 4 est irradié dans une telle manière que M sat(CH4) = 0 et que l'état d'équilibre est atteint pour M sat(NbMe) (dM sat(NbMe) /dt = 0), alors la constante de vitesse k 1 est donné par l'équation 2.1: (2.1) Une fois de plus, si NbMe est irradié dans une telle manière que M sat(NbMe) = 0 et que l'état d'équilibre est atteint pour M sat(CH4) (dM sat(CH4) /dt = 0), alors la constante de vitesse k -1obs , est donnée par l'équation 2

  Afin de favoriser la réaction bimoléculaire du méthane avec A par rapport à sa décomposition et afin d'obtenir des résultats quantitatifs, nous avons recouru à des techniques RMN à haute pression (HP) sur un spectromètre de 400 MHz. 1 (0,100 g, 0,200 mmol) a été chargé dans un tube RMN en saphir d'1 cm (diamètre extérieur) et dissous dans un mélange 1:1 de C 6 F 6 /cyclohexane-d 12 (2 mL). Le tube RMN à HP a été ensuite mis sous pression avec du méthane à environ 60 bar à température ambiante et agité soigneusement pour assurer une dissolution homogène du méthane. L'étude SST a été effectuée dans un spectromètre RMN de 400 MHz à 351 K avec un rapport [CH 4 ]:[1] de 29. La saturation de la résonance du NbCH 3 à δ 0.59 a été suivie par l'observation de la résonance du CH 4 à δ 0.14 (Figure 2.4). L'intégration de la résonance du NbCH 3 avant et après la saturation a révélé une diminution de 12%, ce qui donne une constante de vitesse k -1obs = k -1 C A = (2.67± 0.23) × 10 -2 s -1 , en utilisant l'équation 2.1.

Figure 2 . 4 .

 24 Figure 2.4. Irradiation du signal du Nb-CH 3 et diminution de l'intensité du signal du CH 4 (11%), après le transfert de saturation de spin.

Figure 2 . 5 . 2 . 5 -

 2525 Figure 2.5. Irradiation du signal du CH 4 (δ 0.18 ppm) et très faible diminution de l'intensité du signal du Nb-CH 3 après le transfert de saturation de spin.

Figure 2 . 6 .Figure 2 . 7 .

 2627 Figure 2.6. Spectres RMN superposés pour la réaction du composé 3 avec le méthane pour former 1 dans du cyclohexane-d 12 (1 mL) et du perfluorobenzene (1 mL) à 314 K.

  et d'Eyring. La relation entre le lnk et est linéaire, avec la pente représentant (Figure 2.8), qui permet d'obtenir l'énergie d'activation: E a = 78± 5 kJ/mol.

Figure 2 . 8 .Figure 2 . 9 .

 2829 Figure 2.8. Corrélation d'Arrhenius pour l'abstraction du mesitylene dans 3.

  Table of contents .

  CH 4 , it was shown that this extremely electrophilic

	Scheme 1.7. C-H bond activation of 13 CH 4 by [Cp*Ir(PMe 3 )(CH 3 )(CH 2 Cl 2 )][BArF 4 ]
		Jones and Maguire have taken advantage of the Re(III)/(V) couple, showing that
	[CpRe(PPh 3 ) 2 H 2 ] could catalyze the H/D exchange between C 6 D 6 and CH 4 , with 68 turnovers
	over 3 h, representing one of the few catalytic methane-activating systems based on this
	mechanism (Scheme 1.8).
	Ir(III)	cation	could	activate	methane	even	at	10°C,	yielding
	[Cp*Ir(PMe 3 )( 13 CH 3 )(CH 2 Cl 2 )][BArF 4 ] (Scheme 1.7). Hall et al. has computed an oxidative
	addition-reductive elimination pathway involving an Ir(V) intermediate. 29

30 Scheme 1.8. Mechanism for the H/D exchange between CH 4 and C 6 D 6 , catalyzed by a cyclopentadienylrhenium(I) complex. Other low valent complexes can also promote oxidative addition of methane. The complex [(dmpe) 2 FeH 2 ] is the only first-row transition metal complex shown to cleave a CH bond of methane by oxidative addition. At -100°C, the transient [(dmpe) 2 Fe] species, generated photolytically, activates methane (in liquid xenon) to form [cis-(dmpe) 2 Fe(CH 3 )H].

31

Another group 8 complex, [(PMe 3 ) 4 Os(CH 2 tBu)H], eliminates neopentane at elevated temperatures and pressures to generate [Os(PMe 3 ) 4 ] which adds methane in poor yields (16%).

32

  Mn-CH 4 (39 kJ.mol -1 ), and Re-CH 4 (51 kJ.mol -1 ) in [W(CO) 5 (CH 4 )],

4 , in particular, transient methane adducts such as [W(CO) 5 (CH 4 )], [CpM(CO) 2 (CH 4 )] (M = Mn, Re), and [Cp*Re(CO) 2 (CH 4 )] could be generated and the CH 4 binding enthalpies to the metal center have been obtained: W-CH 4 (30 kJ.mol -1 ),

  CO, PF 3 ; alkane = n-C 5 H 12 , n-C 7 H 16 , c-C 5 H 10 , c-C 6 H 12 , i-C 4 H 10 ), 52 (HEB)W(CO) 2 (pentane), 53 CpRe(CO) 2 (alkane) (alkane = c-C 5 H 12 , 2,2-

	dimethyl-butane)	and	Cp ≠ Re(CO) 2 (alkane)	(alkane	=	c-C 5 H 10 ,	n-C 7 H 16 ), 54
	CpMn(CO) 2 (alkane) 55 (alkane = C 2 H 6 and i-C 5 H 12 ) have been generated by low-temperature
	photodissociation experiments in pure alkane solvents. Although these thermally unstable
	species are produced in modest yields, the σ-alkane complexes are well characterized by low
	temperature 1 H and 13 C NMR spectroscopy. However, this method is not applicable to
	methane due to its very low boiling point. The only full NMR characterization of a transition
	metal σ-methane complex in solution has been reported by Bernskoetter, Brookhart and
	coworkers in a rhodium(I) compound. 56 Protonation of the rhodium(I) methyl complex,
	[(PONOP)Rh(CH 3 )] with H[BAr F 4 ](Et 2 O) 2 in CDFCl 2 solvent at -110°C yields the rhodium(I)
	σ-methane complex, [(PONOP)Rh(CH 4 )][BAr F 4 ] (Scheme 1.14), which was characterized by
	a series of low-temperature 1 H, 2 H,

3 (CH 2 D) scrambling, but it has been observed for the D(H)/C 6 D 2 H 3 (C 6 H 2 D 3 ) scrambling in the coordinatively unsaturated fragment [Cp*Rh(PMe 3 )]. 51 NMR experiments have also been used to characterize σ-alkane complexes. The alkane complexes [Cp'ReL(Cp)(alkane)] (L =

at 23°C and 5 at 80 °C), consistent with methane CH bond cleavage being the rate- determining

  step. Wayland et al. also reported that complexes containing porphyrin ligands joined by a m-xylene tether could activate CH 4 . 66-68 DiMagno et al. has found that a perfluoro and perfluoroaryl substituted porphyrin of Rh(II) activated and ultimately functionalized methane by delivering the methyl carbonium moiety to PPh 3 to give [PPh 3 CH 3 ] + .

69 Scheme 1.18. CH bond activation of methane by Rh(TMP) radical

Scheme 1.20. Degenerate reactions of [Cp 2 Lu(CH 3 )] with 13 CH 4 and CD 4

  CH 4 and Lu 13 CH 3

	signals is accompanied by the concomitant growth of the 12 CH 4 resonance, as observed by
	both 1 H and 13 C NMR (Figure 1.3). In addition, [Cp* 2 Lu(CH 3 )] reacts with CD 4 to give
	[Cp* 2 Lu(CD 3 )], which is characterized by 2 H NMR spectroscopy (Scheme 1.20). Kinetic
	studies show that the reaction rate is first order in [Cp* 2 Lu(CH 3 )] and first order in CH 4 ,
	implying a four-center transition state.

methylpropane and regenerate the methyl complex with a low TON of 4. 79 Attempts to improve the TON met with limited success since the ansa-bridged complex [Op* 2 Sc(CH 2 CH(CH 3 ) 2 )] is more active towards methane activation than

  An intermolecular KIE of 6(2) has been measured when CD 4 is used, demonstrating that cleavage of the methane C-H bond is involved in the rate-determining step.

	Scheme 1.21. CH bond activation of methane by [Cp* 2 Th( c CH 2 C(CH 3 ) 2 CH 2 )]
	In 2003, Sadow and Tilley demonstrated that metallocene-alkyl complexes of
	scandium could activate methane preferentially over benzene at room temperature. By
	coupling σ-bond metathesis and migratory insertion reactions, they have realized the catalytic
	coupling of methane and propene to give 2-methylpropane. As shown in Scheme 1.22(a),
	[Cp* 2 ScCH 3 ] inserts propene to give [Cp* 2 Sc(CH 2 CH(CH 3 ) 2 )]. This complex reacts with CH 4
	to give 2-[Cp* 2 Sc(CH 2 CH(CH 3 ) 2 )] but is less selective for olefin insertion. 80 [Cp* 2 Sc(CH 3 )] is also a
	catalyst for the dehydrocoupling reaction of methane and diphenylsilane by which Ph 2 SiH 2
	and CH 4 are converted into Ph 2 MeSiH and H 2 via two consecutive σ-bond metathesis
	reactions (Scheme 1.22(b)). 81,

82 Scheme 1.22. Productive

  σ-bond metathesis of methane.

Scheme 1.23. CH bond

  complexes also activate methane via this α-CH abstraction / 1,2-CH bond addition scheme. activation of methane by a zirconium imido intermediate.

	reagent such as ethylene, via intermolecular ''CH 2 '' coupling routes. As suggested in this
	study, therefore, 1,2-CH bond addition and tautomerization may constitute a plausible
	pathway for the dehydrogenation of methane, forming in the process a terminal titanium
	methylidene. Unsaturated metal alkylidenes had been known previously to activate CH bonds
	of other hydrocarbons 91-94 but this was the first example for methane activation.
	More recently, the α-CH abstraction / 1,2-CH bond addition has been extended to
	metal carbon triple bonds by Mindiola (Scheme 1.24). 90 A transient titanium alkylidyne
	compound [(PNP)Ti≡CR], generated by α-H abstraction of an alkane from an alkyl alkylidene
	complex [(PNP)Ti(=CH t Bu)(CH 2	t Bu)], cleaves a CH bond of methane following a similar
	pathway. Remarkably [(PNP)Ti(=CH t Bu)(CH 3 )] extrudes methane very slowly at room
	temperature (t 1/2 = 62 h at 25°C). Just like in the zirconium case, there is a strong polarization
	of the Ti≡C bond involving a proton-like transfer in the TS.
	Scheme 1.24. C-H bond activation of methane by a titanium alkylidyne intermediate.
	Isotopic labeling studies, including KIE experiments, show H/D exchange between the
	Ti-CH 3 and Ti=CH t Bu positions. The dehydrogenation of CH 4 to methylidene is especially
	important because it could allow for the conversion of methane to an industrially important

1.2.3-1,3-CH addition.

	Compared to the numerous examples of reversible α-CH abstraction / 1,2-CH bond
	addition reactions, there are few examples of reversible β-CH abstraction / 1,3-CH bond
	addition from dialkyl complexes and only one describing the activation of methane. Erker
	first demonstrated that a relative stable [Cp 2 Zr(η 2 -benzyne)] intermediate, generated by
	reversible intramolecular β-H abstraction from diaryl zirconocenes, was able to cleave the CH
	bond of benzene by 1,3-addition. 95 However, activation of an alkane from a dialkyl precursor
	for example is less favorable. In most cases the intermediate η 2 -alkene complex is not stable
	enough to allow CH bond activation in a subsequent step (Scheme 1.25). It is well known that
	[Cp 2 Zr(n-Bu) 2 ] readily eliminates butane by β-H abstraction to yield the transient η 2 -1-butene
	[Cp 2 Zr(η 2 -CH=CHCH 2 CH 3 )] for which a rich organic chemistry but no CH bond activation
	was developed.

2 - cyclopropene niobium complex Chapter 2: CH bond activation of methane by a transient η 2 -cyclopropene niobium complex 2.1-Introduction

  have been acquired by1 H NMR and analyzed by numerical simulation. Three separate experiments adding 2, 6, and 31 equiv of benzene to solutions of 1 in cyclohexane-d 12 at 323 K were carried out. The concentrations of 1 and 2 as determined by integration of key 1 H NMR signals were plotted as a function of time. The kinetic model shown in Scheme 2.2 was used to simultaneously fit the data. It involves four distinct processes and it indicates that the 1,3-CH bond activation of methane or benzene by the intermediate A are bimolecular

	Previous studies performed in our group indicate that the niobium cyclopropyl
	complex [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] (1) is able to activate the CH bond of benzene by
	β-H abstraction /1,3-addition pathways (Scheme 2.1). 1,2 Complex 1 reacts smoothly at room
	temperature with benzene (t 1/2 ca. 7 h) to give [Tp Me2 NbPh(c-C 3 H 5 )(MeCCMe)] (2) and
	methane. Kinetic studies in cyclohexane-d 12 shows that the reaction converting 1 to 2 is first
	order in 1 and zeroth order in benzene with a rate constant k obs at 303 K of 2.93 ± 0.05 × 10 -5
	s -1 . Temperature-dependent studies (303-323 K) lead to activation parameters ΔH † = 99 ± 5
	kJ/mol and ΔS † =-6 ± 10 J/K.mol. Reaction of 1 with C 6 D 6 instead of C 6 H 6 gives an
	insignificant isotope effect providing a ratio k H /k D of 1.0 at 303 K. These data support that the
	rate-determining step is an intramolecular abstraction of a β-H of the cyclopropyl group to
	form methane, yielding the transient unsaturated η 2 -cyclopropene / metallacyclobutane
	intermediate [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)] A. This is followed by the 1,3-CH bond
	addition of benzene to afford 2. DFT computational studies confirm that the formation of A
	passes through a four-center transition state (1-TS) exhibiting an almost linear C…H…C
	arrangement (CHC angles > 166°) with the hydrogen positioned symmetrically between the
	two carbon centers (Scheme 2.1). This structural rearrangement is very similar to those
	described by Legzdins

3

and is reminiscent of σ-bond metathesis processes.

4

Scheme 2.1. CH bond activation of benzene by a transient η 2 -cyclopropene niobium/niobiacyclobutane complex [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)] (A).

Kinetic experimental data for the activation of benzene by 1 (1→2 via intermediate A)

0 h t = 10 h t = 40 h Tp Me2 CH(1) Tp Me2 CH(1) o-C 6 H 3 (3) o-C 6 H 3 (3) Tp Me2 CH(3) p-C 6 H 3 (3) C 6 H 3 (mesitylene) Figure

  were present in solution. Under these pseudo-first order conditions, the disappearance of 3 was monitored. To determine the rate constant, k, the peak of the p-C 6 H 3 of 3 (δ 6.10) of each 1 H NMR spectrum acquired during the run was integrated and normalized with respect to the sum of the peaks of the C 6 H 3 of free mesitylene (δ 6.56), the p-C 6 H 3 of 3 (δ 6.10), the o-C 6 H 3 of 3 (δ 5.87), the Tp Me2 CH of 3 (δ 5.80, 5.71, 5.66) and the Tp Me2 CH of 1 (δ 5.87, 5.73, 5.59), as an internal reference (Figure2.8). Although adamantane was added to the sample as an internal reference, we were not able to confidently integrate its signals due to their proximity to the Tp Me2 Me signals of 1 and 3. Figure2.9 and Table2.1 showed the kinetic constants at different temperatures.

	Compound 3 (0.072 g, 0.12 mmol) was dissolved in a mixture of cyclohexane-d 12 (1
	mL) and perfluorobenzene (1 mL), and then transferred into a high pressure (HP) single-
	crystal 10 mm sapphire NMR tube by syringe. The HP NMR tube was pressurized with 40 bar Figure 2.8. Low-field expansions of stacked 1 H NMR spectra obtained from reaction of 3 with of methane and the reaction process was monitored by 1 H NMR spectroscopy. From the 1 H NMR spectrum, it was found that 3 reacted with CH 4 to give 1 and methane to give 1 in cyclohexane-d 12 (1 mL) and perfluorobenzene (1 mL) at 314 K.
	mesitylene (Figure 2.8). To carry out a complete kinetic study, the course of the reaction was
	monitored at 290, 303, 314 and 321 K for more than 5 half-lives but for practical reasons was

1-

mesityl bond with a strong CH bond in methane would yield a stronger Nb-methyl bond and a weaker CH bond in mesitylene. An excess of methane should make the reaction even more favorable thermodynamically. Here, we investigated how the CH bond of methane is activated by the mesityl complex [Tp Me2 Nb(CH 2 -3,5-C 6 H 3 Me 2 )(c-C 3 H 5 )(MeCCMe)] (3). treated within 1.8, 2.3, 2.3 and 3.2 half-lives, respectively. Integration of the CH 4 signal compared to the Tp Me2 CH signal of 3 indicated that ca. 12-33 equivalents of methane vs 3 t = 2.9. First-order kinetic plot for the β-H abstraction of mesitylene in 3 as a function of temperature.

Table 2 .

 2

	T (K)	k (s -1 × 10 -6 )	CH 4 equiv.
	290	5.85 ± 0.02	12
	303	20.3 ± 0.2	29
	314	70.2 ± 0.3	27
	314	71.9 ± 0.4	33
	321	132 ± 2	27

1. First order rate constants for β-H abstraction of mesitylene from 3

The reaction of 3 with different concentrations of methane (27 and 33 equivalents) at 314 K gave the same rate constants (Table

2

.1), indicating that the reaction is first order in 3 and zero order in methane (equation 2.11).

(2.11)

  C NMR spectra were recorded using Bruker Avance 400 ( 1 H, 400.13 MHz) and Bruker Avance 500 ( 1 H, 500.33 MHz) spectrometers. 2 H NMR spectra were recorded using a Bruker Avance 600 spectrometer ( 1 H, 600.13; 2 H, 92.12 MHz) equipped with a cryoprobe. For the kinetic and SST studies, the temperature of the probes was calibrated with 80% ethylene glycol (DMSO-d 6 ). For the SST and kinetic experiments 1 H NMR signals of adamantane were used as internal standards. Methane, (C 2 H 6 < 100 ppb, CO 2 < 100 ppb, total C n H m < 50 ppb, O 2 < 500 ppb, N 2 < 2 ppm, H 2 O < 2 ppm, H 2 < 2 ppb) was purchased from Alpha Gaz and used as received. CD 4

(isotope enrichment 99.93%) and

13

CH 4 (isotope enrichment 99.9%) were purchased from Euriso-top and used as received. [Tp Me2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (1) and [Tp Me2 Nb(CH 2 -3,5-Me 2 C 6 H 3 )(c-C 3 H 5 )(MeC≡CMe)] (

  in Chart 2.2) and to seal the tube (8, in Chart 2.2). All metal parts are made of a 6A1, 4V titanium alloy. The sample tube (12, in Chart 2.2) is connected to the titanium-alloy flange (9, in Chart 2.2) by a single component epoxy adhesive (10, in Chart 2.2). The valve body is connected to the flange through four threaded bolts (7, in Chart 2.2). In a typical operation, the sapphire tube is filled in a glove box with the sample solution by syringe. The valve body is then connected to the sapphire tube through the four bolts and sealed by closing the Viton O ring seal (8, in Chart 2.2). The tube is then filled with high pressure methane through the gas inlet port(6, in Chart

  ). This highlights the importance of the C 3 ring in decreasing the thermodynamic driving force for the initial β-H abstraction step, stabilizing the alkyl complex. Moreover,

	3.2-Objectives of this work
	Here, we aimed to demonstrate that A, formed by β-H abstraction from 1, was able to
	regioselectively activate the CH bond of a variety of unsaturated hydrocarbons including 1-
	alkene, 1-alkyne and heteroaromatics but also ferrocene and pentafluorobenzene via 1,3-CH-
	bond activation pathway to generate [Tp Me2 NbX(c-C 3 H 5 )(MeCCMe)] (X = 2-C 4 H 3 O (4a), 2-
	C 4 H 3 S(4b), 1-C 5 H 7
	Scheme 3.3. CH bond activation of furan and thiophene via β-H abstraction/1,3-addition
	pathway

alkene complexes of early transition metals generally decompose rapidly via alkene loss, which is an obstacle for them to activate the CH bond of hydrocarbons. The fact that the ligand exchange reaction A + C 2 H 4 →A et + C 3 H 4 is strongly endergonic (ΔG = +64 kJ/mol), clearly reflects that the interaction between the niobium center and the C=C π bond of cyclopropene is actually considerably stronger than that with ethylene, which allows A to have a lifetime long enough to activate the CH bond of hydrocarbons. In agreement with the energetic data, the Nb-C bond lengths in A (2.15 Å) are shorter than A et (2.23 Å), while the Mulliken populations of +0.9 (Nb) and -0.31 (C) in A vs +0.72 and -0.22 in A et indicate the greater charge transfer for the formation of the Nb-(cyclopropene) bond. Therefore, the unique geometric strain of A allows it to activate the C-H bond of hydrocarbons rather than to decompose via ligand loss. Moreover, our group recently reported that a zirconium analogue of A, [Cp 2 Zr(η 2 -c-C 3 H 4 )], generated from the intramolecular β-H abstraction of Cp 2 Zr(c-C 3 H 5 ) 2 , could selectively activate the CH bond of the more reactive furan or thiophene via 1,3-CH-bond activation (Scheme 3.3).

21

  C NMR spectra, the signals of the Cα, Cβ and Cβ carbons of the cyclopropyl group are observed at δ 74.6, 22.4, and 12.3 ppm, respectively. There is no evidence for C-H agostic interactions with the Nb center. Signals for the thienyl group were observed at δ 136.0 (3-C),

J HH = 12.1Hz J HF = 3.6 Hz J HC = 155.0 Hz

  1 H NMR of 9a in benzene-d 6 at 298 K

	6.150 3 ppm (t1) 6.200	6.100	6.050 1 H{F}	6.000	ppm (t1)	2.650	2.600	2.550	2.500	2.450	2.400
			1.00	1 H
	6.200	6.150	6.100	6.050	6.000
	ppm (t1)					ppm (t1)	2.650	2.600	2.550	2.500	2.450	2.400

  .15).

	' '
	Intramolecular
	H shift
		(7.60,s)
	9a	9b
	1 H NMR (cyclohexane-d 12 )
	7.60
	328 K
	313 K
	0.18		1.00
	298 K
	7.50 7.50 7.50 7.50	7.00 7.00 7.00 7.00	6.50 6.50 6.50 6.50
	ppm (t1) ppm (t1) ppm (t1) ppm (t1)

Table 3 .1. Electrochemical

 3

data for the reduction of 1, 5, 6 and 7 (scan rate = 200 mV)

  which follows the hybridization of the niobium-bound carbon sp (CCPh) > sp 2 (c-C 5 H 7 ) > sp3 (Me), the more the s character, the more S-accepting character of the group, the easier the reduction of the metal center. The Fc substituent fits in between sp 2 and sp hybridized carbons being more electron poor than the cyclopentenyl ligand with E 1/2 = -2.67 V vs. Fc/Fc +. for 7. The niobium fragment [Tp Me2 Nb(c-C 3 H 5 )(MeC≡CMe)] is strongly electron-donating as 7 exhibits a reversible one-electron oxidation process at E 1/2 = -0.26 V vs. Fc/Fc + . The ferrocenyl group in 7 is therefore more easily reducible than ferrocene, its half wave potential being 0.26 V less positive than that of ferrocene. This is 0.04 V less positive than [(C 5 H 5 )(C 5 Me 5 )Fe].

  C, 75.5 MHz), Bruker Avance 400 ( 1 H, 400.13 MHz;13 C, 100.6 MHz) and Bruker Avance 500 ( 1 H, 500.33 MHz; 13 C, 125.8 MHz) spectrometers. Only pertinent 1 J CH are quoted in the13 C spectra. Elemental analyses were obtained from the Analytical Service of our laboratory. Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe) was prepared according to a published procedure.6,20

of Tp Me2 Nb(c-C 3 H 5 )(C≡CPh)(MeC≡CMe) (5).

  st procedure: [TpMe2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (2.00 g, 3.99 mmol) was dissolved in pentane (50 mL) and about 5 equivalents of phenylacetylene (2.15 mL, 2.00 g, 19.58 mmol) were added. After 2 days at room temperature, the orange brown solution is separated by filtration trough a pad of Celite, from an orange yellow sticky solid which was washed 3 times with 10 mL of pentane. The solvent is stripped off, the powder is washed with small among of cold pentane to leave 5 as an orange powder (0.90 g, 1.53 mmol, 38 %), which was stored under Ar at room temperature.

	1 Orange crystals of 5 (0.139 g, 0.237 mmol, 40%) were isolated and stored at room
	temperature under Ar. Anal. Calcd for C 30 H 38 BN 6 Nb: C, 61.45; H, 6.53; B, 1.84; N, 14.33;
	Nb, 15.84. Found: C, 61.79; H, 6.64; N, 14.01.
	1 H NMR (400 MHz, benzene-d 6 , 298 K) δ 7.47 (d, 2 H, o-C 6 H 5 ), 7.02 (t, 2 H, m-
	C 6 H 5 ), 6.94 (t, 1 H, p-C 6 H 5 ), 5.81, 5.76, 5.43 (all s, 1 H each, Tp Me2 CH), 3.25, 2.30 (both s, 3
	as a yellow H each, CH 3 C≡), 2.93, 2.22, 2.21, 2.19, 2.03, 2.02 (all s, 3 H each, Tp Me2 CH 3 ), 2.25 (m, 1 H,
	powder (0.450 g, 0.82 mmol, 62 %), which was stored under Ar at room temperature. Anal. c-C 3 H 5 β'), 2.18 (overlaps, 1 H, c-C 3 H 5 α), 1.37 (m, 1 H, c-C 3 H 5 β'), 1.29, 0.71 (m, 1 H each,
	Calcd for C 26 H 36 BN 6 NbO: C, 56.54; H, 6.57; B, 1.96; N, 15.22; Nb, 16.82; O, 2.90. Found: C, c-C 3 H 5 β). 13 C NMR (100.6 MHz, benzene-d 6 , 298 K) δ 239.3, 233.0 (MeC≡C), 153.2, 151.4,
	55.66; H, 7.33; N, 16.43. 150.6, 144.0, 143.4, 143.3 (Tp Me2 CCH 3 ), 149.2 (br s, C≡C-Ph), 130.9 (m-C 6 H 5 ), 128.1 (o-
	1 H NMR (400 MHz, benzene-d 6 , 298 K) δ 7.35 (d, J 1.5 Hz, 5-H-furyl), 6.49 (d, J 3.0 C 6 H 5 ), 127.1 (ipso-C 6 H 5 ), 126.2 (p-C 6 H 5 ), 125.9 (br s, C≡C-Ph), 107.5, 107.1, 106.9

Hz, 4-H-furyl), 6.21 (dd, J 1.5 Hz and 3.0 Hz, 3-H-furyl), 5.

79

, 5.77, 5.52 (all s, 1 H each, Tp Me2 CH), 3.16, 2.27 (both s, CH 3 C≡), 2.26, 2.20, 2.19, 2.10, 2.07, 1.34 (all s, 3 H each, Tp Me2 CH 3 ), 2.27 (partly obscured m, 1 H, c-C 3 H 5 β'), 2.03 (m, 1 H, c-C 3 H 5 α), 1.51 (m, 1 H, Synthesis 2 nd procedure: [Tp Me2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (0.30 g, 0.60 mmol) was dissolved in C 6 H 12

(4 ml

) and phenylacetylene (1.32 mL, 30 mmol) was added. The solution was heated at 35 °C for 22 h. The yellow solution turned to orange and some blocks formed suggesting polymerization due to phenylacetonitrile. After solvent evaporation, the orange blocks was scratched and the orange solid was extracted with pentane (20 mL). The combined solutions were concentrated until some crystals came out and stored at -40 °C overnight to crystallize.

(Tp Me2 CH), 77.9 (br d, 1 J CH 146 Hz, Nb c-C 3 H 5 α), 23.2, 8.4 (both t , 1 J CH 159, 163 Hz, c-C 3 H 5 β, β), 21.8, 21.1 (CH 3 C≡), 16.2, 15.4, 14.7, 12.7, 12.6, 12.3 (Tp Me2 CH 3 ).

of Tp Me2 Nb(C 10 H 9 Fe)(c-C 3 H 5 )(MeC≡CMe) (7).

  40 °C overnight to crystallize. A yellow powder of 6 Tp Me2 CH), 66.5 (br d, 1 J CH 136 Hz, Nb c-C 3 H 5 α), 43.7 (CH 2 -C=CH), 34.6 (C=CH-CH 2 ), 25.4 (C=CH-CH 2 -CH 2 ), 22.1, 19.1 (CH 3 C≡), 21.1, 12.3 (both t , 1 J CH 159, 161 Hz, c-C 3 H 5 β, β), 15.5, 15.2, 14.1, 12.8, 12.6, 12.5 (Tp Me2 CH 3 ). C 3 H 5 α), 1.41 (m, 1 H, c-C 3 H 5 β'), 1.27, 0.96 (m, 1 H each, c-C 3 H 5 β).

	(0.220 g, 0.40 mmol, 66%) was isolated and stored at -40 °C under Ar. Anal. Calcd for C 27 H 40 BN 6 Nb: C, 58.71; H, 7.30; B, 1.96; N, 15.21; Nb, 16.82. Found: C, 58.63; H, 7.66; N, 15.02. 1 H NMR (400 MHz, benzene-d 6 , 298 K) δ 6.29 (br s, 1H, C=CH), 5.80, 5.70, 5.61 (all s, 1 H each, Tp Me2 CH), 2.94, 2.29 (both s, 3 H each, CH 3 C≡), 2.47, 2.26, 2.19, 2.14, 2.13, 1.70 (all s, 3 H each, Tp Me2 CH 3 ), 2.36 (m, 2 H, C=CH-CH 2 -), 2.09 (m, 1 H, CH 2 -C=CH), 2.03 (m, 1 H, c-C 3 H 5 β'), 1.89 (m, 1 H, CH 2 -C=CH), 1.76 (m, 1 H, c-C 3 H 5 α), 1.65, 1.56 (both m, 1 H each, CH 2 -CH 2 -C=CH), 1.49 (m, 1 H, c-C 3 H 5 β'), 1.35, 0.89 (m, 1 H each, c-C 3 H 5 β). 13 C NMR (100.6 MHz, benzene-d 6 , 298 K) δ 240.2, 239.0 (MeC≡C), 201.6 (NbC=CH), 152.3, 151.1, 149.8, 143.6, 143.2, 143.1 (Tp Me2 CCH 3 ), 136.4 (d, 1 J CH 160 Hz, 1 mmol) were heated in cyclohexane (10 mL) at 313 K overnight. The colour of the solution changed from bright orange to purple. The solvent was evaporated to dryness. The residue was dissolved in 100 ml of pentane and filtered to another one-necked schlenk bottle under Ar. The resulting solution was cooled at 273 K for 1 overnight. Purple crystals of 7 were obtained (140 mg, 0.21 mmol, 42%) and stored at -40 °C under Ar. Anal. Calcd for C 30 H 42 BN 6 Nb: C, 61.03; H, 7.17; N, 14.23. Found: C, 61.55; H, 7.52; N, 13.92. 1 H NMR (500.3 MHz, benzene-d 6 , 287 K) δ 5.79, 5.66, 5.57 (all s, 1 H each, Tp Me2 -CH), 4.32, 3.74 (d, 2 J HH = 1.2 Hz, 1 H each, FcCH), 4.09 (s, 5 H, FcCH), 4.05 (t, 2 J HH = 1.6 Hz, 2 H, FcCH), 4.01 (s, 1 H, FcCH), 3.21, 2.24 (all s, 3 H each, CH 3 C≡), 2.54, 2.37, 2.14, NbC=CH), 107.2, 106.9, 106.7 (Synthesis [Tp Me2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (1) (0.250 g, 0.500 mmol) and Ferrocene (0.186g, 2.10, 2.07, 2.00 (all s, 3 H each, Tp Me2 CH 3 ), 2.21 (overlaps, 1 H, c-C 3 H 5 β'), 1.70 (tt, 1 H, c-

13 C NMR (125.8 MHz, benzene-d 6 , 287

	K) δ 241.9, 241.2 (MeC≡), 150.6 (Cipso), 151.8, 151.1, 150.4, 144.0, 143.8,
	143.5 (Tp Me2 CMe), 130.0 (4-C 6 H 4 ), 128.1, 127.0 (m,o-C 6 H 4 ), 108.2, 107.3, 107.0 (Tp Me2 CH),
	76.5 (br t, 1 J CH = 117 Hz, NbCH 2 Ar), 67.2 (d, 1 J CH = 138 Hz, NbCαH), 22.4 (C 6 H 4 CH 3 ), 21.0,
	20.2 (CH 3 C≡), 21.7, 12.5 (t, 1 J CH = 158, 160 Hz, Cβ, Cβ), 16.5, 15.4, 15.2, 13.0, 12.9, 12.8
	(Tp Me2 CH 3 ).

Synthesis of Tp Me2 Nb(C 6 F 5 )(c-C 3 H 5 )(MeC≡CMe) (8).

  mL) and C 6 F 5 H (3.2 g, 19.04 mmol) was added. After 72h of stirring at room temperature, the colour of the solution changed from yellow-brown to dark brown, with the formation of a yellow precipitate. To favor the precipitation, the solvent was partially evaporated and the resulting suspension was cooled to -40 °C for 1 night. The suspension was filtered off and the yellow solid was washed 2 times with cold pentane (5 ml) and dried under vacuum, affording 8 (0.48 mmol, 60 %) and stored at -40 °C under Ar. The complex must be stored below 0°C.Anal. Calcd for C28 H 33 BF 5 N 6 Nb: C, 51.56; H, 5.10; B, 1.66; N, 12.88. Found: C, 51.86; H, H 5 β'), 1.53, 0.24 (m, 1 H each, c-C 3 H 5 β).

	[Tp Me2 NbMe(c-C 3 H 5 )(MeC≡CMe)] (0.40 g, 0.80 mmol) was dissolved in pentane (6
	5.78; N, 12.12.
	1 H NMR (500 MHz, dichloromethane-d 2 , 240 K) 5.85, 5.82, 5.72 (all s, 1 H each,
	Tp Me2 CH), 2.88 (d, 3 H, J HF = 5.6 Hz, CH 3 C≡), 2.13 (s, CH 3 C≡), 2.47, 2.42, 2.43, 1.87, 1.75,
	1.22 (all s, 3 H each, Tp Me2 CH 3 ), 2.72 (tt, 1 H, c-C 3 H 5 α), 2.47 (overlaps, 1 H, c-C 3 H 5 β'), 1.65
	(m, 1 H, c-C 3

13 C{ 1 H} (125.8 MHz, dichloromethane-d 2 , 240 K) 246

  Tp Me2 CH), 86.6 (s, c-C 3 H 5 α'), 23.2 (d, J CF = 10.7 Hz, CH 3 C≡), 22.4 (s, CH 3 C≡), 24.9 (s, c-C 3 H 5 β 2.1 (d, J CF = 9.2 Hz, c-C 3 H 5 β), 15.4, 15.1, 14.0, 13.1,

	.0 (s , MeC≡) 238.7 (d, J CF = 7.6 Hz, MeC≡), 151.0, 150.7,
	149.4, 144.6, 143.6, 143.5 (Tp Me2 CCH 3 ), 148.9, 146.9 (o-C 6 F 5 ), 138.7 (m-C 6 F 5 ), 135.9 (p-
	C 6 F 5 ), 106.8, 106.7, 106.5 (13.0, 12.7 (all s, Tp Me2 CH 3 ). 19 F NMR (282.4 MHz, benzene-d 6 , 298 K) -100.2, -117.5 (both
	m, 1 F each, o-C 6 F 5 ), -157.6 (m, 1 F, p-C 6 F 5 ), -162.5, -163.2 (both m, 1 F each, m-C 6 F 5 ).

Synthesis of Tp Me2 Nb(C 6 F 5 )[-HC(CH 3 )-HC=C(CH 3 )-(CH 3 )C=Nb] (9a).

	[Tp Me2 Nb(C 6 F 5 )(c-C 3 H 5 ) (MeC≡CMe)] (0.120 g, 0.18 mmol) was heated in benzene (2
	mL) for 24 hours at 45 °C. The colour of the solution changed from yellow to orange. The
	solvent was evaporated to dryness, and the resulting solid was washed 2 times with cold
	pentane (2ml) dried under vacuum to leave 9a and 9b as an orange solid (0.147 mmol, 80 %)
	and stored at room temperature under Ar.
	1 H NMR (400 MHz, benzene-d 6 , 298 K): 6.09 (dd, 1 H, J HF = 3.7, J HH = 12.2 Hz (-
	HC=C(CH 3 )-), 5.83, 5.61, 5.37 (all s, 1 H each, Tp Me2 CH), 2.65 (d, 3 H, J HF = 5.0 Hz -
	HC=C-(CH 3 )), 2.40 (d, 3 H, J HH = 6.1, HC(CH 3 )-), 2.13 (s, 3 H, (CH 3 )C=Nb), 2.30, 2.25,
	2.09, 1.90, 1.22, 0.93 (all s, 3 H each, Tp Me2 CH 3 ), 1.05 (m, 1 H, -HC(CH 3 )-). 13 C{ 19 F, 1 H}
	NMR (125.8 MHz, benzene-d 6 , 298 K) 260.2 (-(CH 3 )C=Nb), 151.8, 148.7, 148.3, 145.8,
	144.8, 143.8 (Tp Me2 CCH 3 ), 149.6, 148.5 (o-C 6 F 5 ), 138.9 (m-C 6 F 5 ), 136.7 (p-C 6 F 5 ), 135.9 (br
	s, ipso-C 6 F 5 ), 120.0 (-HC=C(CH 3 )-) , 107.5, 107.4, 106.7 (Tp Me2 CH), 101.6 (-HC=C(CH 3 )-),
	87.3 (-HC(CH 3 )-), 24.4 (-(CH 3 )C=Nb), 22.9 (-HC=C(CH 3 )-), 21.1 (-HC=C(CH 3 )-), 16.0,
	15.7, 15.1, 13.0, 12.9, 12.7 (Tp Me2 CH 3 ).

19 F (376.5 MHz, benzene-d 6 , 298 K) -

  109.5, (dt, 1 F,3 J FF = 18.9, 4 J FF = 9.2 Hz, o-C 6 F 5 ), -115.7, (dt, 1 F, 3 J FF = 29.9, 4 J FF = 9.6 Hz, o-C 6 F 5 ), -158.3 (m, 1 F, 3 J FF = 20.2, p-C 6 F 5 ), -163.0, -164.1 (both m, 1 F each, m-C 6 F 5 ).prepared in a glove box. Before each measurement, the solutions were degassed by bubbling Ar and the working electrode was polished with a polishing machine. Under the experimental conditions employed throughout this work, the half wave potential (E 1/2 ) of the Fc/Fc + couple in THF occurs at E 1/2 = 0.55 V vs SCE.

Table 3 . 2 .

 32 Crystal data and structure refinement for 4a

	1,00E-06
	5,00E-07
	Current (μA)
	-3,50E+00	-3,00E+00	-2,50E+00	-2,00E+00	-1,50E+00	-1,00E+00	-5,00E-01	0,00E+00	5,00E-01
					E/V vs. Ferrocene

Table 3 .

 3

	3. Bond lengths (Å) for 4a
	C(1)-C(2)	1.513(5)	C(31)-C(32)	1.496(5)
	C(1)-C(3)	1.528(5)	C(32)-N(3)	1.341(4)
	C(1)-Nb(1)	2.179(4)	C(32)-C(33)	1.373(5)
	C(2)-C(3)	1.467(6)	C(33)-C(34)	1.364(5)
	C(4)-C(5)	1.291(5)	C(34)-N(4)	1.357(4)
	C(4)-C(7)	1.488(5)	C(34)-C(35)	1.503(5)
	C(4)-Nb(1)	2.057(3)	C(41)-C(42)	1.494(5)
	C(5)-C(6)	1.498(5)	C(32)-N(3)	1.341(4)

Table 3 . 5 .

 35 Crystal data and structure refinement for 5

	Empirical formula	C30 H38 B N6 Nb
	Formula weight	586.38
	Crystal size (mm)	0.18 x 0.12 x 0.03 mm
	Crystal system	Monoclinic,
	space group	P121/n1
	a (Å)	11.0651(5)
	b (Å)	14.9952(7)
	c (Å)	17.7145(9)
	D (°)	90
	E (°)	93.684(2)
	J (°)	90
	V (Å 3 )	2933.2(2)
	Z	4
	F	1224
	Ucalculated (g.cm -3 )	1.328
	Radiation type	MoKa
	Longueur d'onde (Å)	0.71073
	Temperature	180 K
	Limit of 2T(°)	2.11-26.37
	Nb of the reflections collected	49107
	Nb of the reflections independent	5980 (0.0514)
	Affinement by

Table 3 . 8 .

 38 Crystal data and structure refinement for 6

	Empirical formula	C 27 H 40 BN 6 Nb
	Formula weight	552.37
	Crystal size (mm)	0.35 x 0.22 x 0.12 mm
	Crystal system	Monoclinic
	space group	P 21/n
	a (Å)	12.0657(2)
	b (Å)	16.5205(2)
	c (Å)	14.9268(2)
	D (°)	90
	E (°)	109.051(2)
	J (°)	90
	V (Å 3 )	2812.41(7)
	Z	4
	F	1160
	U calculated (g.cm

Activation de la liaison CH du méthane par un complexe intermédiaire K K 2 -cyclopropène de niobium

  Le manuscrit se concentre sur l'activation de la liaison CH de différents hydrocarbures, en particulier le méthane par un K 2 -cyclopropène intermédiaire de niobium. Seule l'activation de la liaison CH du méthane est ici résumée. , de sorte que la conception moléculaire des catalyseurs et la compréhension détaillée du mécanisme intime des réactions soient possibles. Un résumé des différentes voies de clivage d'une liaison CH par des complexes des métaux de transition est représenté dans le schéma 1.1. Certains complexes de métaux de transition de la gauche et de la droite du tableau périodique sont capables d'activer le méthane par différentes voies mécanistiques, mais seulement quelques-uns d'entre eux ont conduit à des applications catalytiques. Dans les sections suivantes, nous allons décrire par un aperçu général les différents schémas d'activation de la liaison CH selon les lesquels les complexes des métaux de transition peuvent activer le méthane dans des conditions homogènes.

	Chapitre 1: Les processus d'activation de la
	liaison CH du méthane.
	2 ) par steam reforming (CH 4
	+ H 2 O = CO + 3H 2 ), un processus endergonic très exigeant énergétiquement, catalysé par des
	catalyseurs à base de nickel à des températures élevées (ca 700 -1000 ° C) et une pression
	modérée (< 25 bars), qui est utilisé pour la synthèse de toute une gamme de produits
	chimiques de base.
	La fonctionnalisation catalytique du méthane dans des produits chimiques facilement
	transportables et à haute valeur ajoutée selon une voie propre, économique et durable reste un
	objectif à long terme d'un point de vue scientifique, environnementale et économique. Le
	méthane est en effet très inerte, montrant un potentiel d'ionisation de ~ 12,5 eV, avec une
	électronégativité de Pauling χ C = 2,55 et χ H = 2,20, une valeur de pK a estimé à ~ 50 -51 et
	une enthalpie de dissociation homolytique de la liaison C-H de 440 kJ/mol à 25 ° C. Par
	conséquent, il est très difficile d'activer la forte liaison CH du méthane directement, par une
	voie hétérolytique ou homolytique.

Le méthane est une source d'énergie et de produits chimiques en raison de son abondance sous forme de gaz naturel (80-90%) et de gas de schiste. Les utilisations actuelles du méthane comprennent son oxydation complète en CO 2 et H 2 O pour fournir de la chaleur (et d'électricité) et la production de syn gaz (un mélange de CO et H Dans le but de bénéficier de la flexibilité des systèmes homogènes, des applications importantes ont été trouvé dans la catalyse par des complexes des métaux de transition. L'utilisation de complexes de métaux de transition, en outre, permet un réglage précis de divers facteursSchéma 1.1. Les principales voies d'activation de la liaison CH effectué par des complexes des métaux de transition.

1.

1-Activation de la liaison CH du méthane par des complexes de la droite du tableau périodique

	métallique. Dans la section qui suit les principales caractéristiques de chaque mécanisme
	d'activation seront discutées.
	1.1.1-Substitution electrophile
	Les métaux de la droite du tableau périodique et les lanthanides agissent comme des
	acides de Lewis, réagissant avec du méthane par une substitution électrophile en milieu
	polaire (Équation 1.1).
		oxidative addition and V V-CAM	late transition metal complexes
	1,2-addition
	(often Following a-H abstraction		electrophilic substitution
	V-bond
	metathesis
			metalloradical addition
	early
	transition
	metal complexes	carbene insertion

Les complexes contenant des métaux de transition de la droite du tableau périodique sont capables d'activer la liaison CH du méthane par différentes voies telles que la substitution électrophile, l'addition oxydante, l'insertion d'un carbène et l'addition d'un radical Le schéma 1.2 décrit les systèmes capables d'activer le méthane selon un mécanisme de substitution électrophile. Ces systèmes sont remarquables car ils constituent des rares exemples de fonctionnalisation catalytique du méthane avec des complexes métalliques homogènes, mais avec une faible TOF et TON. Schéma 1.2. Fonctionnalisation catalytique de la liaison CH du méthane par la voie de substitution électrophile.

Table 2 . 2 .

 22 Constants de vitesse du premier ordre pour l'abstraction du mesitylene dans 3 La réaction de 3 avec différentes concentrations de méthane (27 et 33 équivalents) à 314 K donne les mêmes constantes de vitesse (Tableau 2.2), ce qui indique que la réaction est du premier ordre par rapport au complexe 3 et d'ordre zéro par rapport au méthane (Equation

	T(K)	K(s -1 × 10 -6 )	CH 4 equiv.
	290	5.85±0.02	12
	303	20.3±0.2	29
	314	70.2±0.3	27
	314	71.9±0.4	33
	321	132±2	27
	2.3).
		(2.3)

A partir des constantes de vitesse obtenues aux différentes températures (Figure

2

.7 et le Tableau 2.2), nous avons pu obtenir les paramètres d'activation en utilisant les équations

H NMR (benzene-d 6 ,

K)

F{ 1 H} NMR (benzene-d 6 , 298 K) p-F m-F m-F

et al. ont calculé une voie d'addition oxydante et d'élimination réductrice impliquant un intermédiaire d'Ir (V) (Schéma 1.5). Schéma 1.5. Activation de la liaison CH du 13 CH 4 par le complexe [Cp*Ir(PMe 3 )(CH 3 )(CH 2 Cl 2 )][BArF 4 ] Jones et Maguire ont profité du couple Re(III)/(V) pour montrer que [CpRe(PPh 3 ) 2 H 2 ] pouvait catalyser l'échange H/D entre C 6 D 6 et CH 4 , avec 68 turnovers en 3h, ce qui représente l'un des rares systèmes capables d'activer le méthane par voie catalytique (Schéma 1.6).Schéma 1.6. Mécanisme d'échange H/D entre le CH 4 et le C 6 D 6 , catalysé par un complexe de cyclopentadienylrhenium(I).En présence de certains précurseurs de Re, Rh et Ir, l'addition oxydante, couplée à des réactions de borylation, a montré un grand potentiel pour la fonctionnalisation des alcanes et des arènes vers la formation de liaisons C-C étendues. Cependant, aucune de ces transformations ont été élargis au CH 4 , probablement en raison des difficultés rencontrées dans la manipulation de ce gaz ainsi que dans la présence de réactions de compétition avec le

Remerciements

Ce travail a été réalisé grâce au financement de la Région Midi-Pyrénées et du PRES-de l'Université de Toulouse.

The work presented has been carried out in the "Laboratoire de Chimie de Coordination" (CNRS, Toulouse) in the

Chapter 3 CH bond activation of heteraromatic and unsaturated hydrocarbons by a niobium methyl cyclopropyl precursor

Complex 7 is very stable but it was found to react slowly with benzene. After 14 h at 287 K, the 1 activate the C-H bond of furan, thiophene, 1-cyclopentene, phenylacetylene, pentafluorobenzene and ferrocene, yielding the corresponding products [Tp Me2 NbX(c- (6) , Fc (7), and C 6 F 5 (8)) at mild conditions. Complexes 4-7 have been isolated and characterized by 1 H, 13 C NMR spectroscopy and by X-ray diffraction analysis. Electrochemical studies on complexes 1, 5, 6, and 7 shows the involvement of the niobium center in a quasi reversible one-electron reduction process. The reducibility of such complexes follows the order:

. This trend follows the order of the hybridization of the niobium-bound carbon: sp 3 (Me) < sp 2 (c-C 5 H 7 ) < sp (CCPh). The reactions between 4a, 5, 6, and 7 with benzene indicate that the Nb-R bond strengths decrease in the order: Nb-alkynyl > Nb-heteraromatic > Nb-phenyl > Nb-vinyl > Nb-Fc bond.

Synthesis of Tp Me2 Nb(C 6 F 5 )[-HC(CH 3 )-(CH 3 )C=C(H)-(CH 3 )C=Nb] (9b).

1 H NMR (400 MHz, benzene-d 6 , 298 K): 6.09 (dd, 1 H, J HF = 3.7, J HH = 12.2 Hz (-

2.2-Objectives du travail

Inspirés par le travail décrit dessus, nous avons cherché à voir si 1 était capable d'activer l'inerte liaison CH du méthane d'une manière similaire que le benzène. Le mécanisme a été d'abord abordé par des expériences de marquage isotopique utilisant du 13