Dr Jacques Andrieu
email: jacques.andrieu@u-bourgogne.fr

Jean Michel Camus

Rinaldo Poli

Philippe Richard

New Chiral -Aminophosphine Oxides and Sulfides: an Unprecedented Rhodium-catalyzed Ligand Epimerization

Keywords: Hydroformylation, phosphine oxides and sulfides, aminophosphines, epimerization, rhodium

New chiral -aminophosphine oxide N,P(O) and sulfide N,P(S) ligands have been prepared in one pot syntheses by addition of Ph2PH to (S)-PhCH=NCH(Ph)CH3 followed by oxidation with O2 or S8. Crystallization from cold methanol leads to the isolation of an enantiomerically pure single N,P(O) diastereomer and to a 1:1 mixture of the two N,P(S) diastereomers. The coordination chemistry of these ligands with [RhCl(COD)]2 and [RhCl(CO)2]2 has been investigated under either argon or syngas. At high temperatures, a P-C oxidative addition of the N,P(O) ligand followed by imine elimination leads to several hydrido rhodium species. The complexes containing N,P(S) ligand undergo the same process at room temperature. Catalytic amounts of the rhodium complexes catalyze the epimerization of the N,P(S) ligand under argon at room temperature, the dicarbonyl complex being 14 times more active than the COD complex.

The same catalyzed epimerization takes place also for the N,P(O) ligand, but only much more slowly at 35°C and under a syngas pressure. Possible mechanisms for this catalytic process are discussed. Catalytic tests in styrene hydroformylation have shown high activities and regioselectivities, but no enantioselectivities, for the N,P(O)-containing rhodium complexes.

Introduction

The challenge of asymmetric hydroformylation is not only related to chemoselectivity (hydroformylation vs. hydrogenation) and regioselectivity (branched vs. normal aldehyde), but also to enantioselectivity. 1,2 The hydroformylation of vinyl aromatics leads mainly to branched aldehydes, which can be further oxidized to the corresponding acids. These compounds are effective nonsteroidal analgesics. 3,4 Rhodium complexes containing chiral diphosphine ligands give higher activity and regioselectivity vs. platinum complexes, with moderate enantiomeric excesses (e.e.). However, by using diphosphite and phosphine-phosphite ligands, the enantioselectivity has been improved to 90 and more than 94 %, respectively. 5,6 Three reviews on asymmetric hydroformylation have been published during the last five years. 1,2,7 If chiral phosphorus ligands seem to be very attractive for this reaction, most of them are air-sensitive and their preparation is rather tedious. Recently described chiral thioureas are the first air stable ligands leading to a significant enantioselectivity (40 % e.e.), albeit with a low conversion of 7 %. 8 Since there is a strong demand for air-stable and easily accessible chiral ligands, we decided to develop new air stable chiral aminophosphine oxide ligands (hereafter abbreviated as N,P(O)) and related sulfide ligand (N,P(S)). In fact, ligands such as A or B in Chart 1 give not only greater catalytic activities in hydroformylation than their corresponding aminophosphines in combination with rhodium, but also higher branched/normal aldehyde ratios. [9][10][11] It is notable that a set of optically pure -aminophosphine oxide ligands (P-C*-N) of type A has recently been obtained by catalytic enantioselective addition of diphenylphosphine oxide to cyclic imines. 12 To the best of our knowledge, the use of such chiral-aminophosphine oxides and sulfides (of type A) has not so far been reported in asymmetric hydroformylation. We note, however, that a good conversion (85 %) with a high enantioselectivity (84 %) has recently been reported for the N,P(O) ligand C in the Ru-catalyzed transfer hydrogenation of phenylisopropyl ketone. 13 In this paper, we present the synthesis of new chiral -aminophosphine oxides and sulfides with secondary amine function, and their use in asymmetric hydroformylation of olefins. Our results show that these ligands are not good chiral auxiliaries for the enantioselective branched hydroformylation and that they epimerize rapidly in the presence of rhodium.

<Insert Chart 1>

Results and Discussion

Synthesis of the aminophosphine oxide and sulfide ligands

In order to obtain optically pure ligands, we based our synthetic approach on the two simple reactions in chart 2. The first step, namely the addition of Ph2PH to an imine, has already been studied and recently reported by us. The addition proceeds to an equilibrium position which was found to be strongly affected by the electron richness of the system. 14,15 The second step consists of an oxidation reaction with either oxygen or sulfur. We used an optically active starting imine compound, namely (S)-PhCH=NCH(Ph)CH3, in order to induce chirality at the central C atom and hopefully lead to a single diastereomer or to two diastereomers that could be easily separated by virtue of their different solubility properties.

<Insert Chart 2> Before realizing the one-pot preparation of the N,P(O) and N,P(S) ligands, we wished to examine in more detail the previously communicated 14 equilibrium between the starting imine and Ph2PH mixture and the two diastereomers of the intermediate aminophosphine 1 (equation 1).

An NMR study in CDCl3 showed that an equilibrium was achieved after 5 hours at room temperature.

<Insert Equation 1>Figure 1 shows the time evolution of the diastereomers syn -1 (R,S) and anti -1 (S,S) from a 1 H NMR monitoring in CDCl3. The absolute configurations were determined by analogy with both diastereomers of compound CH3CH(Ph)NHCH(Ph)CH3, which give signals for the benzylic and methine hydrogens at higher fields for the (S,S) diastereomer than for the (R,S) one. 16 This was explained by assuming a zig-zag conformation together with anti and syn dispositions of the aryl groups in the (S,S) and (R,S) diastereomers, respectively. 17 Hence, in the anti or (S,S) isomer each benzylic hydrogen is subjected to the ring current effect of the non-adjacent aryl group.

Then, the 1 H-NMR spectra (see experimental section) differ significantly, especially for the benzyl and methine signals. 16 <Insert Figure 1> It is interesting to note from Figure 1 that both syn and anti stereoisomers form at approximately the same rate at the beginning of the reaction. However, the diastereomeric ratio changes remarkably after ca. 15 minutes. Indeed, the anti diastereomer continues to form whereas the amount of the syn one decreases until an equilibrium is reached, which corresponds to a diastereomeric excess (d.e.) of 62 % in favor of the anti -P,N ligand. This means that the two imine enantiofaces are equally accessible to the phosphine attack, but the reversible P-C bond formation equilibrium eventually puts the process under thermodynamic control, to yield and equilibrium constant of K anti/syn = 4.25. Clearly, because of the above equilibrium, it is impossible to separate the two diastereomers of 1.

After achieving equilibrium, the reaction mixture was then slowly oxidized (by opening the NMR sample to air). The half time of this oxidation reaction is estimated to be ca. Unfortunately, attempts to separate the two diastereomers of 3 by crystallization from cold methanol as described for the analogous oxide ligand led to a 1:1 diastereomeric mixture in 29% isolated yields. Thus, the solution is completely depleted from isomer syn-3 and pure anti-3 should remain in solution. Isolation of the pure anti-3 ligand from the residual filtrate, however, was prevented by the traces of unreacted Ph2P(S)H.

X-ray structural analyses of ligands 2 and 3

Suitable crystals of syn -2 and syn/anti-3 (1:1) for the X-ray diffraction analyses were obtained under identical conditions (see Experimental section). Nevertheless, quite surprisingly, the unit cell of the N,P(O) ligand contains only the (R,S) diastereomer, whereas the N,P(S) ligand leads instead to the co-crystallization of the (R,S) and (S,S) isomers. The absolute configuration observed for the aminophosphine oxide confirms the assignment made earlier on the basis of NMR. Neither compound shows any inter-or intramolecular NH … E bonding interaction.

Consequently, both are present as monomers in the solid state (see Figures 2a and2b).

<Insert Figure 2> Selected bond lenghts and angles are reported in Table 1. The P-C(1) bond lenghts in the N,P(S) ligand (1.893(2) and 1.856(2) Å for the syn and anti diastereomers, respectively) are longer than those found in its oxide counterpart (1.832(2) Å). This can directly be correlated with the weaker electron withdrawing effect of the sulfur atom relative the oxygen atom. This difference indicates that it should be easier to cleave the P-C(1) bond for the N,P(S) ligand than for the aminophosphine oxide. Moreover, in contrast to the average P-C bond lenght found from 19 other structures containing the Ph2P(S)-C(sp 3 )H moiety in the Cambridge Crystallographic Database, which is 1.83(1) Å, the P-C(1) bond lenght in the syn-3 diastereomer is abnormally longer. This further indicates that this latter P-C bond could be easier to cleave. These assumption will be confirmed in a later section concerning the rhodium catalyzed epimerization. It is also notable that this isomer with the unusually long P-C bond, therefore presumably having a higher relative energy, corresponds indeed to the minor diastereomer at equilibrium according to the NMR analysis. It can be noted that the P-C(1)-N angle is significantly different in the two diastereomers of 3 and that no clear correlation exists between syn-2 and syn-3 for this parameter.

<Insert Table 1>

Interaction of ligands 2 and 3 with rhodium complexes

Before carrying out catalytic tests, the stoichiometric interactions between the N,P(O) and N,P(S) ligands and the rhodium precatalysts [RhCl(COD)]2 and [RhCl(CO)2]2 were investigated.

Stirring a 2 :1 mixture of syn -2 and [RhCl(COD)]2 in CDCl3 solution for 15 minutes at room temperature leads to a single product which, according to the NMR analyses, has an uncoordinated P=O moiety ( 31 P NMR chemical shift identical to the free ligand), see Scheme 2.

The N-coordination is not clearly shown by the 1 H NMR because of overlap of the NH resonance with those of the coordinated cyclooctadiene protons. However, this coordination can reasonably be expected considering the ability of secondary amines to split the chloride bridges in [RhCl(COD)]2, as previously proposed (although without spectroscopic evidence) for analogous -aminophosphonate ligands. 18 The above solution was then heated to 80°C for 15 minutes.

After evaporation of the solvent, the 1 H and 31 P NMR spectra in C6D6 show at least four different hydrido rhodium complexes (D) and the presence of free imine. These products presumably result from a P-C oxidative addition followed by imine elimination (Scheme 2). Note that the Rh(III)-PPh2(O) moiety has recently been identified by X-ray diffraction analysis in dirhodium chloride complexes. 19 At a temperature as low as 55°C, similar hydrido rhodium species begin to appear after 4 hours in very minor quantities. The same NMR resonances are generated in a control experiment involving the addition of Ph2P(O)H to the same rhodium substrate at 80°C in C6D6.

<Insert Scheme 2>

The addition of compound syn-2 to [RhCl(CO)2]2 at room temperature, on the other hand, leads to a rhodium complex containing a bidentate N,P(O) ligand, which is stable in solution for several hours. The bidentate coordination mode is confirmed by a new broad peak of the phosphine oxide moiety at 40.7 vs. 31.5 ppm for the free ligand and by the P=O absorption band at 1184 vs. 1265 cm -1 for the free ligand. In addition, the NH resonance in the 1 H NMR spectrum is also shifted to lower field by coordination. However, two IR absorption bands in CDCl3 at 2086 and 2015 cm -1 indicate the presence of two types of terminal carbonyl ligands. This observation, combined with the broad signals in the 1 H and 31 P NMR spectra, suggest a dynamic isomerization exchange, i.e. (E) in Scheme 2, similar to that reported for complex RhCl(CO)( 2 -N,P(O)) with N,P(O) = Ph2P(O)CH2NMe2. 9 When this reaction was carried out under a CO atmosphere (by preparing [RhCl(CO)2]2 in situ from [RhCl(COD)]2 under CO), the same complexes (E) are obtained. However, a new square planar compound also forms, in which the P=O moiety is uncoordinated due to the hemilabile character of the ligand under CO.

The stoichiometric reaction between the N,P(S) ligand (1:1 mixture of syn and anti diastereomers of 3) with [RhCl(CO)2]2 (or with [RhCl(COD)]2 under CO) at room temperature either in CDCl3 or C6D6 led, after 30 minutes, to a mixture of four rhodium complexes according to the 31 P NMR spectrum. The two major products are characterized by 31 P singlets at 50.42 and 50.25 ppm in CDCl3, i.e. close to the chemical shifts of the two diastereomers of the free ligand. Integration shows that the d.e. has surprisingly changed from 0% in the free ligand to 70% in the Rh products. The N-coordination mode is confirmed by the 1 H NMR chemical shift of the NH proton from 3.55 for the free amine to 3.80 ppm in C6D6 solution. Therefore, in contrast to the same reaction of the N,P(O) ligand described above, ligand 3 binds rhodium in the  1 -N fashion. This is surprising because the reaction of the very similar ligand Ph2PCH2P(S)Ph2 with [RhCl(CO)2]2 was reported to lead to RhCl(CO)( 2 -Ph2PCH2P(S)Ph2) in which the 31 P NMR resonance for the P(S)Ph2 moiety shifts to 52 ppm from the free ligand value of 40 ppm. 20,21 The other two reaction products exhibit 31 P NMR doublets at 114 (with JPRh = 179 Hz) and at 71 ppm (with JPRh = 147 Hz), showing the direct interaction of the rhodium center with phosphorus rather than with sulfur. These signals can likely be assigned to two types of Rh-P species that we have not been able to fully characterize, likely formed by a P-C oxidative addition process. These chemical shifts and the high Rh-P coupling constants may be consistent with terminal or bridging (µ:P,S) modes for the P(S)Ph2 ligand, both of which have previously been established. 22,23 At this point, we conclude that rhodium complexes containing the N,P(S) ligand 3 are unstable in solution even at room temperature. Finally, since the complexes containing the N,P(O) ligand syn -2 are unstable only at high temperatures and since low temperatures are known to decrease the conversion rate but at the same time favor higher enantioselectivities in hydroformylation reactions, 2,6,24 we decided to examine the catalytic activity of Rh complexes containing the ligand syn -2 at relatively low temperatures (≤55 °C).

Rhodium-catalyzed hydroformylation of styrene with ligands syn-2 and 3

The results of all catalytic studies are assembled in Table 2. All runs were carried out at a temperature of 55 or 35°C, and at a total pressure of 400 or 500 psi for the 1:1 CO/H2 mixture, in chloroform or in benzene as solvent. A control catalytic run without ligand (entry 1) shows complete conversion after 20 h with a b/l regioselectivity of 89/11. By using Eu(hfc)3 as a chemical shift reagent, no e.e. was observed, as expected. In the presence of ligand syn-2 (run 2) the conversion rate is five times higher with the same regioselectivity, but with zero enantioselectivity. The use of the less polar solvent C6H6 (run 3) decreases the activity.

Unfortunately, the regioselectivity is moderate and the catalyst is still non-enantioselective even though benzene is known to be the best solvent to favor high regioselectivities and to prevent the aldehyde epimerization. 5,6 Slightly lower activities and zero enantioselectivity are also observed by working at a lower temperature in chloroform (run 4), however the b/l ratio is greatly increased under these conditions. <Insert Table 2> The promoter effect of ligand syn-2 is further demonstrated by the following additional experiments. The conversion was found to be linear with time (zero order dependence on substrate) and dependent of the ligand/metal ratio as illustrated in Figure 3 for experiments carried out in chloroform at 35°C. In all cases, the regioselectivity is greater than 95/5 and the enantioselectivity is zero. The highest activity corresponds to a ligand/metal ratio of 1:1, while it is much lower for a ratio of 0.5:1, slighly lower for 2:1, and then it decreases dramatically when a ratio of 10:1 is used. The TOF for the experiment with 0.5 equivalents of ligand (10.2 mol mol(Rh) -1 h -1 ) is approximately one half the TOF found with 1 equivalent of ligand (21.6 mol mol(Rh) -1 h -1 ). A possible interpretation of these results is that only one half the amount of rhodium forms the complex containing the aminophosphine oxide ligand and subsequently the activity is decreased by a factor of two, while higher ratios likely force the coordination of two or more ligands to the rhodium center via equilibrium processes, consequently decreasing the number of coordination sites and leading to less active species.

<Insert Figure 3> Since ligand syn -2 promotes the activity of the rhodium catalyst for the hydroformylation of styrene, other olefins have also been tested under the same conditions. Methylmethacrylate did not yield aldehydes after one day, and a conversion of only 7 % was achieved with -methylstyrene after 70 h.

This catalytic system seems to be best suited for the hydroformylation of electron-deficient vinyl aromatics. Since, in some cases, better e.e.'s have been obtained with vinyl naphthalene instead of styrene, 24,25 this olefin was also tested, yielding results comparable to styrene. The activity is slightly lower, the regioselectivity is about the same (entry 5), and the e.e. is quite small (3%).

Use of the N,P(S) ligand 3 with neutral or cationic rhodium precursors gave dramatically lower activities (see entries 6 and 7 in Table 2), albeit with surprisingly high regioselectivities, by analogy with the previous studies with the Ph2P(E)CH2NMe2 (E = S vs. O) ligands. 9,10 The rapid decomposition of the Rh complexes with ligand 3 mentioned in the previous section may rationalize these results. In other cases, on the other hand, cationic precursors displayed increased activities. 8,26 The last experiment in Table 2 (run 8) shows that the use of the phosphine oxide Ph2P(O)H leads to a poorer catalytic performance relative to ligand syn-2 (entry 4). The reaction of this phosphine oxide with the rhodium precursor, as shown in the previous section, leads to the same products originating from the thermal decomposition of the rhodium complex with ligand syn -2 (form D in Scheme 2). This result further demonstrates that the coordinated aminophosphine oxide is an efficient ligand for styrene hydroformylation catalysis under conditions where this decomposition do not occur.

The observed racemic mixture of the branched aldehyde product could well result from an aldehyde racemization reaction 8 but, considering the relatively short reaction time, we concluded that this can not be the only reason to rationalize the absence of enantioselectivity. Thus, we wished to investigate the interaction between the rhodium center and ligands 2 and 3 in more detail.

Rhodium-catalyzed epimerization of aminophosphine oxide and sulfide ligands

We have mentioned above that the aminophosphine sulfide is not efficient in hydroformylation. However, an NMR investigation of the interaction between 3 and [RhCl(CO)2]2 surprisingly showed that rhodium induces ligand epimerization. Upon Rh coordination, the anti/syn ratio (as determined by 31 P NMR integration) changes from 50:50 to 85:15 after ca. 30 minutes. The final ratio of the two coordinated ligand diastereomers corresponds to the ratio of the two free diastereomers before crystallization (see Scheme 2).

Since all signal intensities do not further change after several hours (including those of the other by-products, see section 3), all compounds are considered to be in a thermodynamic equilibrium situation. We were then stimulated to interact the ligand 3 with a catalytic amount of [RhCl(COD)]2. The complete equilibration was achieved after 8 hours instead of 30 minutes (see Figure 4), to yield again the same anti/syn ratio of 85:15. As mentioned earlier (see X-ray structure analysis discussion in section 2), the smaller stability of the syn diastereomer may be the consequence of its longer P-C(1) bond length.

<Insert Figure 4> Under the same conditions, the chlorodicarbonylrhodium(I) dimer exhibits a higher activity leading to a complete equilibration after less than one hour (see Figure 4). To the best of our knowledge, there is only one example of ligand epimerization (for the bifunctional S,O ligand (R)-2-[(R)-phenylsulfinyl]propionic acid) upon stereoselective and stoichiometric coordination to hemimetallocenic Rh III , Ir III , Ru II or Os II complexes. 27 Here, we report the first case where such a process is Rh-catalyzed. By applying the kinetic analysis of reversible first order reactions to our experimental measurements (eq. 2), a satisfactory fitting is obtained (see Figure 5), giving rate constants kobs of 8.6(2) . 10 -3 (t1/2 = 80 min, R 2 = 0.996) and of 0.117(4) min -1 (t1/2 = 5.9 min, R 2 = 0.994) for systems using [RhCl(COD)]2 and [RhCl(CO)2]2, respectively. This means that the conversion rate with the latter catalyst is ca. 14 times faster, which may be related to the higher electrophilic character of the rhodium center in this catalytic system.

kobst = ln[%syneq/(%synt-%syneq)] (2)
(kobs = kobs+ + kobs-and %syneq = 15%) <Insert Figure 5> From the values of the equilibration rate constants (kobs++kobs-) and the equilibrium constant (kobs+/kobs-), the individual values of kobs+ and kobs-may be derived. Experiments with different catalyst concentrations have not been carried out. However, under the reasonable assumption that both direct and reverse reactions are first order in the catalyst (a catalyst influence on the rate is shown by the shorter equilibration time when the rhodium complex was used stoichiometrically), namely kobs+/-= k+/-[Rh], the second order rate constants k+ and k-for the [RhCl(COD)]2 catalyzed process are 7.31(3) x 10 -2 and 1.29(3) x 10 -2 mol -1 l min -1 , while those for the [RhCl(CO)2]2 catalyzed process are 0.995(6) and 0.175(6) mol -1 l min -1 , respectively (see Scheme

3). <Insert Scheme 3>

In the case of the N,P(O) ligand, no epimerization could be detected by using [RhCl(COD)]2 even after 24 hours. However, a slow epimerization was observed when the same experiment was conducted under a monoxide pressure and even faster under a syngas pressure (see Figure 6).

<Insert Figure 6> The increased rate in the presence of hydrogen suggests that a hydride species formed in situ catalyzes more efficiently this epimerization process. This state of affairs is analogous to that described for a rhodium-catalyzed reversible P-C bond cleavage of PPh3. 28,29 The more difficult epimerization for the N,P(O) ligand is probably the consequence of the stronger P-C bond (see Xray structure analysis discussion), resulting from the stronger electron withdrawing effect of the oxygen atom vs. the sulfur atom.

Furthermore, Figure 6 shows a faster epimerization of the oxide ligand 2 when using a greater metal/ligand ratio, as also observed for the analogous sulfide ligand 3. Because of the more stringent epimerization conditions, further studies to establish the equilibrium position were not carried out. The d.e. obtained (12%) after 24 h with a 5:1 ligand/metal ratio and under a syngas pressure is identical to the d.e. of the free ligand before crystallization (see Section 1). By comparison with the results of the sulfide ligand epimerization, this may well correspond to a thermodynamic equilibrium.

We now remark that the conditions leading to the extensive epimerization of ligand syn-2 (r.t., 24 h) are comparable to the conditions used for the hydroformylation catalysis (35°C, 3 h).

In addition, an equimolar amount of Rh and ligand are used under the hydroformylation conditions, leading to an expected much faster epimerization. Indeed, a run of syn-2:Rh = 1:1 under catalytic conditions (Table 2, entry 4) in the absence of styrene was stopped after 3 h and the recovered catalyst, upon treatment with tmeda, gave free 2 with a syn/anti ratio of 59:41 (18% d.e.). It is then reasonable to propose that the failure to observe any e.e. for this catalytic hydroformylation may be related to the ligand epimerization under syngas conditions.

Mechanistic considerations of the rhodium-catalyzed epimerization

As the observed rhodium-catalyzed ligand epimerization appears to be an unprecedented phenomenon, we have considered its possible mechanism. Some suggestions may be formulated For the P-C pathway (Scheme 5), a reversible P-C oxidative addition from (R)-F leads to the 16-electron intermediate (R)-J which may be capable of deinserting the imine to lead to (R)-K. Inversion of configuration may be achieved at this point by  2 / 1 imine rearrangement through L, without a need to decoordinate the imine. On the other hand, we have shown that the imine is expelled from the N,P(O) complex at high temperatures, with formation of a mixture of hydride products (see Section 3). As already stated above, a Rh(III)-PPh2(O) fragment in dirhodium chloride complexes have recently been proved by X-ray diffraction analysis. 19 This experimental observation is the only tenuous criterion that currently makes us favour the P-C pathway.

Conclusions and perspectives

We have described what appears to be the first example of an interesting rhodium catalyzed epimerization process. The aminophosphine oxide ligand 2 is efficient and regioselective in the rhodium-catalyzed hydroformylation of styrene, but is not an asymmetric inducer ligand. The epimerization of the chiral center between the nitrogen and phosphorus atoms under the catalytic hydroformylation conditions may be related to this lack of enantioselective induction. The epimerization occurs by a mechanism that appears to involve a reversible P-C bond cleavage.

This observation shows that this type of N,P(O) ligand, albeit sufficiently active and regioselective, can not be used in asymmetric hydroformylation with rhodium catalysts.

Furthermore, the corresponding N,P(S) ligand 3 generates a less efficient catalyst, because of an easier P-C oxidative addition and catalyst deactivation.

The development of enantioselective branched hydroformylation catalysts with this class of ligands requires structural modifications that block or strongly disfavour these epimerization and deactivation pathways. Efforts in this direction are currently being pursued in our laboratory. We also note that all epimerization mechanisms (except via direct tautomerization to intermediate I in Scheme 4) require an oxidative addition process. In the absence of rhodium, both ligands 2 and 3 are configurationally stable. Therefore, we are also interested in applying these ligands to enantioselective catalytic reactions with metals that cannot undergo oxidative addition processes.

Experimental Section

All reactions were carried out in Schlenk-type flasks under argon atmosphere. Solvents were purified and dried under dinitrogen by conventional methods. The 1 H, 31 were carried out on a Fisons EA 1108 apparatus. The imine compound was prepared by a slow addition of stoechiometric amount of distilled benzaldehyde to a dichloromethane solution of (S)methylbenzylamine at 0°C, according to literature. 31 The water formed was removed under vacuum at 120°C for 3 hrs. Compounds (S)-methylbenzylamine, sulfur, [RhCl(COD)]2 (COD :

1,5-cyclooctadiene) and [RhCl(CO)2]2 were obtained from commercial sources and used as received, while Ph2PH was prepared by the conventional method. 32 .

Formation of (R,S)-and (S,S)-Ph2PCH(Ph)NHCH(Ph)CH3, 1.

To a solution of (S)-PhCH=NCH(Ph)CH3 (0.060 g, 0.287 mmol) in CDCl3 (1 ml) was added Ph2PH (50 l, 0.287 mmol). The mixture was stirred for 12 hrs, yielding an equilibrium of 1 with the starting materials. NMR properties of compound 1: 1 H NMR (CDCl3):  = 7.85-6.87 (m, aromatics) and 2.00 (s, br, NH), for (R,S)-1: 4.70 (d, PCH, 2 J(PH) = 4.9 Hz), 3.74 (q, NCH,

Air Oxidation of (R,S)-and (S,S)-1. NMR monitoring

To a solution of (S)-PhCH=NCH(Ph)CH3 (0.060 g, 0.287 mmol) in CDCl3 (1 ml) was added Ph2PH (50 l, 0.287 mmol). After the equilibrium of 1 with the starting materials was attained (3 h), an air stream was directly introduced into the CDCl3 solution which was then stirred for 20 hrs. The 31 P{ 1 H} NMR spectrum showed that the oxidation reaction was nearly complete yielding both diastereomers of 2 with a d.e. (syn/anti) of 12%. The NMR data of syn-2 are given in the next section. NMR data of anti-2: 1 H NMR (CDCl3):  = 7.45-6.88 (m, aromatics), 4.10 (d, PCH, 2 J(PH) = 9.9 Hz), 3.61 (q, NCH, 2 J(HH) = 6.9 Hz), 2.57 (s, b, NH, exchange with D2O), 1.24 (d, CH 3 , 2 J(HH) = 6.9 Hz). 31 P{ 1 H} NMR (CDCl3):32.85 (s).

Synthesis of (R,S)-Ph 2 P(O)CH(Ph)NHCH(Ph)CH3, 2.

To a solution of (S)-PhCH=NCH(Ph)CH3 (0.940 g, 4.50 mmol) in Et2O (15 ml) was added Ph2PH (0.8 ml, 4.50 mmol). After a few secondes a white precipitate was formed. The suspension was stirred for 5 h under an inert atmosphere. An air flow was then introduced into the Et2O solution for 10 minutes in order to displace all inert gas, and stirring was continued for 20 h in air to complete the oxidation. The white powder was collected by filtration (1.51 g), dried under vacuum and dissolved in hot methanol (50 ml). After several days at -30°C, white needles were obtained and identified as the pure (R,S) isomer of 2 (0.966 g, 56%). Single crystals of syn To a solution of (S)-PhCH=NCH(Ph)CH3 (0.739 g, 3.54 mmol) in CH2Cl2 (10 ml) was added Ph2PH (0.63 ml, 3.54 mmol). The mixture was stirred for 3 h under an inert atmosphere, leading of Table 2). The 1:1 CO/H2 mixture was prepared by mixing the pure gases in a 500-ml stainless steel cylinder before introduction into the autoclave at the desired pressure (see Table 2). The zero time for the kinetic runs (see Figure 3) was considered as the time at which the desired temperature (Table 2) was reached inside the autoclave. In order to maintain as constant temperature and pressure conditions as possible during each kinetic run, only a few ml of the catalytic solution mixture were carefully withdrawn each time from the autoclave under pressure.

The CHCl3 or C6D6 solvent was then rotary evaporated at room temperature and the yelloworange viscous residue was analysed by 1 H and 31 P NMR spectroscopies in CDCl3 solution.

Crystal structure analyses of compounds syn -2 and syn/anti-3

Single crystals of syn-2 and syn/anti-3 were obtained by slow diffusion of an ethanolic solution of 2 or 3 onto the same volume of water. Intensity data were collected on a Nonius Kappa CCD at 293 K for syn -2 and 110 K for syn/anti-3. Both structures were solved by direct methods and refined by full-matrix least-squares methods (SHELXL-97) [START_REF] Sheldrick | SHELX97 (Includes SHELXS-97, SHELXL-97)-Programs for Crystal Structure Analysis (Release 97-2)[END_REF] with the aid of the WINGX program. [START_REF] Farrugia | [END_REF] Non-hydrogen atoms were anisotropically refined. With the exception of the NH hydrogen atom (for both compounds), which was located in the final difference Fourier maps and refined freely, all hydrogen atoms were included in a riding model. Crystallographic data for compounds syn -2 and syn/anti-3 are reported Table 3.

la Recherche Scientifique (CNRS) and the Conseil Regional de Bourgogne for support of this work and Mr Cédric Balan for technical assistance. 35,36 .

Captions for Figures

  4 hours and the oxidation process is irreversible. This irreversibility is demonstrated by the stability of the pure (R,S) amino-phosphine oxide (obtained by crystallization, see below) toward epimerization in CDCl3 solution. In a preparative scale process carried out in Et2O as solvent, the air-stable aminophosphine oxide ligand Ph2P(O)CH(Ph)NHCH(Ph)CH3, 2, forms with a poor d.e. (syn) of 12 % (Scheme 1). As diastereomers should exhibit different solubility properties, a separation by fractional crystallization was attempted. White needles of the pure syn-2 diastereomer were obtained quantitatively by a single crystallization from cold methanol. The 1 H and 31 P{ 1 H} NMR analyses confirmed the presence of the single syn -2 diastereomer. The (R,S) configuration of the chiral carbon atoms is confirmed by the X-ray structural analysis (see below). When the oxidation reaction was carried out in a CH2Cl2 solution with sulfur instead of oxygen, the 1 H and 31 P{ 1 H} NMR analyses of the crude product showed complete conversion after one week. The irreversibility of this transformation is shown, like for the oxide derivative, by the stability of the crystallized sample toward epimerization (vide infra). The aminophosphine sulfide ligand Ph2P(S)CH(Ph)NHCH(Ph)CH3, 3, forms with a moderately high d.e. (anti) of 70 % (Scheme 1). The different d.e.'s in the oxygen and sulfur oxidations must result from a kinetic control during the addition reaction of Ph 2 P(E)H (E = O or S) to the imine. Indeed, the 31 P NMR spectra of aliquots withdrawn during both oxidation reactions never show the presence of free Ph 2 PH. Rather, signals corresponding to Ph 2 P(E)H (E = O or S) are observed. <Insert Scheme 1>

  on the basis of previously established reactivity. Under appropriate conditions Car-H, Csp3-H or P-C oxidative additions involving PPh3 or other functionalized phosphines take place with i.e. Co(I), Rh(I) or Ir(I)[28][29][30] . Two possible catalytic pathways have therefore been considered, involving reversible C-H or P-C bond cleavages as shown in Schemes 4 and 5, respectively. <Insert Schemes 4 and 5> For the N,P(O) epimerization, the best activities were observed under CO at high pressures, where the complex probably exists under the monodentate form RhCl(CO)2( 1 -N,P(O)) according to the stoichiometric studies (see Section 3). An equilibrium with the bidentate form, RhCl(CO)( 2 -N,P(O)), however, is established at low CO pressures. For the N,P(S) epimerization, on the other hand, the monodentate form RhClL2( 1 -N,P(S)) [L2 = COD or (CO)2] seems present for both catalysts. It seems likely that the bidentate coordination is not necessary for this catalytic transformation.For the C-H pathway (Scheme 4), a reversible Csp3-H oxidative addition on the starting complex (R)-F would lead to the formation of intermediate (R)-G, stabilized by coordination of the P=E function in a pseudo-allylic moiety. Since the stereogenic carbon atom is blocked during this process, its reversal could only lead back to the starting absolute configuration. However, epimerization could occur via an  3 / 1 equilibration of the pseudo allyl ligand to give intermediate H, for which free rotation around the P-E single bond is possible. An alternative possibility is a reversible E-H reductive elimination to generate intermediate I, for which an alternative N-bonded isomeric form is also possible. Reversal of all steps leads to (S)-F. It is interesting to remark that the N-bonded isomer of I is a tautomeric form of the starting compound (R)-F. Therefore, transformation of (R)-F to (S)-F could also be achieved via this intermediate I without the need for the hydrogen atom to transit via the Rh center.

  P{ 1 H} and 13 C{ 1 H} NMR spectra were recorded at 200.13, 81.03 and 51.32 MHz, respectively, on a Bruker AC 200 instrument in CDCl3 solution. All chemical shifts are given in ppm. The infrared spectra were recorded in CH2Cl2 or in CHCl 3 solution with an IFS 66 Perkin-Elmer instrument. The  rotation polarization angle was measured on the Perkin-Elmer 141 Polarimeter. The elemental analyses

2

  J(HH) = 6.4 Hz), 1.36 (d, CH 3 , 2 J(HH) = 6.4 Hz), for (S,S)-1: 4.16 (d, PCH, 2 J(PH) = 5.4 Hz), 3.61 (q, NCH, 2 J(HH) = 6.4 Hz), 1.28 (d, CH 3 , 2 J(HH) = 6.4 Hz). d.e. (anti/syn) = 62%. 31 P{ 1 H} NMR (CDCl3):1.49 (s) for (R,S)-1and 0.89 (s) for (S,S)-1.

- 2

 2 were obtained by slow diffusion of an EtOH solution of 2 onto the same volume of water. NMR data of pure syn-2: 1 H NMR (CDCl3):  = 7.98-7.09 (m, aromatics), 4.62 (d, PCH, 2 J(PH) = 10.3 Hz), 3.64 (q, NCH, 2 J(HH) = 6.8 Hz), 2.50 (s, b, NH, exchange with D2O), 1.26 (d, CH 3 , 2 J(HH) = 6.8 Hz). 31 P{ 1 H} NMR (CDCl3):31.64 (s). 13 C{ 1 H} NMR (CDCl3):  = 148.32-128.82 (m, Caromatics), 60.40 (d, PCH, 1 J(PC) = 78.6 Hz), 55.25 (d, NCH, 3 J(PC) = 11.4 Hz), 21.95 (s, CH 3 ). IR: P=O = 1270 cm -1 (CH2Cl2), 1265 cm -1 (CHCl3). mp = 187°C. []D (CH2Cl2, 25°C, c = 3.27 g/100 ml) = -63°. Elemental analysis: Calcd for C27H26NOP: C 78.81, H 6.37, N 3.41; Found: C 78.83, H 6.45, N 3.77. Synthesis of (R,S)-and (S,S)-Ph2P(S)CH(Ph)NHCH(Ph)CH3, 3.

Figure 1 .

 1 Figure 1. Kinetic and thermodynamic controls in the formation of chiral -P,N (1) ligands.

Figure 2 .

 2 Figure 2. ORTEP views of compounds syn-2 (a) and syn/anti-3 (b). The two diastereomers in

Figure 3 .

 3 Figure 3. Conversion as a function of time for the hydroformylation of styrene at various syn-

Figure 4 .

 4 Figure 4. Rhodium-catalyzed epimerization of [Ph2P(S)CH(Ph)NHCH(Ph)Me]. Reaction

Figure 5 .

 5 Figure 5. Fitting of the kinetic data from Figure 4 by equation 2.

Figure 6 .

 6 Figure 6. Rhodium-catalyzed epimerization of syn-2. Reaction conditions: [RhCl(COD)]2

Table 1 .

 1 Selected bond distances (Å) and angles (°) for compounds syn -2 and syn/anti-3.

		syn -2 (E = O)	syn -3 (E = S)	anti -3 (E = S)
	P-E	1.4870(13)	1.9642(9)	1.9651(8)
	P-C(1)	1.832(2)	1.893(3)	1.856(2)
	N-C(1)	1.462(2)	1.452(3)	1.463(3)
	E-P-C(1)	116.66(8)	111.17(8)	112.69(8)
	P-C(1)-N	104.82(12)	111.32(16)	103.90(14)
	H-C(1)-P-E	170(1)	157(1)	163(1)

Table 2 .

 2 Hydroformylation of styrene catalyzed by [RhCl(COD)]2 and aminophosphine oxide

	(syn-2) or sulfide (syn/anti-3) ligands a
	Entry solvent	Ligand	T (°C) P (psi) % conversion b (time) % b/l b
	1	CHCl3	---	55	400	4 (3 h) 100 (20 h)	89/11
	2	CHCl3	2	55	400	88 (3 h)	100 (4 h)	89/11
	3	C6H6	2	55	400	16 (3 h) 100 (21 h) c 85/15
	4	CHCl3	2	35	500	60 (3 h) 100 (6 h) c	96/4
	5 d	CHCl3	2	35	500	35 (3 h) 100 (8 h) e	96/4
	6	CHCl3	3	55	400	0 (3 h)	30 (70 h)	95/5
	7 f	CHCl3	3	55	400	0 (3 h)	30 (67 h)	93/7
	8	CHCl3 Ph2P(O)H	35	500	3 (3 h)	50 (21h)	96/4

a Reaction conditions: [RhCl(COD)]2 (0.025 mmol), solvent (40 ml), CO+H2 (1:1), styrene/Rh/ligand: 167/1/1. b Determined by 1 H NMR spectroscopy, no ethylbenzene detected. c The e.e. at complete conversion is 0%. d Vinylnaphthalene used instead of styrene under similar conditions. e The e.e. at complete conversion is 3%. f [Rh(COD)2](CF3SO3) used instead of [RhCl(COD)]2 under similar conditions.

Table 3 .

 3 Crystal data and structure refinement for syn -2 and syn/anti-3.

	Compound	syn -2		syn/anti-3
	Formula	C27H26NOP		C27H26NSP
	M	411.46		427.52
	T; K	293(2)		110(2)
	Crystal system	orthorhombic		monoclinic
	Space group	P212121		P21
	a; Å	5.804(1)		9.0532(4)
	b; Å	16.911(3)		18.5975(9)
	c; Å	23.518(6)		13.4262(8)
	a; °	90		90
	b; °	90		92.218(2)
	g, °	90		90
	V; Å 3	2308.3(8)		2258.8(2)
	Z	4		8
	F(000)	872		904
	Dcalc; g/cm 3	1.184		1.257
	diffractometer	Kappa CCD		Kappa CCD
	scan type	mixture of  rotations	mixture of  rotations
		and  scans		and  scans
	; Å	0.71073		0.71073
	; mm -1	0.137		0.228
	Crystal size; mm 3	0.35 x 0.30 x 0.24	0.30 x 0.18 x 0.12
	sin()/ max; Å -1	0.61		0.65
	Index ranges	h: -6; 7		h: -11; 11
		k: -20; 20		k: -24; 23
		l: -28; 28		l: -17; 17
	RC = Refl. Collected	8469		14691
	IRC = independent RC	4099 [R(int) = 0.025]	9440 [R(int) = 0.033]
	IRCGT = IRC and [I>2(I)]	3776		7329
	Refinement method	Full-matrix L.S. on F 2	Full-matrix L.S. on F 2
	Data / restraints / param.	4099 / 0 / 278		9440 / 1 / 549
	R for IRCGT	R1 a = 0.036, wR2 b = 0.086	R1 a = 0.042, wR2 b = 0.075
	R for IRC	R1 a = 0.041, wR2 b = 0.089	R1 a = 0.069, wR2 b = 0.083
	Goodness-of-fit c	1.045		0.992
	Flack parameter d	0.00(9)		0.01(5)
	, max, min; e . Å -3	0.129 and -0.152		0.271 and -0.261
	a R1=(|F o |-|F c |)/|F o |. b wR2=[w(F o 2 -F c 2 ) 2 /[w(F o	2 ) 2 ] 1/2 where w=1/[ 2 (Fo 2 + (0.036P) 2 +
	0.49P] for syn -2 and w=1/[ 2 (Fo 2 +(0.031P) 2 +0.0P] for syn/anti-3 where P = (Max(Fo 2 ,0) +
	2*Fc 2 )/3. c Goodness of fit =[w(F o 2 -F c

2 ) 2 /(N o -N v )] 1/2 . d See ref.

Acknowledments

We are grateful to the Ministère de l'Education Nationale, de la Recherche et de la Technologie (MENRT) for support and for a PhD fellowship (to J.-M. C.), the Centre National de

to a pale yellow solution. Molecular sulfur (0.139 g, 4.34 mmol) was introduced into the solution and stirring was continued for seven days at room temperature. The solvent was then removed under vacuum leaving an orange viscous oil. The NMR analysis of this crude product showed both diastereomers of 3 with a d.e.(anti) of 70 %, slightly contaminated by the imine and the diphenylphosphine sulfide. Crystallization from methanol at -80 °C afforded the pure ligand (1:1 diastereomeric mixture) as a white powder (0.144 g, 29%). Single crystals were obtained by slow diffusion of an ethanolic solution of 3 onto the same volume of water. NMR data of anti-3: 1 H NMR (C6D6): 8.01-6.69 (m, aromatics), 4.53 (d, PCH, 2 J(PH) = 11.7 Hz), 3.60 (q, NCH, 2 J(HH)= 6.2 Hz), 3.23 (s, br, NH), 1.21 (d, CH 3 , 2 J(PH) = 6.5 Hz). 31

Catalytic runs

The hydroformylation reactions were carried out in a 300-ml stainless-steel Parr autoclave equipped with a magnetic drive and an internal glass vessel. The temperature was controlled by a rigid heating mantle and by a single loop cooling coil. The autoclave was purged three times under vacuum/argon before introducing the catalytic solution (for the amounts used, see footnote


Erwan Le Grognec

Rinaldo Poli
email: rinaldo.poli@u-bourgogne.fr

Philippe Richard

Prof Rinaldo Poli

A Reinvestigation of compound CpMo(PMe3)2(CH3)2: Alkylation by Single Electron Transfer and Radical Addition?

The thermally unstable compound CpMo(PMe3)2(CH3)2 has been isolated and characterized by X-ray crystallography, low temperature EPR, and cyclic voltammetry. Its formation from CpMo(PMe3)2Cl2 appears to proceed via an initial electron transfer followed by radical addition.

Introduction

A few years ago, the reaction of CpMo(PMe 3 ) 2 Cl 2 with different alkylating reagent was investigated in our laboratory. 1 The formation of the desired dialkyl compound CpMo(PMe 3 ) 2 (CH 3 ) 2 was only witnessed when using CH 3 Li as the alkylating agent, while CH 3 MgBr, Al(CH 3 ) 3 and CH 3 ZnI reacted extremely slowly or not at all or gave different products (CH 3 ZnI gave rise to a Cl/I exchange process). All attempts to isolate the product failed because of its thermal instability. Thus, the precise formulation of the compound was not proven unambiguously, as the spectroscopic characterisation in solution (limited to EPR spectroscopy) showed only the presence of two equivalent phosphorus atoms in the co-ordination sphere. Similar difficulties for the alkylation of phosphine-containing half-sandwich Mo(III) compounds were also encountered by Fryzuk et al. for the alkylation of [ 5 :P,P'-C 5 H 3 -1,3-(SiMe 2 CH 2 P i Pr 2 ) 2 ]MoCl 2 2 and by us for CpMo(Ph 2 PCH 2 CH 2 SMe)Cl 2 . 3 More recently, we have reported the synthesis, characterisation and reactivity studies of the first example of stable paramagnetic molydenum(III) complexes containing alkyl ligands.

These complexes have the general formula CpMo( 4 -diene)R 2 , (diene = butadiene, 2methylbutadiene or 2,3-dimethylbutadiene and R = methyl, benzyl or trimethylsilylmethyl). 4,5 Permethylated cyclopentadienyl analogues of these complexes have also been subsequently prepared and have shown activity in ethylene polymerisation. 6 All these alkyl compounds containing a diene ligand exhibit thermal stability in common organic solvents, 5 in remarkable contrast with the previously reported instability of CpMo(PMe 3 ) 2 (CH 3 ) 2 . In addition, the alkylation of CpMo( 4 -diene)Cl 2 compounds works equally well with lithium and Grignard reagents, while the bis-phosphine analogue could only be alkylated with the lithium reagent. For these reasons, we have decided to reinvestigate the synthesis of this phosphine-containing alkyl compound. The successful isolation and full characterisation of this compound are reported in this contribution, and the mechanism of its formation reaction is discussed in comparison with the alkylation of the diene analogues.

Experimental

General procedures. All reactions involving air-and moisture-sensitive organometallic compounds were carried out in a Jacomex glove box or by the use of standard Schlenck techniques under an argon atmosphere. Toluene and diethyl ether were purified by distillation under argon after drying over sodium benzophenone ketyl. THF was dried over sodium benzophenone ketyl and then over Na/K alloy. Pentane was dried over sodium. EPR measurement were carried out at the X band microwave frequency on a Bruker ESP 300 spectrometer, equipped with an ER 4111 VT unit. The spectrometer was calibrated with DPPH (g =2.0037). EPR spectra simulations and fittings were carried out with WinSim. 7 The resulting solution was stirred while the temperature was slowly raised. No reaction occurred until ca. -20°C, when the solution started to change colour from dark red to orange within a few minutes. Stirring was continued at a temperature between -20 and -10°C for 3 h with no further colour change, to insure complete conversion. After evaporation to dryness at -10°C, the product was extracted with cold pentane (-10°C, 15 mL). The mixture was filtered through Celite at -10°C and the resulting solution was concentrated ca 3 mL and stored at -80°C. This led to the crystallisation of the product CpMo(PMe 3 ) 2 (CH 3 ) 2 (85 mg; 55%) within a few days.

Reaction of CpMo(PMe3)2Cl2 with MeLi in presence of

PMe3. An alkylation reaction was carried under similar conditions as in the previous section, from CpMo(PMe 3 ) 2 Cl 2 (100 mg, 0.29 mmol) and MeLi (0.6 mmol) in a 1:1 THF:Et 2 O mixture (20 mL) in presence of 1 equiv. of PMe 3 (0.3 mL, 1M in THF). The alkylation was much slower as shown by the colour change and by EPR monitoring. After 9 hours at a temperature between -20°C and -10°C, solvents were evaporated and the CpMo(PMe 3 ) 3 (CH 3 ) formation as the major product was attested by 1 H NMR after comparison to his previously reported spectrum. 1 The desired CpMo(PMe 3 ) 2 (CH 3 ) 2 was found to be only the minor product by EPR (ratio 85:15). Synthesis of CpMo(PMe3)2(CD3)2. By using a procedure identical to that described above for the dimethyl compound, CpMo(PMe 3 ) 2 (CD 3 ) 2 was synthesised starting from CpMo(PMe 3 ) 2 Cl 2 (195 mg, 0.57 mmol) and CD 3 Li (1.25 mL, 0.5 M in Et2O

Crystal structure determination of complex CpMo(PMe3)2(CH3)2.

A dark red crystal of CpMo(PMe 3 ) 2 (CH 3 ) 2 (0.4x0.4x0.4 mm 3 ) suitable for an X-ray analysis was obtained from a saturated pentane solution of CpMo(PMe 3 ) 2 (CH 3 ) 2 at -80°C. The crystal was rapidly mounted on an Enraf-Nonius Kappa CCD using Mo K radiation under a cold nitrogen flux (110 K). 13262 reflections (3760 unique) were collected up to sin()/ = 0.65 at 110 K.

Absorption corrections were applied to the data during integration by the SCALEPACK 9 algorithm. The structure was solved via a Patterson search program [START_REF] Sheldrick | SHELXS and SHELXL97[END_REF] and refined (space group P21/c) with full-matrix least-squares methods 10 based on |F 2 |. All non hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms of the complex were found in Fourier difference maps and freely refined with isotropic temperature factors. Final agreement indices are reported in Table 1.

CCDC reference number xxxxxx.

See http://www.rsc.org/suppdata/xxxxxx for crystallographic files in .cif format.

<Table 1>

Results and Discussion

(a) Synthesis

As our previous study 1 showed that only CH 3 Li is able to convert CpMo(PMe 3 ) 2 Cl 2 to the dimethyl species (equation 1), we have continued our investigations with this reagent. In addition, it was previously shown that the use of THF as solvent leads to a smaller conversion to the paramagnetic product relative to toluene, this being tentatively attributed to a competition between substitution and single electron transfer pathways. With the idea in mind of limiting competing redox processes and facilitating solvent removal at low temperature, while maintaining convenient conversion rates and substrate solubility, we used a 1:1 mixture of THF and diethyl ether as solvent.

CpMo(PMe 3 ) 2 Cl 2 + 2 CH 3 Li CpMo(PMe 3 ) 2 (CH 3 ) 2 + 2 LiCl (1)
A smooth reaction occurs in the -20°C to -10°C temperature range. By carrying out all manipulations at or below -10°C (see experimental section), the product was obtained by crystallisation from cold pentane as orange crystals in 50% yield. All attempts to work at a temperature higher than -10°C invariably failed due to the thermal instability of CpMo(PMe 3 ) 2 (CH 3 ) 2 . 1 An EPR spectroscopic monitoring of the reaction gave no evidence for the presence, at any time, of an intermediate mixed compound, CpMo(PMe 3 ) 2 (CH 3 )Cl. The characteristic triplet of CpMo(PMe 3 ) 2 Cl 2 is replaced by a new triplet due to the dialkyl species, see Figure 1.

<Figure 1>

The formation reaction was found to be strongly hindered by the presence of free PMe 3 .

When the synthetic procedure was carried out in the presence of 1 equivalent of PMe 3 , the EPR monitoring showed a much slower disappearance of the starting dichloride compound. On the other hand, this reaction affords another compound as the major product, namely the reduced CpMo(PMe 3 ) 3 (CH 3 ) compound, in agreement with the previous report. 1 This retardation effect will be discussed later, together with the reaction mechanism, in section (e).

(b) EPR spectroscopic study

As shown in Figure 1b, the EPR spectrum exhibits a discernible coupling only to the Mo and the two P nuclei at 0°C. The availability of the isolated, crystalline product has now allowed a more detailed spectroscopic study in a dilute pentane solution and at lower temperatures. The resulting spectrum at -80°C shows additional superhyperfine splitting, see Figure 1c. By analogy with the CpMo( 4 -diene)R 2 complexes, 4,5 we considered that the unpaired electron could couple with the 6 methyl hydrogen atoms. However, the observed pattern contains many more lines than expected for a triplet of septets. Therefore, we considered also the possible coupling to the remote PMe 3 hydrogen nuclei. Simulations and analytical fittings 7 permitted the full interpretation of the observed spectrum as the result of coupling with the two equivalent phosphorus nuclei (aP = 25.81 G), the six equivalent hydrogen atoms of the methyl groups (aH = 6.89 G), and the eighteen equivalent hydrogen atoms of the phosphine ligands (aH = 2.04 G).

Any other coupling model (for instance including a coupling to the 5 equivalent Cp protons) did not match the experimental spectrum. Examples of hyperfine coupling to Cp protons are known, for instance for compounds CpCr(NO)(PPh 3 )(CH 2 SiMe 3 ) [START_REF] Herring | [END_REF]12 or Cp 2 Zr(allyl). 13 However, this kind of coupling was not observed for compounds CpMo( 4 -diene)R 2 . 4,5 In order to further confirm our spectral interpretation, the analogous compound CpMo(PMe 3 ) 2 (CD 3 ) 2 was also prepared by using a CD 3 Li solution. EPR spectra were recorded in diluted solutions under the same conditions used for the homologous bis-CH 3 complex. However, the larger broadness of the spectrum caused by the smaller a D hyperfine coupling and by the larger number of lines allowed only the visual observation of the phosphorus coupling. Even by working at low temperature in pentane, the spectrum of the deuterated compound is broader than that observed for the bis-CH 3 compound at room temperature and shown in Figure 1b. No reliable information on the values of the aD and a H hyperfine coupling could be obtained from the digital fitting procedure. 14

(c) Molecular structure

On the basis of this EPR spectrum and literature precedents, [START_REF] Poli | [END_REF] the "four-legged piano stool" geometry appears most likely for the complex. The EPR spectrum does not permit, however, to distinguish between the cis and trans configurations, both being characterised by equivalent PMe 3 and equivalent CH 3 ligands. Suitable crystals for an X-ray analysis were obtained from a saturated pentane solution at -80°C. The structural determination shows that the molecule adopts in fact the trans four-legged piano stool geometry, as shown in Figure 2. Table 2 collects the relevant bond distances and angles.

<Figure 2 and Table 2> The complex crystallised in the monoclinic space group P21/c with the entire molecule in the asymmetric unit. The geometry is similar to that of its precursor CpMo(PMe 3 ) 2 Cl 2 . 16 The 16 The two methyl ligands are stronger -donors than the two chloride ligands and make the Mo(III) centre electron-richer (this is confirmed by the electrochemical study, vide infra), thus an increase of metal radius is expected on going from the dichloride to the dimethyl compound. The Mo-P bond shortening can therefore be interpreted as a sign of an increased Mo-P π back-bonding caused by the greater metal electron richness. Significant Mo(III)-P π back bonding is in fact already present in the dichloride compound. 16 The discussion are also the angles between the "basal" Mo-L bonds and the Mo-CNT bond. These angular distortions have been interpreted on the basis of π and  interaction schemes. 17,18 The angle related to the X-type donors decreases slightly from X = Cl (average 117.4°) 16 to X = CH 3

[average 115.9(4)°], while the angle related to the L-type donors (PMe 3 ) increases on going from the dichloride (average 113.2°) to the dimethyl compound [average 115.0(2)°]. This trend is consistent with both the  scheme (the trans pair of ligands with stronger -donor properties yields lower angles; CH 3 is a stronger  donor than Cl) 18 and with the π scheme (better π acceptors tend to give greater angles in 17-electron compounds; PMe 3 is a better π acceptor in the dimethyl compound). 17

(d) Electrochemical study

Compound CpMo(PMe 3 ) 2 (CH 3 ) 2 was investigated by cyclic voltammetry since halfsandwich molybdenum(III) complexes usually display a rich electrochemical behaviour. [START_REF] Poli | [END_REF] The study was carried out at -20°C due to the thermal instability of CpMo(PMe 3 ) 2 (CH 3 ) 2 . The voltammogram is shown in Figure 3 and potentials are given in Table 3 in comparison with those of the parent CpMo(PMe 3 ) 2 Cl 2 compound and the analogous CpMo( 4 -C 4 H 6 )X 2 (X = Cl, CH 3 ).

<Figure 3 and Table 3> The complex displays one reductive and two oxydative one-electron, reversible processes at -2.51, -1.27 and -0.05 V, respectively. The reversibility of all processes (i a /i c = 1)

is maintained at scan rates as low as 100 mV s -1 . This is rather remarkable in view of the fact that the analogous dichloride system shows only one reversible oxidation, while the reduction (at a less extreme potential) and the second oxidation give no sign of chemical reversibility even at much higher scan rates. 16 Compound CpMo( 4 -C 4 H 6 )(CH 3 ) 2 , as well as analogues with different diene and alkyl groups, displays a reversible reduction, while the first oxidation is completely irreversible even at 1000 V . s -1 and a second oxidation is not observed 19 (see Table 3).

Therefore, no rapid chemical process seems to occur after conversion to the 18-electron complex [CpMo(PMe 3 ) 2 (CH 3 ) 2 ] -or to the 16-electron complex [CpMo(PMe 3 ) 2 (CH 3 ) 2 ] + and even to the 15-electron complex [CpMo(PMe 3 ) 2 (CH 3 ) 2 ] 2+ . It can be observed from Table 3 that all electrochemical processes are shifted towards more negative potentials relative to both CpMo(PMe 3 ) 2 Cl 2 and CpMo( 4 -C 4 H 6 )(CH 3 ) 2 , attesting to the greater electron-releasing power of CH 3 vs. Cl and 2PMe 3 vs.  4 -diene. The negative shift on going from the dichloro to the dimethyl system is smaller for the bis-phosphine system (-0.36 V) than for the butadiene system (-0.98 V). This may be attributed to an electronic "saturation" phenomenon: while the diene ligand can buffer the excess electron density received from the methyl ligands by a more effective Mo-diene π-back bonding interaction, the less π acidic and more strongly -donating phosphine ligands are less capable of establishing an equally efficient buffering effect.

The different chemical reversibility of the reductive processes for the different compounds in Table 3 can be tentatively interpreted on the basis of the possible loss of a chloride ligand, resulting from unfavourable filled-filled repulsions in the reduced 18-electron complex [CpMo(PMe 3 ) 2 Cl 2 ] -. These repulsions are related to the -donor nature of the chloride ligand and are therefore absent in the dimethyl systems. In addition, Cl -is a much better leaving group than CH 3 -. Conversely, the 16-electron complex formed by oxidation should be stabilised to a greater extent by -donation in the case of the dichloride ligand and this process is indeed reversible. On the other hand, the oxidation of the dimethyl complex still produces a stable species, in spite of the lack of π-stabilisation. Previous studies by us [20][21][22][23][24][25] and others [26][27][28][29][30] have shown that 16-electron complexes may be stabilised by adopting a triplet ground state. This phenomenon may prevent co-ordination of donor molecules including solvents such as THF, since this necessitates an expensive electron pairing process to vacate the necessary orbital for co-ordination. 20 Although we have made no attempt to isolate this oxidation product, we believe that it probably adopts a triplet ground state. Previous examples of triplet 16-electron halfsandwich Mo(IV) compounds are known, 16,[31][32][33] although none contains alkyl ligands. Even more remarkable is the relative stability (on the CV time scale) of the 15-electron [CpMo(PMe 3 ) 2 (CH 3 ) 2 ] 2+ species. This is a d 1 species and possesses a vacant orbital which can be used to accommodate an additional 2-electron donor. It is isoelectronic with the stable CpMoCl 4 and its Cp* analogue, which also resist co-ordination of THF but do in fact add stronger donors such as phosphine ligands. 31,34,35 Compound Cp*Mo(CH 3 ) 4 is also a stable 15electron compound. 36 The lack of reversibility for the second oxidation of CpMo(PMe 3 ) 2 Cl 2 could indeed be ascribed to solvent co-ordination to the highly unsaturated [CpMo(PMe 3 ) 2 Cl 2 ] 2+ species, while the analogous dimethyl complex may resist or undergo a slower solvent co-ordination process for steric and/or electronic reasons.

(e) Formation mechanism

The retardation effect of free PMe 3 on the alkylation reaction clearly indicates the intervention of a phosphine dissociation pre-equilibrium, leading to the reactive species. Another example of an alkylation reaction retarded by free phosphine has been previously reported. 37 Thus, CH 38,39 Now, two competing reactions may take place. Co-ordination of PMe 3 affords an 18-electron complex [CpMo(PMe 3 ) 2 Cl 2 ] -while the methyl radical diffuses away from the metal centre and decays. Subsequently, the resulting Mo(II) complex continues its path to afford the observed reduction product, CpMo(PMe 3 ) 3 (CH 3 ). Alternatively, trapping of the methyl radical by the 16-electron Mo(II) intermediate affords again a Mo(III) species, [CpMo(PMe 3 )Cl 2 (CH 3 )] -, with a 17-electron configuration. Subsequent ligand exchange processes complete a formal Cl/CH 3 exchange process and a second exchange may subsequently take place by the same succession of events. This course of actions would be determined by the following circumstances.

<Scheme 1>

(i) Alkylating agents are not able to attack the 17-electron CpMo(PMe 3 ) 2 Cl 2 species by nucleophilic addition. In fact, this species was shown previously to resist the addition of nucleophiles, because the singly occupied orbital is engaged in a 2c-3e π interaction with the chloride lone pairs which raises the orbital energy. Compounds of type CpMoX 2 L 2 have been shown to undergo dissociative ligand exchanges via 15-electron intermediates 40,41 rather than the typical associative exchanges for organometallic 17-electron compounds. [42][START_REF] Trogler | Organometallic radical processes[END_REF][START_REF] Baird | [END_REF] (ii) Neither CH 3 MgBr nor the stronger reductant methyllithium is able to transfer an electron to CpMo(PMe 3 ) 2 Cl 2 , because of the large negative reduction potential of this species (see Table 3). However, phosphine dissociation renders the metal less electron-rich and therefore more easily reducible. We are not aware of electrochemical studies on pairs of stable complexes that are related by a phosphine association/dissociation process, thus we cannot estimate the potential shift caused by the PMe 3 dissociation. A Cl -dissociation was shown to shift the potential by ca. +1.2 V for the [Cp*Mo IV/V Cl n (PMe 3 )] (n = 3 vs. 4) system. 45 Evidently, the reduction potential of the CpMo(PMe 3 )Cl 2 15-electron intermediate must be accessible only to the more strongly reducing lithium reagent.

(iii) Phosphine dissociation from CpMo(PMe 3 ) 2 Cl 2 is a relatively facile process as shown by both experimental 41 and theoretical studies. [46][47][48] A reason for this favourable dissociation is a spin change, the 15-electron intermediate adopting a spin quartet ground state configuration. According to the DFT calculations of the spin crossover point, 48 the activation energy for this dissociation process would be of the order of 12 kcal/mol, corresponding to a dissociation rate of ca. 10 4 s -1 at room temperature.

(iv) Nucleophilic addition of the alkylating agent to the 15-electron intermediate, although possible, does not compete with the SET process for the methyllithium reagent. It is not excluded that the less reducing Grignard reagent could in fact slowly alkylate CpMoCl 2 (PMe 3 ). However, this process is not synthetically useful because of the thermal instability of the product. (vi) The one-electron reduction of a spin quartet CpMo(PMe 3 )Cl 2 species leads directly to the 16-electron product in the triplet spin state, i.e. the expected ground state for this species.

Under these circumstances, the recombination between 16-electron [CpMo(PMe 3 )Cl 2 ] -and spin doublet CH 3

• should be essentially barrierless. Phosphine co-ordination, on the other hand, requires a spin pairing and could be kinetically retarded by the need to attain a higher-energy crossover point. 21,23 These may be favourable circumstances leading to a preference for this radical recombination pathway.

It must be remarked that the reduction of transition metal halides by organolithium and magnesium reagents is a well known occurrence, often representing a nuisance to the synthetic organometallic chemist. However, the subsequent fast recombination of the organic radical with the metal centre leading to the formal substitution product has not been previously established to the best of our knowledge. On the other hand, an analogous mechanism has been demonstrated

for the conversion of [Cp*Mo(CO) 3 (PR 3 )] + to Cp*Mo(CO)(PR 3 ) 2 H by the action of LiAlH 4 . 49 Furthermore, SET processes followed by radical recombination are quite well established for Wurtz coupling reactions involving easily reducible halide substrates and organolithium or magnesium reagents. [START_REF] March | Advanced Organic Chemistry[END_REF] These mechanisms have been evidenced by ESR [START_REF] Russel | [END_REF] and CIDNP [52][53][54][55][56] techniques. In our case, it would be impossible to use these techniques, because the starting Mo(III) compound is itself paramagnetic.

On the basis of the above mechanistic proposal, we are led to reconsider the recently reported alkylation process for the analogous CpMo( 4 -diene)Cl 2 compounds. 5 As shown in There are additional considerations leading to a proposal of an SET-radical addition mechanism for the alkylation of CpMo( 4 -diene)Cl 2 complexes. While all alkylation reactions gave good yields for the desired substitution product without the observation of reduction byproducts by using either lithium of Grignard reagents, the corresponding arylations with arylmagnesium bromide gave mixtures of substitution and reduction (yielding [CpMo( 4diene)Br] 2 ), while the allylation reaction with allylmagnesium bromide gave reduction exclusively. 57 Thus, counterintuitively, the more weakly reducing allyl Grignard gives exclusively reduction while the more strongly reducing alkyllithium gives exclusively substitution. This observation would in fact be rationalised if a mechanism analogous to that shown in Scheme 1 also operates for the reaction of the diene complexes with alkylating agents (both lithium and Grignards). Following the SET step, the fate of the reduction products is determined by the reactivity and stability of the radical. The radical addition reaction prevails for the more reactive alkyl radicals, whereas the stabilised and less reactive allyl radical escapes the "radical cage" leading to reduction. The aryl radicals have an intermediate reactivity and lead to both pathways.

Conclusion

The alkylation product from the reaction between CpMo(PMe 3 ) 2 Cl 2 and CH 3 Li has been isolated and fully characterised. Its formation reaction necessitates phosphine dissociation and appears to proceed by a SET process followed by radical addition. Transition metal reduction by organolithium and magnesium reagents is a well known process, but cases where recombination of the organic radical with the metal fragment to afford the product of formal alkyl/halide exchange have apparently not been described This is, on the other hand, a well established reaction sequence in organic Wurtz coupling reactions. In the presence of excess phosphine, the rate of alkylation decreases and the reaction is diverted toward the formation of reduction products. The EPR spectrum of CpMo(PMe 3 ) 2 (CH 3 ) 2 shows an unusual multiplicity due to four kinds of atoms, including the remote PMe 3 protons. The cyclic voltammetry exhibits three reversible one electron processes at more negative potentials than the corresponding waves of the parent dichloro complex. The mechanism of decomposition of this thermally unstable compound and the nature of its decomposition products will be the topics of further investigations.

Captions for Figures

  Cyclic voltammograms were recorded with an EG&G 362 potentiostat connected to a Macintosh computer through MacLab hardware/software. The electrochemical cell is a locally modified Schlenk tube. The cell is fitted with a Pt counter-electrode, a Ag/AgCl reference electrode and a Pt working electrode. Bu 4 NPF 6 was used as supporting electrolyte at a concentration of 0.1 M. All potentials are reported vs the Cp 2 Fe/Cp 2 Fe + couple which was introduced under argon into the cell at the end of each measurement. Elemental analyses were performed with a Fisons EA 1108 apparatus. CH 3 Li (1 M in THF/cumene 10/90) and CD 3 Li (0.5 M in Et 2 O) were used as received from commercial sources. Complex CpMo(PMe 3 ) 2 Cl 2 was synthesised according to the literature procedure. 8 Synthesis of CpMo(PMe3)2(CH3)2. All the steps of this procedure were carried out at a temperature lower or equal to -10°C. To solid CH 3 Li (1 mmol obtained after evaporation of 1 mL of the 1 M THF/cumene solution), CpMo(PMe 3 ) 2 Cl 2 (155 mg, 0.45 mmol) was added. A THF:Et 2 O (1:1) solution (30 mL) pre-cooled to -50°C was transferred into the reaction flask.

  (v) A small concentration of free PMe 3 leads to a preferential recombination of the methyl radical with the 16-electron [CpMo(PMe 3 )Cl 2 ] -intermediate, whereas the co-ordination of PMe 3 favourably competes when this is present in larger concentrations. This scheme is in perfect agreement with the observed outcome of the alkylation process as a function of free [PMe 3 ].

  diff. peak and hole; e.Å -3 0.52 and -0.65 a R1=(|Fo|-|Fc|)/|Fo|. b wR2=[w(Fo 2 -Fc 2 ) 2 /[w(Fo 2 ) 2 ] 1/2 where w=1/[ 2 (Fo 2 )+(0.0190P) 2 +1.66P] where P=(Max(Fo 2 ,0)+2*Fc 2 )/3 c Goodness of fit =[w(Fo 2 -Fc 2 ) 2 /(No-Nv)] 1/2 .

  Figure 1. EPR spectra of: (a) CpMo(PMe 3 ) 2 Cl 2 at 0°C in THF; (b) CpMo(PMe 3 ) 2 (CH 3 ) 2 at 0°C in THF; (c) CpMo(PMe 3 ) 2 (CH 3 ) 2 at -80°C in pentane: experimental (above) and best fitting (below).

Figure 2 .

 2 Figure 2. An ORTEP 58 view of compound CpMo(PMe 3 ) 2 (CH 3 ) 2 with thermal ellipsoids drawn at the 30% probability level.

Figure 3 .

 3 Figure 3. Cyclic Voltammogram of CpMo(PMe 3 ) 2 (CH 3 ) 2 . Solvent = THF; T = -20°C; v = 200 mV . s -1 .

Figure 1

 1 Figure 1

  Anal. Calcd. for C13 H 29 P 2 Mo: C, 45.49; H, 8.52. Found: C, 45.22, H, 8.47. The compound decomposes slowly in solution at room temperature, but is indefinitely stable as a solid if kept at -10°C or below under an inert atmosphere. EPR (pentane, -80°C): multiplet with Mo satellites, g = 2.003; a P = 25.81 G; a H = 6.89 G (6H); a H = 2.04 (18H); a Mo = 29.81 G.

  10 two structures are in fact isomorphous. The Cp ligand is oriented in such a way that one C atom is eclipsed with atom C(6). Thus, the molecule is pseudo-Cs-symmetric, the mirror plane passing through atoms Mo, C(6), and C(7). Like the dichloride precursor, the two phosphine ligands are related by this pseudo symmetry operation. The Mo-C(alkyl) distances are significantly longer than in CpMo( 4 -C 4 H 4 Me 2 -2,3)(CH 3 )2 (average 2.224(9) Å),5 which is only other structurally characterised alkylmolybdenum(III) compound with a 17-electron configuration. In turn, those distances were found longer than all previously determined Mo(III)-alkyl distances. This trend can be related to the electron donating properties of the neutral ligands (2PMe 3 >  4 -C 4 H 4 Me 2 -2,3), thereby affecting the atomic radius of the molybdenum centre. The Mo-P distances, on the other hand, are significantly shorter than those in the parent CpMo(PMe 3 ) 2 Cl 2 structure (average 2.484(2) Å).

  PMe 3 ) 2 Cl 2 and 2.020(5) Å for CpMo( 4 -C 4 H 4 Me 2 -2,3)Me 2 ]. Parameters of interest for

Mo-CNT (Cp ring centroid) distance is intermediate between those of the above mentioned compounds [1.939(2) Å for CpMo(

  PMe 3 )Cl 2 ] -and a methyl radical. The 16-electron complex most likely adopts a spin triplet configuration, by analogy with the previously described Cp*Mo(PMe 3 ) 2 Cl and Cp*Mo(dppe)Cl complexes.

3

Li does not react with the 17-electron CpMo(PMe 3 ) 2 Cl 2 compound directly or at least not at a significant rate. This observation is rather enlightening when compared with the following additional observations available from previous studies: (i) compound CpMo(PMe 3 ) 2 Cl 2 is not alkylated by Grignard reagents; 1 (ii) compounds of type CpMo( 4diene)Cl 2 are alkylated equally effectively by both lithium and Grignard reagents; 5 (iii) compounds CpMo( 4 -diene)Cl 2 are more easily reducible than CpMo(PMe 3 ) 2 Cl 2 (see Table

3

);

5

(iv) alkyllithium reagents are stronger reductants than the corresponding Grignard reagents.

On the basis of the above observations, we propose an alkylation mechanism for the CpMo(PMe 3 ) 2 Cl 2 species as shown in Scheme 1. This starts with phosphine dissociation to generate the 15-electron CpMo(PMe 3 )Cl 2 intermediate. Methyllithium undergoes a single electron transfer (SET) process with this intermediate to produce the 16-electron complex [CpMo(

Table 3 ,

 3 the diene ligands makes the metal more easily reducible relative to two PMe 3 ligands. The reduction potential of CpMo( 4 -C 4 H 6 )Cl 2 is about one Volt less negative than that of CpMo(PMe 3 ) 2 Cl 2 . It is therefore possible that both CH 3 Li and CH 3 MgBr are able to reduce the diene compound directly or, even more favourably, after partial deco-ordination of the diene ligand to afford a 15-electron CpMo( 2 -C 4 H 6 )Cl 2 .

Table 1 .

 1 Crystal data and structure refinement for CpMo(PMe 3 ) 2 (CH 3 ) 2 .

	Formula	C13H29P2Mo
	M	343.24
	T; K	110(2)
	Crystal system	monoclinic
	Space group	P21/c
	a; Å	13.0760(4)
	b; Å	10.0550(5)
	c; Å	12.7220(4)
	; °	93.191(2)
	V; Å 3	1670.08(11)
	Z	4
	F(000)	716
	Dcalc; g/cm 3	1.365
	diffractometer	Enraf-Nonius KappaCCD
	scan type	mixture of  rotations and  scans
	; Å	0.71073
	; mm -1	0.955
	Crystal size; mm 3	0.4 x 0.4 x 0.4
	sin()/ max; Å -1	0.65
	Index ranges	h: -16; 16

Table 3 .

 3 Cyclic voltammetric data of CpMo(PMe 3 ) 2 (CH 3 ) 2 and comparison to Values are potentials in Volts versus Cp2Fe/Cp2Fe + : E 1/2 for reversible processes, E p,a or E p,c for irreversible oxidation or reduction processes, respectively, measured at 200 mV . s -1 . b In THF at -20°C. c In CH 2 Cl 2 at room temperature. d In THF at room temperature.

	CpMo(PMe 3 ) 2 Cl 2 a
		Mo(II)/Mo(III) Mo(III)/Mo(IV) Mo(IV)/Mo(V)	ref.
	CpMo(PMe 3 ) 2 (CH 3 ) 2 b	-2.51	-1.27	-0.05	this work
		reversible	reversible	reversible
	CpMo(PMe 3 ) 2 Cl 2 c	-2.15	-0.52	+0.68	16
		irreversible	reversible	irreversible
	CpMo( 4 -C 4 H 6 )(CH 3 ) 2 d	-2.09	-0.17	-	19
		reversible	irreversible	-
	CpMo( 4 -C 4 H 6 )Cl 2 d	-1.11	0.73	-	19
		reversible	irreversible	-

a

Acknowledgement

We are grateful to the MENRT for a doctoral fellowship to ELG, and to the CNRS and the Conseil Régional de Bourgogne for support of this work.