Dolores Morales

Rinaldo Poli
email: poli@u-bourgogne.fr

Jacques Andrieu

Protonation and oxidation chemistry of a pentaethylcyclopentadienyl-containing molybdenum(IV) trihydride complex

Keywords: Protonation, Oxidation, Molybdenum(IV) complexes, Trihydride complexes, Pentaethylcyclopentadienyl complexes

Compound Cp Et MoCl4 (Cp Et = η 5 -C5Et5) (1) can be transformed into Cp Et MoH3(dppe) (2) and Cp Et MoD3(dppe) (2-d 3 ) [dppe = 1,2-(diphenylphosphino)ethane] by reaction with LiAlX4 (X = H and D, respectively). The protonation and oxidation studies of these two compounds, in comparison with previously reported studies on (C5Me5) analogs, show important differences that may be attributed to a kinetic stabilization of the products, which is steric in nature. Protonation of 2 with HBF4 in acetonitrile affords [Cp Et MoH4(dppe)] + (3), which only slowly decomposes to [Cp Et MoH2(MeCN)(dppe)] + (4). Further protona-tion of the latter affords the monohydride species [Cp*MoH(dppe)(MeCN)2] 2+ in three different forms, 5 -7. Direct protonation of 2 with 2 equiv. of HBF4 shows the formation of all of the above compounds, plus a new compound, [Cp Et MoH3(dppe)(MeCN)] 2+ (8), to which a classical structure is assigned. The protonation of 2-d 3 indicates reversibility for the proton transfer processes. The oxidation of 2 in MeCN affords [2] '+ , which decomposes slowly in MeCN to afford a mixture of 4 and 5 as major products. No compound 8, on the other hand, is obtained by oxidation of 2, neither with 1 nor with 2 equiv. of oxidizing agent. Mechanistic schemes that rationalize all these observations are proposed.

Introduction

Polyhydride complexes have long attracted considerable attention for a variety of reasons, both practical and fundamental, such as hydrogen storage, hydrogenation catalysis, molecular dynamics, chemical bonding, and so forth [1][START_REF] Parshall | Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes[END_REF][START_REF]Transition Metal Hydrides[END_REF][START_REF] Crabtree | [END_REF][5]. Our own interest is the kinetic stabilization of paramagnetic complexes and their eventual transformation into highly unsaturated, reactive intermediates. The half-sandwich derivatives of type (η 5 -C5R5)MH3L2 for the heavy Group 6 metals (Mo and W) were initially developed with unsubstituted or singly substituted cyclopentadienyl ligands by the Green group [6][7][8]. We have more recently reported that the use of the sterically encumbering Cp* ligand for Mo systems provides kinetic stabilization to reactive derivatives, such as the protonation and the one-electron oxidation products. Thus, protonation of Cp*MoH3(dppe) affords complex [Cp*MoH4(dppe)] + , which is stable in THF and has a short lifetime in MeCN before decomposing to [Cp*MoH2(MeCN)(dppe)] + with loss of H2 [9,10]. Less substituted Cp derivatives, on the other hand, do now allow the observation of the tetrahydrido intermediate in MeCN. In addition, oxidation of the same compound affords [Cp*MoH3(dppe)] '+ , which slowly decomposes in THF or CH2Cl2 by H2 reductive elimination and in MeCN by disproportionation or proton transfer, depending on conditions [11,12]. Again, similar derivatives with less sterically encumbering cyclopentadienyl rings do not lead to observable one-electron oxidation products. We have therefore resolved to pursue our search for sterically more encumbering systems in order to control the rate of decomposition of the above-mentioned reactive products and possibly trap other previously unobserved intermediates. We report here our protonation and oxidation studies on the pentaethylcyclopentadienyl complex Cp Et MoH3(dppe) (Cp Et =C5Et5) and on the corresponding deuteriated analog, Cp Et MoD3(dppe), which have allowed us to obtain new information on the protonation mechanism and to identify a previously undetected product of double protonation, [Cp Et MoH3(MeCN)(dppe)] 2+ .

Experimental

General procedures

All the manipulations were carried out under an atmosphere of argon by the use of Schlenk line or glovebox techniques. Solvents were dried and distilled by conventional methods (MeOH from Mg, THF from Na/K/benzophenone, Et2O, toluene and pentane from Na-benzophenone, MeCN from CaH2 and CH2Cl2 from P2O5) prior to use. All deuteriated solvents were degassed by three freeze-pump-thaw cycles prior to use. All except MeOD, CF3COOD and D2O were previously dried over molecular sieves. HBF4•OEt2 was used as received, without further purification. 1 H and 31 P{ 1 H} NMR measurements were carried out on a Bruker AC-200 spectrometer. The peak positions are reported with positive shifts downfield of SiMe4 as calculated from the residual solvents peaks ( 1 H) or downfield of external 85% H3PO4 ( 31 P). EPR measure-ments were carried out at the X-band microwave frequency on a Bruker ESP300 spectrometer. The spectrometer frequency was calibrated with diphenylpicrylhydrazyl (g =2.0037). Cyclic voltammograms were carried out at 20°C in a three-electrode cell with an EG&G 283 potentiostat connected to a Macintosh computer via a MacLab analog/digital converter. The working electrode was a 3 mm diameter carbon disk or a 0.5 mm diameter platinum disk. Bu4NPF6 (0.1 M) was used as supporting electrolyte. All potentials are reported relative to the ferrocene standard, which was added to each solution and measured at the end of the experiments. The elemental analyses were carried out by the analytical service of the Laboratoire de Synthe `se et d'Electrosynthe `se Organome ´talliques with a Fisons Instruments EA1108 analyzer. PhICl2 [13] and [Cp2Fe][BF4] [14] were prepared according to the literature procedures.

[Cp2Fe][BAr '4] (Ar' = 3,5-CH3(CF3)2) was prepared according to the previously reported protocol for the preparation of [Cp2Fe][BPh4] [14] from Cp2Fe and Na[BAr '4] [15].

Synthesis of Cp Et MoCl4 (1)

This method is a straightforward adaptation of the previously reported [16,[START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] synthesis of Cp*MoCl4. To a solution of Cp Et Mo(CO)3CH3 (3.02 g, 7.54 mmol) in 50 ml of CH2Cl2 was slowly added a suspension of PhICl2 (5.186 g, 18.85 mmol) in 50 ml of CH2Cl2 causing CO evolution. During the addition the color changed from yellow to red. After stirring for 1 h at room temperature (r.t.), the solvent was evaporated to approximately 50 ml and pentane (50 ml) was added causing the precipitation of 1 as a red crystalline solid. The product was filtered off, washed with pentane (5 ×10 ml) and dried in vacuo. Yield: 2.624 g, 78%. Anal. Calc. for C15H25Cl4Mo: C, 40.66; H, 5.69. Found: C, 40.41; H, 5.71%. EPR (CH2Cl2): g =1.990 (s, with Mo satellites, aMo=38.8 G).

Synthesis of Cp Et MoH3(dppe) (2)

This synthesis follows the previously reported protocol for the preparation of Cp*MoH3(dppe) [10]. To a solution of Cp Et MoCl4 (2.258 g, 5.09 mmol) in a mixture of toluene-ether (60 ml/20 ml) was added a solution of dppe (2.03 g, 5.09 mmol) in 30 ml of toluene, causing a color change from red to brown. LiAlH4 (5.09 g, 49.37 mmol) was added resulting in gas evolution and a color change to orange within 15 min. The mixture was stirred overnight to give a yellow solution. MeOH (approximately 12 ml) was added dropwise with stirring until H2 evolution ceased, causing a darkening of the solution. The stirring was continued for 1 h at r.t., followed by evaporation to dryness. The residue was extracted with pentane (approximately 100 ml) and filtered through Celite. Concentration of the solution to approximately 5 ml yielded 2 as a microcrystalline yellow precipitate. The product was filtered off, washed with cold pentane and dried under reduced pressure. Yield: 2.945 g, 82%. Anal. Calc. for C41H52MoP2: C, 70.07; H, 7.46. Found: C, 70.24; H, 7.68%. 31 P{ 1 H} NMR (C6D6, d): 90.9. 1 H NMR (C6D6, d): 7.8 -7.0 (m, 20H, dppe Ph), 2.30 (q, JHH=7.5 Hz, 10H, Cp Et CH2), 1.95 (d, JPH=15.1 Hz, 4H, dppe CH2), 1.11 (t, JHH=7.3 Hz, 15H, Cp Et CH3), -5.42 (t, JPH=48.3 Hz, 3H, Mo H). Cyclic voltammetric inves-tigations showed a reversible oxidation at E1/2 =-0.76 V (MeCN) or -0.82 V (CH2Cl2), followed by a second irreversible oxidation with Ep,a=+0.66 (MeCN) or +0.72 (CH2Cl2).

Synthesis of Cp Et MoD3(dppe) (2-d 3 )

This compound was prepared following the same procedure reported previously [10] for Cp*MoD3(dppe) and above for compound 2. To a solution of Cp Et MoCl4 (0.366 g, 0.82 mmol) in a mixture of toluene-ether (30 ml/5 ml) was added dppe (0.329 g, 0.82 mmol) and LiAlD4 (0.340 g, 7.95 mmol) causing gas evolution. The mixture was stirred overnight resulting in a yellow solution. MeOD (approximately 3 ml) was added dropwise causing D2 evolution and a darkening of the solution. The stirring was continued for 1 h and then the mixture was evaporated to dryness. The residue was extracted with pentane (approximately 50 ml) and filtered through Celite. Concentration of the solution under vacuum to approximately 2 ml precipitated 2-d 3 as a microcrystalline yellow solid, which was washed with CH3CN and dried in vacuo. Yield: 0.368 g, 64%. 31

Synthesis of [Cp Et MoH4(dppe)][BF4] (3)

To a solution of Cp Et MoH3(dppe) (0.071 g, 0.10 mmol) in 10 ml of Et2O at r.

Synthesis of [Cp Et MoH2(dppe)(CH3CN)][BF4] (4)

Compound 3 (0.020 g, 0.025 mmol) was dissolved in CH3CN (2 ml) and the solution was stirred for 2 h, filtered through Celite and concentrated under reduced pressure to approximately 0.5 ml. Diethyl ether (5 ml) was added and the solution was cooled to -20°C for 12 h, causing the precipitation of 4 as a microcrystalline brown solid. The product was isolated by filtration, washed with pentane and dried in vacuo. Yield: 0.015 g, 72%. Anal.

Synthesis of [Cp Et MoH(dppe)(CH3CN)2][BF4]2 (5)

To a suspension of Cp Et MoH3(dppe) (0.054 g, 0.076 mmol) in 3 ml of CH3CN was added HBF4•OEt2 (21 ml, 0.126 mmol) at r.t. The resulting brown solution was stirred for 3 h, resulting in a color change to red. The solution was filtered through Celite and evaporated to dryness. The residue was washed with toluene and redissolved in CH3CN (1 ml). Slow diffusion of Et2O (5 ml) To a suspension of Cp Et MoH3(dppe) (0.078 g, 0.011 mmol) in 0.5 ml of CD3CN at r.t. was added HBF4•OEt2 (1.5 ml, 0.011 mmol). A brown solution was immediately obtained. Monitoring this solution by 1 H NMR spectroscopy showed the presence of only two complexes, the hydride species 3 and 4, and the slow conversion of the former into the latter (see Results and Discussion, Sections 3 and 4).

With 2 equiv. of HBF4•OEt2

Various experiments analogous to that described in the previous section were carried out by using com-pounds Cp Et MoH3(dppe) and HBF4•OEt2 in a 1:2 ratio in CD3CN. The reactions were monitored by 1 H, 31 P NMR spectroscopy, and various other reagents (i.e. NEt3 or D2O) were added at different times after the beginning of the reaction. Compound 5 (NMR proper-ties identical with those reported above) and the new compounds 6, 7 and 8 were observed, as detailed in Section 3. Compound 6: 31

Protonation of compound 4

2.9.1. In THF To a solution of [Cp Et MoH2(dppe)(MeCN)][BF4] (0.018 g, 0.022 mmol) in 1 ml of THF was added HBF4•OEt2 (3 ml, 0.022 mmol) at r.t. The brown solution was stirred for 3 min and the solvent was evaporated in vacuo, followed by redissolution into CD3CN for 1 H NMR analysis, which showed the presence of compounds 5, 6 and 7 (see Results, Section 3).

In CD3CN

Scheme 1.

To a solution of [Cp Et MoH2(dppe)(MeCN)][BF4] (obtained directly by decomposition of 3 (0.025 g, 0.031 mmol) in 0.5 ml of CD3CN) was added HBF4•OEt2 (3 ml, 0.022 mmol), followed by direct monitoring of the reaction by1 H NMR spectroscopy (see Results, Section 3).

Protonation of compound 2-d 3 in acetonitrile 2.10.1. With 1 equiv. of HBF4•OEt2

HBF4•OEt2 (6.5 ml, 0.05 mmol) was added to a suspension of Cp Et MoD3(dppe) (0.032 g, 0.05 mmol) in 0.5 ml of CD3CN at r.t. in a 5-mm NMR tube, causing the formation of a brown solution. The evolution of the NMR properties is reported in the Results. After the conversion was complete (approximately 3 h), a second equivalent of HBF4•OEt2 (6.5 ml, 0.05 mmol) was added to the NMR tube (see Results, Section 3).

With 2 equi6. of HBF4•OEt2

HBF4•OEt2 (5.5 ml, 0.04 mmol) was added to a suspension of Cp Et MoD3(dppe) (0.014 g, 0.02 mmol) in 0.5 ml of CD3CN at r.t., causing the formation of a brown solution. The solution was transfered to an NMR tube and the reaction was monitored by 1 H NMR spectroscopy (see Results, Section 3).

Chemical oxidation of compound 2 by [Cp2Fe][BF4] in CD3CN

2.11.1. With 1.5 equiv.

To a suspension of Cp Et MoH3(dppe) (0.017 g, 0.024 mmol) in 0.5 ml of CD3CN at -20°C was added [Cp2Fe][BF4] (0.010 g, 0.036 mmol). An immediate reaction occurred completely dissolving the yellow starting material and yielding a red-brown solution. An EPR spectrum recorded at r.t. exhibited a complex pattern, indicating a mixture of paramagnetic products. These signals disappeared within 1 h and were replaced by a triplet at g = 1.984, aP = 27.0 G, aMo = 47.6 G. A 1 HNMR spectrum of the solution revealed the presence of compounds 4 and 5 in an approximately 1:1 ratio, plus an unidentified non-hydridic product characterized by a 31 P{ 1 H} NMR signal at δ 41 ppm.

With 2 equiv.

A mixture of Cp Et MoH3(dppe) (0.024 g, 0.034 mmol) and Cp2FeBF4 (0.019 g, 0.068 mmol) was dissolved in 0.5 ml of CD3CN. An NMR spectrum recorded after approx-imately 5 min showed a mixture of the hydride products 3 and 5. Compound 3 decomposes to 4 in approximately 90 min. At this time, the 4:5 ratio was approximately 6:4. An EPR spectrum of this solution showed a binomial triplet (g =1.986, aP=26.2). The subsequent addition of NEt3 (5 ml, 0.034 mmol) to this solution did not cause anyvariationof NMRproperties, while the EPRspectrum became silent.

Results

Synthesis and characterization of Cp Et MoH3(dppe) and Cp Et MoD3(dppe)

The starting compound for our studies, Cp Et MoH3(dppe), has been prepared by a straightforward adaptation of the method which had previously been developed for the analogous Cp* system (see Scheme 1) [10]. Oxidation of Cp Et MoMe(CO)3 with PhICl2 affords Cp Et MoCl4 (1) by analogy with the PCl5 oxidation of the corresponding Cp* system [START_REF] Murray | [END_REF]. This compound is more soluble than its Cp* counterpart but otherwise shows identical chemical, physical and spectroscopic properties. Treatment of 1 with LiAlH4 and dppe affords the trihydride derivative Cp Et MoH3(dppe) (2) in good yields. Use of LiAlD4 according to the same protocol affords 2-d 3 , which shows no visible contamination by 2-d n (n < 3) species by 1 H NMR. The NMR properties of compounds 2 and 2-d 3 , particularly the hydride chemical shift and JHP coupling constant in the 1 H NMR, closely parallel those reported for the Cp* analogs [10,12].

The electronic properties of 2 also parallel those of the Cp* analog; a reversible one-electron oxidation is observed at -0.76 V in MeCN and -0.86 V in CH2Cl2. Thus, the Cp Et ligand has essentially the same electron donating power as the Cp* ligand for this system. The oxidation product, however, is much less susceptible to decomposition. The oxidation wave of the Cp* system was reported to be fully reversible only in CH2Cl2, while reversibility was achieved in MeCN only at scan rates greater than approximately 100 mV s -1 . On the other hand, the oxidation wave of the pentaethylcyclopentadienyl analog is reversible in both solvents at scan rates as low as 10 mV s -1 . The EPR spectroscopic properties of the oxidation product, [2] +' , which was generated in a dry 1 THF or dichloromethane solution by chemical oxidation with Cp2Fe + , matched closely those of the Cp* analog [11,12]. Although the oxidation of 2 in MeCN is chemically reversible on the timescale of the cyclic voltammogram, the chemical oxidation leads neverthe-less to decomposition products relatively rapidly. Be-fore examining the nature of these products, we shall present the results of the protonation experiments.

Protonation studies

For practical reasons, all species containing the acetonitrile ligand will be assigned the same numeric label whether they were synthetized in regular CH3CN and isolated, or simply observed by NMR by carrying out the reactions in CD3CN. The deuterium substitution at the hydride positions, on the other hand, will be explicitly shown by a -d n suffix.

Protonation of compound 2

As for the Cp* analog, the protonation of 2 by HBF4 in Et2O immediately affords analytically and spectroscopically pure [Cp Et MoH4(dppe)][BF4] (3) as a pale brown solid in good yields. Once again, the spectroscopic properties of this compound parallel those of the Cp* analog. However, complex 3 is more inert in MeCN relative to the Cp* analog. The protonation of Cp*MoH3(dppe) with 1 equiv. of HBF4 in MeCN was shown to lead directly to [Cp*MoH2(MeCN)(dppe)] + with gas evolution, without the observation of the tetrahydrido intermediate. The protonation of 2 under the same conditions, on the other hand, leads to the formation of the tetrahydrido derivative 3 and to its subsequent slow convertion (t1/2 =22 min at r.t.) to the dihydrido compound [Cp Et MoH2(MeCN)(dppe)][BF4] (4), as shown by careful NMR monitoring of a reaction carried out in CD3CN (see Fig. 1). The shape of the hydride resonance of compound 4, as well as the chemical shift and the JHP, is identical to that of the corresponding Cp* compound. The previously reported variable temperature NMR studies on the Cp* system show that this is the result of the freezing out of an exchange between inequivalent hydride sites. Fig. 2 shows the normalized integral (1/4 of the hydride signal for 3 and 1/2 of the hydride signal for 4) as a function of time. Fitting the data to the integrated first-order rate laws (1) and ( 2), respectively, provides the same firstorder rate constant within experimental error (3.2(1) ×10 -2 and 3.2(8) ×10 -2 min -1 , respectively). The sum of the concentrations of 3 and 4 is approxi-mately constant and its least-squares value (y =1.76(2) in the normalized integral units of Fig. 2) corresponds within experimental error to the optimized asymptote for the concentration of 4 [y (4) =1.80 (14)]. Extrapola-tion to zero time yields a non-zero value for the concen-tration of 4 [y0(4) =0.20 (11)] and a concentration of 3 [y0(3) =1.47(5)], which is significantly lower (difference greater than 5 s units) than the optimized total. The sum of these two numbers [y0(3) +y0(4) =1.67 (12)], however, is within one standard deviation of the optimized total. In a separate experiment, this solution was treated with NEt3 before the complete conversion of 3 to 4, causing the resonances of 3 to disappear and to be replaced by those of 2, demonstrating the reversibility of the proton transfer process. The resonances of 4, on the other hand, are not affected; thus, the latter complex is not sufficiently acidic to be deprotonated by NEt3 (see Scheme 2). The difference in reactivity be-tween the Cp* and the Cp Et systems is also shown by other observations. The protonation of the Cp* deriva-tive, even when the strictly stoichiometric amount of HBF4 was employed, always gave subsequent protonation products, identified as three different, slowly inter-converting isomers of the monohydride complex [Cp*MoH(MeCN)2(dppe)] 2+ [10]. The only possible way to isolate pure [Cp*MoH2(MeCN)(dppe)][BF4] was to dissolve the isolated (from the protonation in Et2O) tetrahydrido derivative in MeCN. For the C5Et5 system, on the other hand, the direct protonation of 2 in CD3CN with 1 equiv. of HBF4 leads to the selective formation of 4. By carrying out the protonation on a larger scale in regular acetonitrile, the dihydrido complex 4 was isolated as an analytically pure product as a tetrafluoroborate salt.

Further protonation of complex 4 with 1 additional equiv. of HBF4 in MeCN shows again a chemical behavior identical to but slower rates than the previously investigated Cp* system. For the Cp* system, protonation of [Cp*MoH2(dppe)(MeCN)] + resulted in the immediate disappearance of the starting material and the formation of the three isomeric forms of [Cp*MoH(dppe)-(MeCN)2] 2+ [10]. The Cp Et system affords the same three products (5, 6 and 7), whose hydride 1 H and 31 P NMR resonances parallel those of the corresponding Cp* systems (see Fig. 3). However, NMR monitoring shows that the starting material is only slowly converted into these products, the complete disappearance requiring over 1/2 h at r.t. The nature of the final products does not change when the protonation is carried out in THF, followed by drying and redissolution in CD3 CN. Treatment of the final solution with NEt3 results in the disappearance of 6 and 7 2 , while the resonances of 5 remain unaltered. Complex 5 has also been isolated as an analytically pure BF4 salt by selective crystallization from the solution containing 5, 6 and 7.

The direct protonation of 2 in MeCN with 2 equiv. of HBF4 affords an unexpected result. For the Cp* system [10], as stated above, the tetrahydrido derivative was not observed (even when only 1 equiv. of acid was used), leading directly to the products of double protonation The time evolution of the 1 H NMR spectrum in this double-protonation experiment is shown in Fig. 4. The persistence of 3 in the presence of excess H + (Fig. 4(a)) indicates that its further protonation is thermodynamically unfavored and/or kinetically slow. The same can 3 2 initially, as shown by a 1 H NMR monitoring in

Protonation of compound 2-d 3

The addition of 1 equiv. of HBF4 to compound 2-d3 provides results consistent with those of the protonation study of 2. The immediate formation of [Cp Et MoDnH4-n(dppe)] + (n ≤ 3) species (3-d ≤3 ) is shown by the appearance of a triplet hydride resonance, whose chemical shift and JHP are identical with those of complex 2 within experimental error. Thus, the H/D isotopic substitution at the hydride position does not affect the chemical shift for this particular hydride complex. The absence of an isotopic shift does not conclusively disprove the presence of dihydrogen ligands 3 [19], but is rather more in agreement with a classical polyhydride formulation. The absence of any noticeable H/D couplings is also in favor of a classical structure. One rather surprising observation is the immediate appearance of H2, as well as HD, in the 1 H NMR spectrum (see Fig. 5(a)). This observation is a clear indication of reversibility in the protonation process of 2 to afford 3 (see Section 4). The hydride resonance of 3-d ≤3 subsequently decreases (t1/2 B10 min), while the broadened triplet resonances of 4-d ≤1 replaces it. As for system 3, different isotopomers could not be distinguished by 1 H NMR for system 4.

Once the conversion of 3-d ≤3 to 4-d ≤1 was complete (approximately 1/2 h), the addition of 1 additional equiv. of HBF4 revealed the slow conversion of The direct addition of 2 equiv. of HBF4 to compound 2-d 3 produced the same species obtained from the analogous reaction of compound 2, as shown by the 31 P NMR spectrum. However, the 1 H NMR spectrum in the hydride region (Fig. 5(b)) shows only peaks corresponding to compounds 3-d ≤3 and [Cp Et Mo-DnH3-n(dppe)(CD3CN)] 2+ (8-d ≤2 ). The hy-dride resonance of 8-d ≤2 is not significantly shifted from that of 8 and does not show indications of H-D coupling, again suggesting a classical structure. Since the corresponding experiment with the trihydride precursor 2 also leads to the observation of 4, 5, 6 and 7 under the same conditions, it must be concluded that all these products are either fully deuterated or they form more slowly when obtained from 2-d 3 . The 1 H NMR spectrum also shows the formation of a mixture of H2 and HD (Fig. 5(b)). The initial H2:HD ratio is greater when using 2 equiv. of HBF4 (0.70, Fig. 5(b)) than when using 1 equiv. (0.20, Fig. 5(a)). The spectra were recorded with long relaxation delays (90 s) to ensure full magnetization recovery for HD.

The evolution of the 1 H NMR spectrum for the 2d 3 /2H + reaction parallels that of the 2/2H + reaction with regards to the disappearance of species 3 and 8 (both became unobservable after approximately 6 h from the HBF4 addition). However, signals corresponding to species 5, 6 and 7 started to appear after approximately 30 min, while a signal attributable to 4-d ≤1 was never observed for this experiment.

Chemical oxidation of Cp Et MoH3(dppe) in acetonitrile

As mentioned above, compound 2 is oxidized at about the same potential as its Cp* analogue. The oxidized product 2 +' , however, decomposes much more slowly as shown by the reversibility of the cyclic voltammetric wave down to 10 mV s -1 , even in MeCN. Chemical oxidation by 1.5 equiv. of ferrocenium in MeCN allows the observation of the transient EPR spectrum of 2 +' , while the one-electron oxidation product for the Cp* analog could only be observed in THF and CH2Cl2. An EPR monitoring of the 2 +' solution shows that this signal is slowly replaced by a triplet (g =1.984, aP=26.9 G, aMo=47.6 G) within 1 h, whereas the Cp* analog gave an EPR silent solution. The magnitude of the hyperfine constant is diagnostic for phosphorus coupling. This and the absence of any additional coupling attributable to H nuclei make us suppose that the spectrum corresponds to a diphosphine, non-hydridic complex, such as [Cp Et -Mo(dppe)(MeCN)2] 2+' , whose formation can be rationalized on the basis of our working mechanistic hypotheses (see Section 4). We have not, however, further pursued the identification of this material. The NMR spectrum of the final solution reveals the presence of the same types of products that are also obtained by oxidation of Cp*MoH3(dppe) in MeCN, namely complexes 4 and 5 in an approximately 1:1 ratio, plus another non-hydridic product, which remains unidentified. The NMR experiment also shows that complex 3 is initially present during this experiment, but later disappears transforming into 4, as already established by the protonation experiment. It is to be remarked that at no time during this experiment is there any evidence for the accumulation of complex 8.

An analogous experiment carried out with 2 equiv. of ferrocenium reveals that only 3 (major) and 5 (minor) are obtained initially. No compound 4 is present during the initial moments of the reaction, but it forms later as the resonances of 3 disappears. The unassigned non-hydridic product, on the other hand, is absent at all times, whereas the strong EPR signal attributed to [Cp Et Mo(dppe)(MeCN)2] 2+• is present.

Discussion

Mechanism of the protonation process

The results of the protonation studies on compounds 2 and 2-d 3 are rationalized by the mechanistic Scheme 4, which follows and expands the previously reported one from the protonation studies of the Cp* analog, Cp*MoH3(dppe) [10]. The results of the protonation experiment of 2 with 1 equiv. of HBF4 (Fig. 2), notably the proportion of the products 3 and 4 at zero time, would tend to suggest that protonation of 2 can occur by two competitive pathways, formally corresponding to direct protonation at the metal (to afford 3, major pathway) and protonation at the hydride position to afford a dihydrogen ligand, which is immediately re-placed by acetonitrile to afford 4 (minor pathway). It cannot be excluded, however, that the protonation al-ways occurs kinetically at the hydride position to afford a nonclassical intermediate I, as suggested previously for other protonation reactions [20][21][22]. If this is the case, the collapse of the nonclassical intermediate to the classical product must be much faster than the replacement of the H2 ligand by acetonitrile. One way or the other, the results of this experiment indicate that proto-nation of compound 2 occurs, at least in part, at the hydride site.

The results of the protonation of 2-d 3 , notably the immediate formation of H2, are a clear indication of reversibility for the protonation process. An irreversibility in the formation of both 3-d 3 and I-d 3 could only lead to the formation of HD. On the other hand, reversibility could occur either for the hydride protonation, or for the metal protonation (if this occurs), or for both. Jia and Morris have studied protonation/deprotonation reactions on both classical and non-classical polyhydride complexes that are in equilibrium with each other and have established that the non-classical form has a greater kinetic acidity [22,23]. A reversible protonation process at the hydride position seems therefore more likely. The formation of a mixture of H2 and HD can thus be rationalized as shown in Scheme 5. A fast dynamic exchange process between classical and non-classical sites could equilibrate the isomers shown and lead to the formation of a mixture of HD, D2, 4-d 2 and 4-d 1 from I-d 3 , and to a mixture of H2, HD, D2, 4-d 2 , 4-d 1 , and 4 from I-d 2 . Complex I-d 2 should lead to the preferential elimination of H2 if the H atoms have a preference for the non-classical sites, as indicated by isotopic perturbation of resonance (IPR) experiments on another polyhydride complex [24]. The results also show that the deprotonation from I-d 3 must be faster than the replacement of HD by MeCN or than the collapse to 3-d 3 . Therefore, further reversible proton transfer processes will probably occur from intermediate I-d 2 to also afford I-d 1 and I. In agree-ment with the proposed scheme, the initial H2:HD ratio increases from 0.20 to 0.70 when the H + /2-d 3 ratio is increased from 1 to 2.

The protonations of 3 and 4 are rather slow processes, as shown in Figs. 2 and4. A slower protonation for positively charged complexes relative to neutral species is expected on the basis of the Coulombic repulsive effect. On the other hand, the corresponding cationic Cp* systems are protonated immediately. We attribute this difference to steric effects, since the C5Me5 and C5Et5 rings are electronically equivalent, as shown by the similarity of the NMR and redox proper ties. All Scheme 5. Scheme 6.

available crystal structures of C5Et5 derivatives reveal the same conformation for the ring, with all five ethyl substituents bent toward the same side of the ring away from the metal [25][26][27][28]. This conformation does not impose any additional steric encumberance to an attack of the other ligands by external reagents. However, it is likely that the ethyl groups are freely rotating around the ring-C2H5 axes in solution, yielding a much greater effective cone angle for the C5Et5 ligand.

The protonation of 4 yields directly the monohydride products, without any evidence for the intermediacy of the trihydride derivative 8. The most reasonable explanation of this result is to invoke the protonation of a hydride ligand to afford intermediate II (Scheme 4), which immediately expels the H2 ligand rather than collapse to the classical tautomer. Thus, no direct protonation at the metal occurs for system 4, suggesting that the Mo H bond presumably retains a Mo-H δ-bond polarity in agreement with the inability of NEt3 to deprotonate the complex. According to this scheme, therefore, the behavior of intermediates I and II is completely different, the first one preferring to rear-range to the classical tautomer over the H2/MeCN exchange by at least a 4:1 margin, whereas the second one leads exclusively to substitution of the H2 ligand. This difference can be rationalized on the basis of the differences between the two substrates. Both I and II are formally Mo(IV) systems but the charge increase on going from the former to the latter (caused by the formal replacement of H -by acetonitrile) renders the metal a weaker p base, weakening the Mo H2 interac-tion in favor of substitution and in disfavor of oxidative rearrangement.

The trihydride complex 8 is only observed by direct double protonation of 2 (or 8-d ≤2 from double proto-nation of 2-d 3 ). This signifies that its formation requires a direct protonation of 3 without the intermediacy of 4. As suggested above for the protonation of 2 and 4, we propose that the protonation of 3 occurs at a hydride ligand to afford intermediate III, as shown in Scheme 4. As a matter of fact, there is no choice in this case because there are no lone pairs remaining on a metalbased orbital. Thus, intermediate III is unable to rearrange to a classical tautomer and can only undergo substitution of the H2 ligand with an acetonitrile molecule, to afford 8. The slow rate of disappearance of 3 suggests that its protonation to III is probably easily reversed, allowing the formation of mixtures of III-d 3 , III-d 2 , etc. from 3-d ≤3 by deprotonation/protonation processes by analogy with the reversible protonation of 2 in Scheme 5. Once formed, complex 8 is quite inert showing that, in spite of the high metal charge, the barrier to the reductive elimination process is relatively high.

Compound 8, however, shows a relatively high acidity; it is deprotonated by NEt3 and by D2O, while compound 3 is deprotonated only by NEt3, its 1 H NMR hydride resonance remaining unaffected in the presence of D2O. Since 3 and 8 are formally related by the replacement of a H -ligand with a MeCN ligand, the above comparison is in agreement with a stronger donating power for H -, which is also independently indicated by the trend of oxidation potentials (E1/2: 2 < 4 < 5).

Mechanism of the oxidation process

The results of the chemical oxidation in MeCN can be accommodated by the mechanism illustrated in Scheme 6, which follows and expands the previously reported one from the oxidation studies of the Cp* analogue [12]. Compound 2 is oxidized by ferrocenium to afford the radical species 2 +' in the classical form. The decomposition, however, proceeds via the nonclassical form whose relative energy becomes kinetically accessible as a result of the oxidation. This conclusion was attained from a comparative stability and decomposition mechanism study for the Cp* Mo and W systems [12]. Acetonitrile then accesses the open coordination site to afford equilibrium amounts of a 19-electron species (intermediate IV in Scheme 6), which can be oxidized by the residual 17-electron 2 +' in a precedented [12,[29][30][31] ligand-induced disproportionation reaction. Electrochemical and kinetics studies on the analogous Cp* system have led to the above conclusions [12].

No assumption on the intimate structure of the oxidation product, which results from the disproportionation step, namely the doubly cationic trihydrido species, was made in the previously reported Cp* study. However, the classical form of this compound corresponds to 8, which is shown by the protonation studies to be a kinetically inert species. Since this is not observed as a product of the oxidation reaction with either 1 or 2 equiv. of ferrocenium, we must assume that the disproportionation of [START_REF] Parshall | Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes[END_REF] '+ or the direct double oxidation of 2 leads to the nonclassical tautomer, i.e. II. This assumption is indeed logical if the 19-electron precursor IV also adopts a nonclassical structure. As shown above, intermediate II evolves to compound 5 rather than collapsing to the classical tautomer 8 (Scheme 4). The rest of the Scheme again parallels what was previously proposed for the Cp* study. The selective formation of 5 (and not 6 or 7) parallels what has been observed for the Cp* system and the same rationalization should hold here, namely that intermediate II is obtained in a single isomeric form during the electron transfer process, while it may be obtained under different forms during the protonation process.

The EPR active species with a triplet signal at g = 1.984 has been attributed to the 17-electron complex [Cp Et Mo(dppe)(MeCN)] 2+ , whose formation can in principle be imagined in two different ways. The simplest one involves deprotonation of 5 by 2, followed by oxidation by residual ferrocenium (Scheme 7). However, a control experiment has shown that complex 5 is not deprotonated by complex 2. Thus, this mechanistic hypothesis must be abandoned.

A second mechanistic hypothesis involves oxidation of product 4 by excess ferrocenium to afford intermediate VI, which would lead directly to the product by reductive elimination of H2 (Scheme 8). A parallel electrochemical experiment shows the feasibility of this process, because complex 4 exhibits a reversible oxidation wave at E1/2 =-0.06 V. Thus, excess ferrocenium has sufficient oxidizing power to convert 4 into its oneelectron oxidation product. On the other hand, complex 5 is oxidized at a much higher potential and 3 is not oxidized before the solvent discharge. More detailed electrochemical studies of complexes 2, 4 and 5 are underway and will be reported separately [32].

Conclusions

Protonation and oxidation studies on the pentaethylcyclopentadienyl trihydride complex 2 show evidence for steric protection by the increased bulk of the cylopentadienyl ring in solution, resulting in slower decomposition rates for several derivatives, such as the primary oxidation product, [START_REF] Parshall | Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes[END_REF] '+ , toward disproportionation and the conjugate acid, 3, toward H2 reductive elimination in MeCN. The inertness of 3 has allowed us to provide evidence of a process of further protonation with formation of the hiterto unknown classical trihydride compound 8, which formally corresponds to the product of double oxidation of 2. However, 8 cannot be accessed via the double oxidation process because the irreversible rearrangement to a non-classical structure takes place after the first oxidation step.

Fig. 1 .

 1 Fig. 1. Representative 1 H NMR spectra in the hydride region of the reaction between Cp Et MoH3(dppe) and 1 equiv. of HBF4 in CD3CN at r.t. (a) t=5 min. (b) t=15 min.

y( 3 )Fig. 2 .

 32 Fig. 2. Concentrations of complexes 3 and 4 from the reaction between 2 and HBF4 (1 equiv.) in MeCN at r.t., as obtained from the integration of the 1 H NMR hydride signals, a function of time. White circles: 3; black circles: 4; grey circles: total. The lines correspond to the leastsquares fittings of the data to the pertinent integrated first-order rate laws.

  Scheme 2.

Fig. 3 .

 3 Fig. 3. Representative 1 H NMR spectra in the hydride region of the reaction between [Cp Et MoH2(dppe)(MeCN)] + and 1 equiv. of HBF4 in CD3CN at r.t. (a) t=2 min. (b) t=25 min.

Fig. 4 .

 4 Fig. 4. Representative 1 H NMR spectra in the hydride region of the reaction between Cp Et MoH3(dppe) and 2 equiv. of HBF4 in CD3CN at r.t. (a) t=20 min. (b) t=1 h. and double H2 elimination. For the Cp Et system, on the other hand, six distinct products were formed initially, as shown by a 1 H NMR monitoring in the hydride region of the reaction carried out in CD3CN (see Fig. 4). These are the tetrahydrido derivative 3, the dihydrido derivative 4, the three different forms of [Cp Et MoH(CD3CN)2(dppe)] 2+ , 5, 6 and 7, and a new compound characterized by a broad, low intensity 1 H NMR triplet resonance at δ -3.11 (JHP = 31.2 Hz) and by a single 31 P NMR resonance at δ 59.51. We shall present compelling evidence below that this new compound corresponds to the 'classical' trihydride species [Cp Et MoH3(CD3CN)(dppe)] 2+ (8).The time evolution of the 1 H NMR spectrum in this double-protonation experiment is shown in Fig.4. The persistence of 3 in the presence of excess H + (Fig.4(a)) indicates that its further protonation is thermodynamically unfavored and/or kinetically slow. The same can

Scheme 3 .

 3 Scheme 3.

7 .

 7 Fig. 5. 1 H NMR spectrum in the hydride region of the solution obtained by adding HBF4 to Cp Et MoD3(dppe) in CD3CN at r.t. (a) HBF4/Cp Et MoD3(dppe) =1; t=2 -7 min. (b) HBF4/Cp Et MoD3(dppe) =2; t=10 -15 min. The starred peaks correspond to unknown impurities.

  Scheme 7.

  P{ 1 H} NMR (CD3CN, δ): 69.61 (br), 62.83 (br). 1 H NMR (CD3CN, δ): -2.09 (dd, JP%H = 21.3 Hz, JP¦H = 82.3 Hz). Compound 7: 31 P{ 1 H} NMR (CD3CN, δ): 71.37 (d, JPP=28.2 Hz), 46.55 (d, JPP=28.2 Hz). 1 H NMR (CD3CN, δ): -4.01 (dd, J =9.6 Hz, J =88.2 Hz). Compound 8: 31 P{ H} NMR (CD3CN, d): 59.51 (br). 1 H NMR (CD3CN, δ): -3.11 (t, JPH=31.2 Hz).

When water is deliberately added to the solvent or when the latter is incompletely dried, decomposition of [Cp Et MoH (dppe)] + ' leads to the formation of Cp Et Mo(OH)2(dppe), as described elsewhere[28].

The product(s) of this deprotonation process could not be isolated or characterized by NMR in solution and their identification was not further pursued.

For instance, no significant isotopic shift is observed for the nonclassical complexes [Cp*MH4(H2)(PMe3)] + (M=Mo, W), see Ref.[19].

Acknowledgements

We are grateful to the CNRS and the MENRT for support of this work. We also thank Professor Helmut Sitzmann for a generous gift of compound Cp Et MoMe(CO)3, and the Conseil Re ´gional de Bourgogne and the II Plan Regional de Investigation del Principado de Asturias (Spain) for postoctoral fellowships to D.M.


Erwan Le Grognec

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

Philippe Richard

Synthesis, characterisation, and molecular and electronic structure of CpMoCl2(R1C≡CR2) (R1, R2 = Ph, Et, Me): a new class of half-sandwich 17electron molybdenum(III) organometallics

    Synthesis, characterisation, and molecular and electronic structure of CpMoCl2(R1C̸ ≡CR2) (R1, R2 = Ph, Et, Me): a new class of half-sandwich 17-electron molybdenum(III) organometallics

Introduction

Alkyne organometallic chemistry has always attracted much attention because of the versatility of alkynes in metalmediated organic synthesis and catalysis [1][START_REF] Collman | Principles and Applications of Organotransition Metal Chemistry[END_REF][START_REF] Buhro | [END_REF][4][5] and more recently also for their application in materials chemistry. [START_REF] Gibson | Handbook of Conducting Polymers[END_REF][START_REF] Fox | [END_REF][8][9] Group 6 alkyne chemistry is quite extensively studied, the vast majority of the investigations concerning compounds where the metal has the formal oxidation state II. [10][11][12] The interactions of alkynes with Group 6 metals in the oxidation state III are mostly related to dinuclear M2X6-type (M = Mo, W) 1,[START_REF] Buhro | [END_REF]13 and [(C5H4R)WCl2]2 (R = Me, Pr i ) [14][15][16][17][18] compounds. A few mononuclear M(III)-alkyne complexes, however, have been described both for Mo and for W, e.g. WCl3(PMe3)2(PhC≡CPh), 19 [Tp*WX(CO)(MeC≡CMe)]-[BF4] [Tp* = hydrotris (3,5-dimethylpyrazolyl)borate; X = F, Cl, Br or I], 20 [MoCl3(THT)2(PhC≡CR)] (R = Me, Et, THT = tetrahydrothiophene), 21 and Mo(NPr i Ar)3(PhC≡CPh) (Ar = 3,5-C6H3Me2). 22 Paramagnetic half-sandwich compounds of general formula (η 5 -C5R5)MCl2(alkyne) (M = Group 6 metal) appear limited to two related examples for tungsten. The reaction between W(≡CBut)Cl3(dme) and excess RC≡CR yields (η 5 C5R4Bu t )-WCl2(RC≡CR) (R = Me, Et), 23 the structure of the 2-butyne adduct being confirmed by an X-ray analysis. 24 These reactions are not selective and do not appear of potentially general synthetic utility. In addition, the reduction of Cp*WX4 (X = OPh, Cl) with sodium amalgam in the presence of alkynes was reported to afford the corresponding W(II) species Cp*WX(alkyne)2 with no mention of W(III) intermediates . 25 For molybdenum, it has been shown that the treatment of CpMoCl(CF3C≡CCF3)2 with dienes involves a disproportionation process resulting in the isolation of the Mo(III)diene products CpMoCl2(diene). [26][27][28] Whether this transformation involves Mo(III)-alkyne or Mo(II)-diene intermediates does not appear to have been the subject of investigations. We have more recently prepared the same diene compounds by the high-yield, straightforward addition of diene to [CpMoCl2]2. 29,30 It has also been reported that the thermal treatment of CpMoCl(PhC≡CPh)2 affords several rearrangement products, including the 17-electron Mo(III) cyclobutadiene complex CpMoCl2(η 4 -C4Ph4). 31,32 No mention was made, however, whether an alkyne complex of formula CpMoCl2(η 2 -C2Ph2) also forms. Since the direct addition of neutral ligands to [CpMoCl2]2 appears to be a versatile method to obtain half-sandwich Mo(III) derivatives under mild conditions, 33 we have explored this methodology for the preparation of alkyne adducts and report here the successful synthesis and studies of four members of the hitherto unreported CpMoCl2(alkyne) family.

Results and discussion

The addition of alkynes R 1 C≡CR 2 to a THF suspension of [CpMoCl2]2 at the THF reflux temperature yields the soluble adducts CpMoCl2(R 1 C≡CR 2 ) according to eqn. (1). Thermal

[CpMoCl2]2 + 2 R 1 C≡CR 2 → 2 CpMoCl2(R 1 C≡CR 2 )
(1) (1: R 1 =Me, R 2 =Et; 2: R 1 =R 2 =Et; 3: R 1 =Me, R 2 =Ph; 4: R 1 =R 2 =Ph) decomposition of the products does not take place under these conditions and all products were recovered in good to excellent yields. The rate of the reaction decreases in the order 1,2 > 3 > 4, compound 1 being also obtained within a reasonable time (4 h) when the reaction is conducted at room temperature. For compound 4, an excess of alkyne may be used with no detriment to the yield, and no reaction occurs upon prolonged treatment of isolated 4 with a second equivalent of diphenylacetylene in refluxing THF. For compounds 1 and 2, on the other hand, the stoichiometric amount of alkyne must be used in order to avoid the formation of by-products (vide infra).

It is interesting to observe that while the alkyne addition to the corresponding tungsten starting materials, [(C5H4R)-WCl2]2 (R = Me or Pr i ), gave only the dinuclear, metal-metal bonded products (C5H4R)2W2Cl4(μ-C2R'2) or (C5H4R)2-W2Cl4(μ-C4R'4) depending on stoichiometry and conditions, [14][15][16] reaction (1) produces only mononuclear paramagnetic products, in agreement with the reduced strength of the Mo-Mo bond relative to the W-W bond in the starting material. The mononuclear (η 5 -C5R4Bu t )-WCl2(RC≡CR) (R = Me, Et) complexes, 23 analogous to our compounds 1-4, were obtained by a different synthetic strategy starting from a mononuclear carbyne precursor. It is also of relevance that the reaction between [(C5H4Pr i )MoCl2]2 and 2-butyne was mentioned to afford intractable products. 34 Compounds 1-4 are air sensitive but can be stored in the solid state under argon without significant decomposition. They are readily soluble in THF and CH2Cl2, only partially soluble in diethyl ether and completely insoluble in pentane. Satisfactory C,H elemental analyses for 3 and 4 could only be obtained in the presence of V2O5 as a combustion catalyst. The IR spectra confirm the presence of coordinated alkyne ligands, a weak ν(C≡C) absorption being observed at 1730 cm -1 for 1, 1732 for 2, 1717 cm -1 for 3 and 1700 cm -1 for 4, i.e. >400 cm -1 lower relative to the free alkynes. Although the quantitative relationship between stretching frequency and C-C bond order does not appear to have been carefully explored, larger shifts are usually associated with the 4electron (σ + π) coordination mode, the largest observed shifts being around 450 cm -1 . [START_REF] Collman | Principles and Applications of Organotransition Metal Chemistry[END_REF] The mass spectra show the molecular ion for each compound, in agreement with a mononuclear formulation. The mononuclear nature of the products is also consistent with the paramagnetism, as indicated by the EPR spectra in solution at room temperature and confirmed by the magnetic susceptibility measurement for 1, 3 and 4, yielding a magnetic moment close to the spin-only value for one unpaired electron. The EPR spectra are characterised by a sharp isotropic signal at room temperature, while frozen solutions at low temperatures show an essentially tetragonal g tensor. This is as expected for the 17-electron configuration of the complexes (when considering the alkyne as a σ + π 4-electron donor). In this respect, compounds 1-4 are perfectly isoelectronic with the growing class of CpMoX2L2 compounds. 35 Molybdenum satellites due to 95 Mo(15.9%) and 97 Mo(9.5%), both having I = 5/2, are visible for both the isotropic spectrum and the perpendicular component of the frozen solution spectrum (see Table 1). A nearly tetragonal g tensor was also observed for the related alkyne complexes of W(III), [Tp*WX(CO)(MeC≡CMe)][BF4] (X = F, Cl, Br, I). 20 A mononuclear formulation is finally confirmed by the Xray structure of 4, which is illustrated in Fig. 1. The most relevant feature of the geometry is the three-legged piano stool disposition, with the alkyne ligand adopting a conformation almost parallel to the Cp plane. The angle θ between the Mo-C(6)-C (7) plane and the plane defined by Mo, CNT (center of gravity of the Cp ring), and the midpoint of the C(6)-C(7) vector is 85.61°. This tilting away from the ideal parallel conformation (θ = 90°) is less pronounced relative to the previously reported W(III) complex (η 5 -C5Me4Bu t )WCl2(MeC≡CMe) (θ = 77.16°) 24 and slightly more relative to Cp*TaCl2(PhC≡CPh) (θ = 87.27°) 36 and (η 5 -C5H4Me)NbCl2(ArC≡CAr) (Ar = C6H4CH3-4, θ = 86.40°). 37 Table 2 collects relevant bond distances and angles, in comparison with those of the above mentioned Nb analogue (η 5 -C5H4Me)NbCl2(ArC≡CAr) 38 and of geometry optimised HC≡CH model compounds (vide infra). It can be seen from Table 2 that while the M-CNT and M-C(alkyne) distances are significantly shorter for Mo relative to Nb, the M-Cl distances are essentially identical. The same trend can be noticed by comparing the previously described Cp*TaCl2(PhC≡CPh) and (η 5 -C5Me4Bu t )WCl2-(MeC≡CMe). 24,36 A general shortening is expected as a result of the smaller atomic radius of a Group 6 metal relative to the corresponding Group 5 metal in the same row and formal oxidation state. The alkyne C-C distance is essentially identical with that of the Nb complex (Table 2) and of the W complex (1.312(10) Å), 24 while this is longer in the Ta compound (1.337(8) Å). 36 In other structurally characterized Mo(III)-alkyne complexes, e.g. MoCl3(THT)2(PhC≡CR) (THT = tetrahydrothiophene; R = Me, Et), 21 the alkyne C-C distance is less accurately determined but does not appear to be significantly different than for compound 4 (i.e. 1.27(1), 1.31(1) and 1.32(1) Å). The C-C≡C-C framework is quite bent (C-C≡C angles around 145°), only slightly less so than in the above mentioned Ta, W and Nb compounds (averages Together with the C≡C distance, this parameter confirms that the alkyne ligand acts as a four-electron donor.

The bonding features of this class of alkyne complexes has been further investigated at the theoretical level. The electronic structure for CpMCl2(alkyne) systems has only been previously explored by Curtis et al. for M = Nb at the extended Huckel level, at the fixed geometry determined by X-ray diffraction. 37 We have carried out geometry optimisations at the B3LYP/LANL2DZ level on the model compound CpMoCl2(HC≡CH). As is now well established, this level of theory affords satisfactory geometries and energies for medium-size organometallic compounds. A full optimisation indeed yields a geometry with bond distances and bond angles close to the experimental values. As is typical for this computational technique, distances are slightly overestimated (up to 0.07 Å), whereas bond angles are reproduced to within 3°. However, the orientation of the ethyne ligand is quite different from experiment, θ being only 30.78°. Subsequent optimisations have been carried out with the ethyne ligand forced to adopt a parallel and a perpendicular orientation by imposing a Cs symmetry to the molecule. The results (Fig. 2) show that the total energy is almost independent on the alkyne orientation and suggest that the adoption of a close to parallel geometry for compound 4 [and by extension also by the tungsten analogue (η 5 -C5Me4Bu t )WCl2(MeC≡CMe)] is due to the minimisation of the steric repulsion between the cyclopentadienyl and alkyne ligands. The optimised parameters reported in Table 2 are those of the parallel geometry, which is closest to the experimentally determined geometry.

Analogous calculations have been carried out on the niobium model system CpNbCl2(HC≡CH), the results being also shown in Fig. 2. In this case, a full optimisation leads to an essentially Cs-symmetric geometry with a perpendicular ethyne ligand. The Cs-imposed parallel orientation is found, like for Mo, to have an energy very close to that of the perpendicular conformation. The optimised distances and angles reported in Table 2 are those relating to the parallel geometry, which is closest to the experimental structure. A comparison between calculated and experimental parameters shows again the adequacy of the computational method. It seems, therefore, that the choice of geometry for Group 5 CpMCl2(alkyne) compounds, like for the analogous Group 6 compounds, is dictated by the steric requirements.

Additional insights are given by the orbital analysis. The energy and shape of relevant Kohn-Sham orbitals retrieved from the DFT calculations are shown in Fig. 3 for both Nb and Mo systems with the ethyne ligand both in the perpendicular and in the parallel conformation. For the Mo complex, only the α spin orbitals are shown (the shapes of the corresponding β spin orbitals are essentially identical), while the energy reported is the average of α and β. Orbital no. 38 is the HOMO for the Nb system while the Mo system has an additional unpaired electron in orbital no. 39. The latter is essentially a metal-based orbital. However, there is a significant participation of Cl lone pairs for the Mo system, giving rise to a Mo-Cl π* combination. This rationalises why, on going from Nb to Mo, the M-Cl bond length does not significantly shorten: the effect of the metal radius reduction is compensated by the Mo-Cl π* contribution of the additional electron. The alkyne-to-M π interaction (π⊥) gives a bonding combination which is evident in orbital no. 28, and the corresponding antibonding component in orbital no. 40. The alkyne-to-M σ donation is in a deeper energy orbital, while the orbitals no. 29-37 (not shown) contain the M-Cl σ bonds, the Cl p lone pairs, and the M-Cp σ and π interactions. There is heavy symmetry-allowed mixing of various bonding components in individual orbitals. Orbital no. 38 contains the M-alkyne π back-bonding interaction (π||). Its lower energy and greater percentage contribution of metal and alkyne orbitals for M = Nb suggests a greater degree of back-bonding for this metal, in agreement with the observed structural differences (vide supra).

Fig. 3 allows one to appreciate the reason for the small dependence of the total energy on the alkyne configuration. The alkyne ligand is able to establish both π interactions with the metal center no matter what the relative orientation of alkyne and M-Cp axis. In a reference system where z coincides with the M-CNT vector and the alkyne midpoint lies on the xz plane, the perpendicular conformation involves a π⊥ interaction with an essentially xy metal orbital and a π || interaction with a (x 2 -z 2 ) orbital, whereas the same two metal orbitals trade places on going to the parallel conformation. Thus, these two orbitals may generate linear combinations suitable to establish equally efficient π⊥ and π|| interactions for any alkyne conformation, effectively following the rotation of the alkyne ligand around the Mo-alkyne axis. For this reason, little or no electronic barrier is expected against alkyne rotation and the observed conformation is largely determined, for both Group 5 and 6 metals, by the intramolecular steric interactions. The electrochemical investigation of compounds 1-4 by cyclic voltammetry reveals both a one-electron reduction and a one-electron oxidation process. The reduction wave appears at relatively large negative potentials vs. the ferrocene standard (see Table 3) and is reversible (ia/ic = 1, ΔEp ca. 60 mV) for compounds 3 and 4 in the 100-5000 mV s -1 range of scan speeds. For compounds 1 and 2, on the other hand, the reduction wave becomes reversible only above 2000 mV s -1 and, at lower scan speeds, the ia/ic becomes smaller and smaller. Since no new wave appears in the voltammogram at low scan rate, the reduction products [1] -and [2] -must decompose to redox-inactive species which remain so far uncharacterised. The oxidation process, on the other hand, is completely irreversible up to 5 V s -1 for all derivatives. In this respect, these compounds behave similarly to the previously described diene complexes, CpMoCl 2 (η 4 -diene), [26][27][28]30 while derivatives with more electron donating ligands (phosphines, phosphine-thioethers, phosphine-oxazolines, and phosphineamides) 35,39,40 show a reversible oxidation process at much lower potentials and no reduction process or, at the most, an irreversible reduction before the solvent discharge. A closer comparison of the reduction potentials between the isoelectronic complexes with the 4-electron donors alkyne (σ + π) and diene (2σ) ligands in Table 3 indicates that the alkynes transfer about the same amount of electron density to the metal center. The negative shift of the reduction potential on going from the less alkyl substituted alkyne complex 4 to the more alkyl substituted complexes 1 and 2 is in agreement with the greater electron-donating properties of alkyls. The same trend is observed for the oxidation potentials (1,2 < 3 < 4). However, the potentials of the oxidation processes will also reflect the kinetics of the follow-up chemical processes and cannot be used as a thermodynamic gauge of the metal electron density.

When reaction (1) was carried out in the presence of excess diphenylacetylene, no detriment to the yields of 4 and no additional product were observed. However, a second resonance grows in the EPR spectrum for the 2-pentyne and 3-hexyne reactions with an excess of the alkyne under forcing conditions. The 3-hexyne reaction has been subjected to more detailed investigations. A new compound (characterised by an EPR resonance at g = 1.996; aMo = 34.6 G) forms only quite slowly and a complete consumption of 2 was achieved only upon prolonged reflux in neat 3-hexyne (bp 82 °C). The reaction, however, is not selective and the Mo-containing, EPR active product could not be obtained in a pure form. However, a mass spectrum of the crude product mixture exhibits, in addition to the peaks of 2, peaks attributable to a compound of formula CpMoCl2(C4Et4). On the basis of the existence of the cyclobutadiene complex CpMoCl2(η 4 -C4Ph4) 31,32 and on the EPR properties which indicate a 17electron species, we tentatively suggest a cyclobutadiene structure also for this compound. Crystallization of the crude product from this reaction afforded instead a small amount of the previously reported 41 CpMoCl2(η 4 -C4Et4H), 5. The NMR properties for the isolated crystals of 5 match with those previously reported 41 and a single crystal X-ray analysis (see Fig. 4 and Table 4) confirms the structural assignment. In particular, the hydrogen atom of the η 4 (5e)butadienyl ligand was directly located from the difference Fourier map and freely refined. While the quality of the data affords metric parameters of greater precision, the structural parameters of 5 compare quite closely with those previously reported for the analogous CpMoBr2(η 4 -C4Et4H). 41 No further discussion of the structure of 5 is therefore warranted here, except for pointing out that the structural data and the NMR data concur in interpreting the metal-butadienyl bonding mode as mostly I (Fig. 5). We presume that, under the forcing conditions used for the reaction with excess alkyne, a hydrogen atom abstraction from the medium by either the cyclobutadiene product or by a reaction intermediate may be responsible for the formation of 5. This compound was previously obtained starting from a bisalkyne Mo(II) precursor, where the C-C coupling process was presumed to follow the protonation of one alkyne ligand to afford a reactive alkyne-vinyl intermediate. 41 A similar process has also been observed for related dithiocarbamato derivatives of Mo(II) and W(II). 42 The failure to obtain the previously reported tetraphenylcyclobutadiene complex from 4 + PhC≡CPh means that the barrier to the 2 + 2 cycloaddition reaction must be prohibitively high, higher than for the 2 + EtC≡CEt reaction, on this metal system. Thus, the reported formation of CpMoCl2(η 4 -C4Ph4) by thermolysis of CpMoCl-(PhC≡CPh)2 31,32 most likely does not involve compound 4 as an intermediate, while a more likely possibility is oxidative coupling of the alkyne ligands to generate a CpMoCl(η 4 -C4Ph4) intermediate which subsequently disproportionates. It is notable that alkyne dimerization occurs on a mononuclear CpMo(III) species, whereas for related tungsten chemistry the same process occurs on a dinuclear CpW(III) species. [14][15][16]

Conclusions

The alkyne addition to [CpMoCl2]2 provides a facile entry to hitherto unknown half-sandwich Mo(III) alkyne complexes under mild conditions. The electronic structure of these complexes is quite similar to that of the corresponding Group 5 derivatives, the additional electron residing in an essentially pure metal orbital which is not engaged in the metal-alkyne interaction. As a consequence, the nature of the metal-alkyne interaction is rather similar for Group 5 and Group 6 compounds, including the slight dependence of the energy on the alkyne conformation. On the other hand, the oxidation to afford isoelectronic species with the Group 5 complexes is irreversible, while the reduction to anionic 18-electron complexes is reversible on the cyclic voltammetric time scale. The electrochemical investigations further indicate that the CpMoCl2(η 2 -alkyne) system is about as electron-rich as the CpMoCl2(η 4 -diene) system, while the reactivity study shows that a 2 + 2 cycloaddition to afford a cyclobutadiene system has a high activation barrier for this system.

Experimental

All reactions involving air-and moisture-sensitive organometallic compounds were carried out in a Jacomex glove box or by the use of standard Schlenk techniques under an argon atmosphere. The solvents were dried by conventional methods (THF, Et2O, toluene and pentane from sodium benzophenone ketyl and CH2Cl2 from P4O10) and distilled under argon prior to use. EPR measurements were carried out at the X-band microwave frequency on a Bruker ESP300 spectrometer. All EPR spectral parameters for the new compounds are collected in Table 1. The spectrometer frequency was calibrated with DPPH (diphenylpicrylhydrazyl, g = 2.0037). Cyclic voltamograms were recorded with an EG&G 362 potentiostat connected to a Macintosh computer through MacLab hardware/software. The electrochemical cell was fitted with a Ag/AgCl reference electrode, a platinum disk working electrode and a Pt wire counterelectrode. Bu4NPF6 (ca. 0.1 M) was used as supporting electrolyte. All potentials are reported relative to the ferrocene standard, which was added to each solution and measured at the end of the experiments. All the voltammetric results obtained in this study are collected in Table 3. The mass spectra were obtained by electron impact at 70 eV on a Kratos Concept 32S spectrometer by direct introduction of solid samples. The magnetic susceptibility measurements were carried out at room temperature with a Johnson Matthey magnetic balance which operates with a modified Gouy method, whereby the effect of the sample on the weight of the magnet, rather than the effect of the magnet on the weight of the sample, is used to retrieve the susceptibility information. The elemental analyses were carried out by the analytical service of the Laboratoire de Synthèse et d'Electrosynthèse Organométallique with a Fisons EA 1108 apparatus. EtC≡CMe, EtC≡CEt, PhC≡CMe, PhC≡CPh were purchased from Aldrich Chemical Co. and used as received. Compound [CpMoCl2]2 was prepared as described in the literature. 43,[START_REF] Poli | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF]

Synthesis of CpMoCl2(η 2 -EtC≡CMe), 1

To a suspension of [CpMoCl2]2 (0.668 g, 1.44 mmol) in 20 mL of THF was added 2-pentyne (277 μL, 2.88 mmol) via a syringe. The mixture was then refluxed for 30 minutes, transforming the initial pale brown suspension to a dark brown solution. This solution was filtered through Celite and evaporated to dryness to yield an oily residue. Diethyl ether (10 mL) was added and the suspension was vigorously stirred to afford 1 as a brown powder. The supernatant was decanted off and the solid was washed with several portions of pentane (total ca. 30 mL). Yield: 0.604 g, 70%. Anal. Calc.

Synthesis of CpMoCl2(η 2 -EtC≡CEt), 2

To a suspension of [CpMoCl2]2 (0.905 g, 1.95 mmol) in 20 mL of THF was added 3-hexyne (440 μL, 3.9 mmol) via a syringe. The mixture was then refluxed for 2 h, transforming the initial pale brown suspension to a dark brown solution. This solution was filtered through Celite and evaporated to dryness to yield an oily residue. The product 2 was extracted with 40 mL of refluxing diethyl ether. Following filtration of the hot solution, crystals formed upon cooling to 4 °C. The supernatant was decanted off and the crystals were dried in vacuo. Yield: 0.612 g, 50%. Anal. Calc.

Synthesis of CpMoCl2(η 2 -PhC≡CMe), 3

To a suspension of [CpMoCl2]2 (0.720 g, 1.55 mmol) in THF in 20 mL was added 1-phenyl-1-propyne (388 μL, 3.1 mmol) via a syringe. The mixture was then refluxed for 2 hours yielding a red brown solution. The solution was filtered through Celite and concentrated to ca. 3 mL. Diethyl ether (10 mL) was then added under a vigorous stirring, precipitating a fine red-brown powder. The supernatant was cannulated off and the residue was washed with several portions of pentane (total ca. 50 mL). Yield: 0.750 g, 70%. Anal. (obtained in the presence of V2O5 as a combustion catalyst) Calc.

Synthesis of CpMoCl2(η 2 -PhC≡CPh), 4

To a pale brown suspension of [CpMoCl2]2 (1.438 g, 3.1 mmol) in 40 mL of THF was added a THF solution of diphenylacetylene (1.3 g, 7.3 mmol). The mixture was heated to 70-80 °C for 2 hours, yielding a red solution. After cooling to room temperature, the solvent was reduced in volume to ca. 7 mL and pentane (30 mL) was added to complete the precipitation of the product. The supernatant was decanted off, and the brick-red powder was washed with several portions of pentane (total ca. 50 mL). Yield: 2.150 g, 85%.

Recrystallisation by slow diffusion of pentane into a CH2Cl2

Reaction between 2 and excess EtC≡CEt

Compound 2 (50 mg, 0.16 mmol) was dissolved in 3-hexyne (2 mL) and the resulting red solution was refluxed for 15 h with EPR monitoring. The initial EPR signal (g = 1.987, aMo = 37.5) was slowly replaced by a new signal at g = 1.996, aMo = 34.6 G. During this time the color changed to dark yellow and a white precipitate formed. An aliquot of the final solution was evaporated to dryness and the residue was redissolved in the minimum amount of CH2Cl2 for the mass spectral analysis. MS (EI, 70 eV): m/z 397 [M] + (MoCl2 pattern, 5%), 361 {[M] + -HCl} (MoCl pattern, 3.8%). The remainder of the solution was evaporated to dryness and the residue was extracted with diethyl ether (5 mL) followed by filtration through Celite. The solution was concentrated to ca. half volume and a layer of pentane (5 mL) was left to diffuse slowly at room temperature. Red crystals of compound CpMoCl2(η 4 -C4Et4H), 5, formed within 1 week. One of the crystals was used for the X-ray analysis. 1 H NMR (CDCl3): δ 5.32 (s, 5H, Cp), 3.28 (m, 2H, CH2), 2.67 (m, 2H, CH2), 2.58 (m, 2H, CH2), 2.34 (m, 2H, CH3CH2CH), 2.00 (dd, 1H, J1 = 3.5 Hz, J2 = 8.0 Hz, C4Et4H), 1.42 (t, 3H, J = 8.0 Hz, CH3CH2CH), 1.41 (t, 3H, J = 7.5 Hz, CH3), 1.11 (t, 3H, J = 7.5 Hz, CH3), 0.99 (t, 3H, J = 7.5 Hz, CH3). 13 Crystal structure determinations (a) Compound 4. Crystals for the X-ray structure analysis were grown from a slow diffusion of pentane in a CH2Cl2 solution at room temperature. A prism of 0.48 × 0.36 × 0.24 mm 3 was cut from a red needle and sealed under argon in a capillary. A total of 25 reflections was used for an accurate orthorhombic cell determination and three reflections were selected to check the Laue group. A total of 1860 reflections were collected at room temperature up to sin(θ)/λ = 0.623 Å - 1 on an Enraf-Nonius CAD4 diffractometer. The data were corrected for Lorentz and polarization effects [START_REF] Harms | [END_REF] and for absorption (psi-scan method). 46 A 3% decay was linearly corrected. The structure was solved via a Patterson search program and refined in the polar space group Pna21 with fullmatrix least squares methods [START_REF] Sheldrick | SHELXS and SHELXL[END_REF] based on |F 2 |. All nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in their calculated positions and refined with a riding model. At the end of this refinement the agreement indices were wR2 = 0.0526 for all data and R1 = 0.0196 for 1764 intensities with I > 2σ(I), the absolute structure parameter 48 was x = 0.00(5) (x = 0.19 (5) and wR2 = 0.0539 for the inverted structure). The final difference electron density is featurless: Δρ = 0.256 and -0.217 e Å -3 . Crystal data and final refinement parameters are reported in Table 5. (b) Compound 5. A small red crystal of 5 (0.05 × 0.62 × 0.12 mm 3 ) suitable for an X-ray analysis was obtained by diffusion of pentane into a diethyl ether solution at 4 °C. The crystal was mounted on an Enraf-Nonius KappaCCD using Mo-Kα radiation and 10243 reflections (3924 unique) were collected up to sin(θ)/λ = 0.65 at 110 K. Absorption corrections were applied to the data during integration by the SCALEPACK algorithm. 49 The structure was solved via a Patterson search program and refined (space group P-1) with full-matrix leastsquares methods based on |F 2 |. [START_REF] Sheldrick | SHELXS and SHELXL[END_REF] All non-hydrogen atoms were refined with anisotropic thermal parameters. Except for H (9), the hydrogen atoms of the complex were included in their calculated positions and refined with a riding model. The H(9) hydrogen atom attached to the C(9) carbon atom was located in a Fourier difference map and freely refined with an isotropic temperature factor. Final agreement indices are reported in Table 5. CCDC reference number 186/1895. See http://www.rsc.org/suppdata/dt/b0/b000948m/ for crystallographic files in .cif format.

Computational details

All calculations were performed using Gaussian 94 [START_REF] Frisch | Gaussian 94[END_REF] on an SGI Origin200 workstation. The LANL2DZ basis set was employed to perform geometry optimisations with a DFT approach. The three parameter form of the Becke, Lee, Tang and Parr functional (B3LYP) [START_REF] Becke | [END_REF] was used in all cases. The LANL2DZ basis set includes both Dunning and Hay's D95 sets for H, C, N and O, [START_REF] Dunning | Modern Theoretical Chemistry[END_REF] and the relativistic Electron Core Potential (ECP) sets of Hay and Wadt for Mo and W. [START_REF] Hay | [END_REF][54][55] The energies reported for the open shell (doublet) systems correspond to unrestricted B3LYP calculations. The mean value of the first-order wavefunction, which is not an exact eigenstate of S 2 for unrestricted calculations on the doublet systems, was considered suitable for the unambiguous identification of the spin state. Spin contamination was carefully monitored and the values of 〈S 2 〉 for the unrestricted B3LYP systems at convergence were very close to the ideal value of 0.75.

aFig. 1

 1 Fig. 1 An ORTEP 57 view of compound 4 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. of 139.4(9), 140.3(7)° and 143.3(10)°, respectively).Together with the C≡C distance, this parameter confirms that the alkyne ligand acts as a four-electron donor.The bonding features of this class of alkyne complexes has been further investigated at the theoretical level. The electronic structure for CpMCl2(alkyne) systems has only been previously explored by Curtis et al. for M = Nb at the extended Huckel level, at the fixed geometry determined by X-ray diffraction.37 We have carried out geometry optimisations at the B3LYP/LANL2DZ level on the model compound CpMoCl2(HC≡CH). As is now well established, this level of theory affords satisfactory geometries and energies for medium-size organometallic compounds. A full optimisation indeed yields a geometry with bond distances and bond angles close to the experimental values. As is typical for this computational technique, distances are slightly overestimated (up to 0.07 Å), whereas bond angles are reproduced to within 3°. However, the orientation of the ethyne ligand is quite different from experiment, θ being only 30.78°.

Fig. 2

 2 Fig. 2 B3LYP/LANL2DZ total energies for CpMCl2(HC≡CH) (M = Nb, Mo) at different ethyne conformations.

Fig. 3

 3 Fig. 3 Energy and shape of relevant orbitals for CpMCl2(HC≡CH) (M=Nb, Mo).

Fig. 4 Table 4 Fig. 5

 445 Fig. 4 An ORTEP view of compound 5 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms except for H(9) of the η 4 (5e)-butadienyl ligand are omitted for clarity. Table 4 Selected bond distances (Å) and angles (°) for compound 5 Mo-CNT 1.986(3) Mo-C(8) 2.444(2) Mo-Cl(1) 2.4652(7) Mo-C(9) 2.303(3) Mo-Cl(2) 2.4874(6) C(6)-C(7) 1.438(4) Mo-C(6) 1.934(2) C(7)-C(8) 1.404(4) Mo-C(7) 2.329(2) C(8)-C(9) 1.440(4) CNT-Mo-Cl(1) 112.6(2) C(9)-Mo-Cl(1) 137.51(7) CNT-Mo-Cl(2) 111.7(2) C(7)-Mo-Cl(1) 84.55(7) CNT-Mo-C(6) 106.5(2) C(6)-Mo-Cl(2) 140.61(8) CNT-Mo-C(9) 109.7(2) C(9)-Mo-Cl(2) 80.85(7) Cl(1)-Mo-Cl(2) 80.36(2) C(7)-C(6)-Mo 86.0(2) C(6)-Mo-C(9) 77.52(10) C(8)-C(9)-Mo 77.8(2) C(6)-Mo-C(7) 38.02(10) C(10)-C(6)-Mo 147.9(2) C(9)-Mo-C(7) 63.06(9) C(16)-C(9)-Mo 126.2(2) C(3)-Mo-C(7) 131.65(9) C(8)-C(9)-C(16) 122.9(2) C(6)-Mo-C(8) 65.26(9) C(7)-C(6)-C(10) 126.1(2) C(9)-Mo-C(8) 35.16(9) C(8)-C(7)-C(6) 115.2(2) C(7)-Mo-C(8) 34.11(9) C(7)-C(8)-C(9) 116.8(2) C(6)-Mo-Cl(1)93.97(8)

  for C10H13Cl2Mo: C, 40.03; H, 4.37. Found: C, 40.32; H, 4.22%. IR (Nujol mull, cm -1 ): 1730 [m, ν(C≡C)], MS (EI, 70 eV): m/z 301, [M] + (MoCl2 pattern, 2%); 233 [CpMoCl2] + (MoCl2 pattern, 6%), 68 [EtC≡CMe] + (47%). μeff = 1.60μB (diamagnetic correction = -148 × 10 -6 cgsu).

  for C11H15Cl2Mo: C, 42.07; H, 4.80. Found : C, 41.63; H, 4.75%. IR (Nujol mull, cm -1 ): 1732 [m, ν(C≡C)], MS (EI, 70 eV): m/z 315, [M] + (MoCl2 pattern, 13%); 233 [CpMoCl2] + (MoCl2 pattern, 67%), 82 [EtC≡CEt] + (56%).

  for C14H13Cl2Mo: C, 48.31; H, 3.76. Found: C, 48.16; H, 3.65%. IR (Nujol mull, cm -1 ): 1717 [m, ν(C≡C)], MS (EI, 70 eV): m/z 349 [M] + (MoCl2 pattern, 4%), 233 [CpMoCl2] + (MoCl2 pattern, 6%), 116 [PhC≡CMe] + (80%). μeff = 1.64μB (diamagnetic correction = -157 × 10 -6 cgsu).

  C{ 1 H} NMR (CDCl3): δ 294.8 (Mo=CEt), 139.1 (CEt), 116.6 (CEt), 102.6 (Cp), 76.5 (CHEt), 36.4 (CH2), 24.4 (CH2), 23.9 (CH2), 20.3 (CH2), 17.4 (CH3), 14.5 (CH3), 13.3 (CH3), 9.2 (CH3).

Table 1 EPR

 1

	parameters for compounds 1-4 in CH2Cl2:THF (1: 2)
		T=298 K		T=100 K
	g iso	|Aiso|	g||	g⊥	|A⊥ Mo |
	1 1.987	37.5	2.043	1.970	63.0
	2 1.987	37.5	2.043	1.969	63.2
	3 1.988	38.6	2.040	1.971	61.5
	4 1.989	38.9	2.038	1.971	61.7

Table 2

 2 Comparison between experimental and calculated bond distances and angles for compounds CpMCl2(alkyne) (M = Nb, Mo)

		M=Nb		M=Mo
		Experimental a	Optimised b	Experimental c	Optimised b
	M-CNT	2.113	2.180	2.010(5)	2.087
	M-Cl	2.365(2)	2.426	2.375(9)	2.423
	M-Cl	2.353(2)	2.426	2.370(9)	2.423
	M-C	2.079(5)	2.096	2.034(3)	2.056
	M-C	2.063(5)	2.096	2.021(3)	2.056
	C≡C	1.307(7)	1.328	1.303(4)	1.324
	CNT-M-Cl	115.7	114.38	120.6(3)	117.29
	CNT-M-Cl	115.2	114.38	116.0(4)	117.29
	CNT-M-C	111.9	114.88	113.3(3)	116.09
	CNT-M-C	114.1	114.88	116.0(4)	116.09
	Cl-M-Cl	99.4(1)	101.69	89.12(4)	91.93
	C-M-C	36.8(2)	36.93	37.47(11)	37.56
	M-C≡C	72.3(3)	71.54	71.8(2)	71.22
	M-C≡C	70.9(3)	71.54	70.7(2)	71.22
	C-C≡C	142.3	145.97	145.1(3)	143.53
	C-C≡C	144.3	145.97	145.8(3)	143.53

Table 3

 3 Cyclic voltammetric data for compounds 1

-4 in

				THF at
	room temperature and comparison with related η 4 -diene
	complexes
	Compound	E1/2(Mo II /	Ep.a(Mo III /	Ref.
		Mo III )	Mo IV ) a
	CpMoCl2(η 2 -EtC≡CMe), 1	-1.32	0.14	This work
	CpMoCl2(η 2 -EtC≡CEt), 2	-1.33	0.16	This work
	CpMoCl2(η 2 -PhC≡CMe), 3	-1.20	0.22	This work
	CpMoCl2(η 2 -PhC≡CPh), 4	-1.14	0.32	This work
	CpMoCl2(η 4 -C4H6)	-1.11	0.73	28,56
	CpMoCl2(η 4 -C4H5Me-2)	-1.19	0.75	56
	CpMoCl2(η 4 -C4H4Me2-2,3)	-1.24	0.66	56
	a All potentials are measured at a scan rate of 200 mV.

  solution afforded dark red crystals. Anal. (obtained in the presence of V2O5 as a combustion catalyst) Calc. for C19H15Cl2Mo: C, 55.64; H, 3.69. Found: C, 55.29; H, 3.65%. IR (Nujol mull, cm -1 ): 1700 [m, ν(C≡C)], MS (EI, 70 eV): m/z 411 [M] + (MoCl2 pattern, 2%), 233 [CpMoCl2] + (MoCl2 pattern, 6.5%), 178 [PhC≡CPh] + (100%). μeff = 1.61μB (diamagnetic correction = -178 × 10 -6 cgsu).

Table 5

 5 Crystal data and structure refinement for compounds 4 and 5 =0.0198, wR2 b =0.0512 R1 a =0.0312, wR2 b =0.0673 R for IRC R1 a =0.0231, wR2 b =0.0526 R1 a =0.0378, wR2 b =0.0716

	Compound	4	5
	Formula	C19H15Cl2Mo	C17H26Cl2Mo
	M	410.15	397.22
	T/K	293(2)	110(2)
	Crystal system	Orthorhombic	Triclinic
	Space group	Pna21	P1 ¯
	a/Å	18.958(6)	7.4272(4)
	b/Å	12.485(1)	8.5344(3)
	c/Å	7.187(1)	13.7739(6)
	α/°		89.929(3)
	β/°		79.954(3)
	γ/°		86.459(2)
	V/Å 3	1701.1(6)	858.01(7)
	Z	4	2
	λ/Å	0.71073	0.71073
	Diffractometer	Nonius CAD4	Nonius Kappa CCD
	µ/mm -1	1.078	1.065
	Crystal size/mm 3	0.48 × 0.36 × 0.24	0.050 × 0.062 × 0.118
	RC=reflections collected	1860	10243
	IRC=independent RC	1860	3924 [R(int)=0.04]
	IRCGT=IRC and [I> 2σ(I)]	1764	3546
	R for IRCGT	R1 a

Acknowledgements

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


Christian Bessière
email: bessiere@lirmm.fr

Assef Chmeiss
email: chmeiss@cril.univ-artois.fr

Lakhdar Saïs
email: sais@cril.univ-artois.fr

Neighborhood-Based Variable Ordering Heuristics for the Constraint Satisfaction Problem

One of the key factors in the efficiency of backtracking algorithms is the rule they use to decide on which variable to branch next (namely, the variable ordering heuristics). In this paper, we give a formulation of dynamic variable ordering heuristics that takes into account the properties of the neighborhood of the variable.

Introduction

Constraint satisfaction problems (CSPs) are widely used to solve combinatorial problems appearing in a variety of application domains. They involve finding solution in a constraint network, i.e., finding values for network variables subject to constraints on which combinations are acceptable.

The usual technique to solve CSPs is systematic backtracking. But if we want to tackle highly combinatorial problems, we need to enhance this basic search procedure with clever improvements. An improvement that has shown to be of major importance is the ordering of the variables, namely, the criterion under which we decide which variable will be the next to be instantiated. Many variable ordering heuristics for solving CSPs have been proposed over the years. However, the criteria used in those heuristics to order the variables are often quite simple, and concentrate on characteristics inherent to the variable to be ordered, and not too much on the influence its neighborhood could have. Those that use more complex criteria, essentially based on the constrainedness or the solution density of the remaining subproblem, need to evaluate the tightness of the constraints, and so, need to perform many constraint checks.

The goal of this paper is to propose heuristics that take into account properties of the neighborhood in the criterion of choice of a variable, while remaining free of any constraint check.

Preamble

Numerous criteria have been proposed to find good variable orderings for backtrack search procedures. Among them, dynamic1 variable orderings (DVOs) have always shown better average performances than static ones. In [START_REF] Haralick | Increasing tree seach efficiency for constraint satisfaction problems[END_REF], Haralick and Elliott introduced dom, the DVO choosing as next variable the one with the smallest remaining domain.

Since dom can be completely fooled by the structure, especially at the beginning of the search, when domains have more chances to be of equal size, other heuristics have been proposed. dom+futdeg is the one derived from the Brélaz heuristic (proposed for graph coloring) [START_REF] Brélaz | New methods to color the vertices of a graph[END_REF]. It breaks ties in dom by preferring the variable with highest future degree [START_REF] Smith | The Brélaz heuristic and optimal static orderings[END_REF]. Smith also improved dom+futdeg by adding to it a second and a third tie breakers, namely, the size of the smallest neighbor, and the number of triangles in which the first chosen variable is involved. She called this DVO BZ3.

However, both dom+futdeg and BZ3 use the domain size as the main criterion. The degree of the variables is considered only in case of ties, which can again fool the heuristic. Combined heuristics [START_REF] Bessière | MAC and combined heuristics: two reasons to forsake FC (and CBJ?) on hard problems[END_REF] do not give priority to the domain size or degree of variables, but use them equally in the criterion. DD chooses as the next variable, the variable X i minimizing the ratio "size of domain/degree". DD has extensively been studied in [START_REF] Smith | Trying harder to fail first[END_REF].

To give an insight into the state of the art, we performed experiments with some of these well-known heuristics: dom, the oldest and most well known DVO, DD and BZ3, the best current VOs for CSPs [START_REF] Smith | Trying harder to fail first[END_REF][START_REF] Smith | The Brélaz heuristic and optimal static orderings[END_REF]. On random problems with 10 values per domain, an average degree of 5 (i.e., 5/2 times more constraints than variables), and the tightness fixed at the cross-over point (which is stable at 55 forbidden tuples per constraint), we increased the number N of variables by steps of 10, and could see the following:2 -when N = 110, DD needs less than 1 sec., BZ3 less than 10 sec., and dom less than 100 sec., -when N = 120, dom goes above 100 sec., -when N = 150, DD needs less than 10 sec., and BZ3 less than 100 sec., -when N = 160, BZ3 goes above 100 sec., -when N = 210, DD goes above 100 sec.

Multi-level DVOs

One of the key features for the efficiency of a backtrack search method lies in its branching strategy. At each step of the search process, a problem P is reduced into a finite number of sub-problems (P 1 , P 2 , . . . , P |D(Xi)| ), where X i is the chosen variable. Following ideas developed for the DP procedure on SAT, we think that a good DVO should reduce both the number and the difficulty of such subproblems.

We propose a general formulation of DVOs which integrates in the selection function a measure of the constrainedness of the given variable. The constrainedness of a variable can be defined as a function of the constraints involving the variable. One could choose semantical constraints-based measures (e.g., number of allowed tuples) or syntactical ones (e.g., size of the Cartesian product of the domains). Choosing the most constrained variable should have a great impact on the search space, leading the search to the most constrained parts of the CSP, and thus provoking early detection of local inconsistencies.

A General Criterion Free of Constraint Checks

From now on, we will denote by Γ (X i ) the set of variables sharing a constraint with the variable X i . Let us first define W (R ij ) as the weight of the constraint R ij and,

(1) W (X i ) = X j ∈Γ (X i ) W (Rij ) |Γ (Xi)|
as the mean weight of the constraints involving X i . In order to maximize the number of constraints involving a given variable and to minimize the mean weight of such constraints, the next variable to branch on should be chosen according to the minimum value of

(2) H(X i ) = W (Xi) |Γ (Xi)|
over all uninstantiated variables (numerator to minimize the weight, and denominator to maximize the number of constraints).

For reasons of efficiency of computation, the weight we will associate to a constraint must be something cheap to compute (e.g., free of constraint checks). It can be defined by W (R ij ) = α(X i ) α(X j ), where α(X i ) is instantiated to a simple syntactical property of the variable such as |D(X i )| or |D(Xi)| |Γ (Xi)| , and ∈ {+, ×}. For α(X i ) = |D(X i )|, and = ×, the weight associated to a given constraint R ij represents an upper bound of the number of tuples allowed by R ij .

We obtain the new formulation of (2):

(3) H α (X i ) = X j ∈Γ (X i ) (α(Xi) α(Xj )) |Γ (Xi)| 2

Multi-level Generalization

In the formulation of the DVOs presented above, the evaluation function H(X i ) considers only the variables at distance one from X i (first level or neighborhood). However, when arc consistency is maintained (MAC), the instantiation of a value to a given variable X i could have an immediate effect not only on the variables of the first level, but also on those at distance greater than one.

To maximize the effect of such a propagation process on the CSP, and consequently to reduce the difficulty of the subproblems, we propose a generalization of the DVO H α such that variables at distance k from X i are taken into account. This gives what we call a "multi-level DVO", H (k,α) . To obtain this multi-level DVO, we simply replace α(X j ) in formula ( 3) by a recursive call to H (k-1,α) (X j ). This means that to compute H (k,α) on a given variable, we need to compute H (k-1,α) on all its neighbors, and so on. The recursion terminates with H (0,α) , equal to α. This is formally stated as follows:

(4) H (0,α) (X i ) = α(X i ) (5) H (k,α) (X i ) = X j ∈Γ (X i ) (α(Xi) H (k-1,α) (Xj )) |Γ (Xi)| 2
In the following, H (0,dom) and H (0,DD) are denoted by their classical name, dom and DD, respectively. H (k,dom) and H (k,DD) are denoted by H k dom and H k DD respectively.

Experiments

We have compared experimentally the behavior of the new DVOs defined above (and others) on several classes of random CSPs and on real instances from the FullRLFAP archive3 . We give here a brief snapshot of the results. Extensive experiments can be found in [START_REF] Bessière | Neighborhood-based variable ordering heuristics for the constraint satisfaction problem[END_REF]. In all our experiments, we stopped search after the first solution is found. The search procedure used maintains arc consistency (MAC).

We ran the H 1 DD ( ∈ {+, ×}) on the experiment described in Section 2. The gap between H 1 DD and DD grows with N . At N = 230, H 1 DD × is more than 5 times faster than DD, which was by far the best DVO known so far. We performed other experiments, fixing the number of variables to 100, and increasing the number of constraints in the network. H 1 DD × becomes better and better when density grows. (As opposed to H 1 DD + which was even better than H 1 DD × on sparse problems, but which becomes slower on denser problems.)

If we increase the number of variables or the domain size, the gain of the H (1,α) heuristics continues to grow compared to DD.

As a synthesis of the results on different classes of random CSPs, we can say that, except H 1 dom ×, the first level DVOs improve significantly DD. Furthermore, in general, H 1 DD are better than H 1 dom . This is not surprising because the former take into account the connectivity of the neighborhood of the chosen variable.

We also compared the behavior of these DVOs on the real instances of the FullRLFAP archive. Since these are optimization problems, we built a series of satisfaction problems for each instance of optimization problem. In the table below, we report results for all instances on which a significant difference has been observed among the DVOs tested. The cpu time limit was put to one hour on a PC Pentium II 300MHz. No conclusion can be drawn on a so small number of pertinent instances. However, it seems that H 1 DD are better on inconsistent problems, and H 1 dom on satisfiable ones. But, more extensive tests should be run to draw definite conclusions.

Conclusion

In this paper, a general formulation of dynamic variable ordering heuristics has been proposed. It admits numerous advantages, -the constrainedness of a given variable is computed without any constraint check, thanks to simple syntactical properties, -it takes advantage of the neighborhood of the variable, with the notion of distance as a parameter, -it can be instantiated to different known variable ordering heuristics, -it is possible to use other functions to measure the weight of a given constraint.

This work was partially supported by the IUT of Lens, the Nord/PasdeCalais region, and the European community.

A variable ordering is dynamic when it can change the order of the variables from one branch to the other.

These experiments have been run on a PC Pentium III 667 MHz under Linux. 100 instances for each value of the parameters.

We thank the Centre d'Electronique de l'Armement (France).