Li-Sheng Wang

James C Fettinger

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

Prof Rinaldo Poli

Cyclopentadienylmolybdenum(II) and -(III) Complexes Containing Diene and Allyl Ligands. 1. Isomeric Preferences and Isomerization Rates in a Pair of Redox-Related Organometallic Complexes

Treatment of CpMoCl 2 (-C 4 H 6 ) (1) with 1 equiv of allylmagnesium bromide yields the dinuclear complexes [CpMo(-C 4 H 6 )(µ-Br)] 2 (2, major) and Cp 2 Mo 2 (-C 4 H 6 ) 2 (µ-Br)(µ-Cl) (3, minor). A solid solution of compounds 2 and 3 adopts an anti geometry in the solid state, as shown by X-ray crystallography, whereas both anti and syn isomers are observed in benzene solution by 1 H NMR spectroscopy. The reaction of 1 with 2 equiv of allylmagnesium bromide yields [CpMo(-C 3 H 5 )(-C 4 H 6 )] as an equilibrium mixture of a major (98%, 4a) and a minor (2%, 4b) isomer. NOE-NMR studies indicate the CpMo(prone-C 3 H 5 )(supine-C 4 H 6 ) orientation for the major isomer 4a, which is also found in the solid state by X-ray crystallography. The orientation of 4b is suggested by the 1 H-NMR chemical shifts as

The green compound 5a converts into the more stable red-violet 5b with an estimated half-life of < 20 s in THF. It can be observed, however, at low temperature by EPR spectroscopy. The [CpMo(supine--C 3 H 5 )(supine--C 4 H 6 )][PF 6 ] configuration for 5b has been confirmed by Xray diffraction methods. Upon reduction with cobaltocene, 5b is converted selectively to 4b, followed by slow equilibration (t 1/2 = 6.5 h) with 4a. Refluxing or photolyzing a solution of 4a/b in benzene generates a third isomer, 4c, which adopts a CpMo(supine-C 3 H 5 )(s-trans-C 4 H 6 ) configuration as confirmed by an X-ray analysis. The distribution of 4a and 4c at equilibrium is approximately 1:1 either by starting from 4a/b or from pure 4c and independent of equilibration method (thermal/photochemical).

Oxidation of 4c generates the corresponding 1-electron oxidation product 5c, which rapidly isomerizes to 5b. It can be observed, however, by EPR in THF together with a fourth isomer 5d (ca. 1:1), believed to differ from 5c only in the orientation of the allyl ligand. Equilibrium, rate, and electrochemical data allow to sort out most of the thermodynamic and kinetic parameters related to the transformation of the different compounds. The faster s-trans-to s-cis-butadiene isomerization for 5 relative to 4 indicates the easier accessibility of the unsaturated 15-electron vs. 16-electron intermediate. Possible reasons for this trend are analyzed.

Introduction

Transition metal complexes containing allyl ligands and/or conjugated diene ligands have attracted considerable attention, due to the importance of these complexes as starting materials or reactive intermediates in catalytic processes, [START_REF] Jolly | The Organic Chemistry of Nickel[END_REF] and as reaction substrates in organic synthesis. [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] On the other hand, the coordination chemistry of allyl and diene ligands is of interest in terms of the variable coordination modes and different reactivities. [START_REF] Erker | [END_REF]5 In addition, compounds that contain contemporarily a butadiene and an allyl ligand are important models for diene polymerization catalysts or are themselves catalytically active. [START_REF] Boor | Natta Catalysts and Polymerization[END_REF][START_REF] Gabassi | Transition Metal Catalyzed Polymerizations[END_REF][START_REF] Teyssié | Transition Metal Catalyzed Polymerizations[END_REF][START_REF] Porri | Comprehensive Polymer Science[END_REF][START_REF] Porri | [END_REF][11] As a continuation of our studies of cyclopentadienylmolybdenum(II) complexes having unsaturated hydrocarbons as ligands, 12 our attention has focused on the direct synthesis of previously unreported diene-allyl complexes. In this first paper of a series, we present our results on the preparation and characterization of the diamagnetic complex CpMo(-C 3 H 5 )(-C 4 H 6 ) in three different isomeric forms, and its paramagnetic one-electron oxidation product, [CpMo(-C 3 H 5 )(-C 4 H 6 )] + , in four different isomeric forms. The isolation and characterization of a halide-bridged dinuclear Mo(II) intermediate is also reported. Later contributions will deal with the chemical reactivity of these compounds and with their activity as butadiene polymerization catalysts.

Experimental

General Procedures. All reactions were conducted by standard Schlenk-line techniques under a dinitrogen atmosphere. Solvents were dried by conventional methods (THF and Et 2 O on Na/K/benzophenone, toluene and heptane on Na, CH 2 Cl 2 and CHCl 2 CH 2 Cl from P 4 O 10 ) and distilled directly from the drying agent under dinitrogen or argon. All routine NMR experiments were carried out on a Bruker AM400 spectrometer, while 2D-NMR and 13 C-NMR data were obtained from a Bruker AMX500 spectrometer. EPR spectra were recorded on a Bruker ER200 spectrometer upgraded to ESP300 and IR spectra on a Perkin-Elmer FTIR 1600 spectrophotometer. Cyclic voltammograms were recorded with an EG&G 362 potentiostat connected to a Macintosh computer through MacLab hardware/software; the electrochemical cell was a locally modified Schlenk tube with a Pt counterelectrode sealed through uranium glass/Pyrex glass seals. The cell was fitted with a Ag/AgCl reference electrode and a Pt working electrode. All potentials are reported vs. the Cp 2 Fe/Cp 2 Fe + couple which was introduced into the cell at the end of each measurement. Elemental analyses were performed by M-H-W Laboratories, Phoenix, AZ. Compound CpMoCl 2 was prepared as previously described. 13 The C 3 H 5 MgBr solution in Et 2 O (1 M, Aldrich) was used as received. All 1 Hand 13 C-NMR data are given in Table 1.

Preparation of CpMoCl 2 (-C 4 H 6 ) (1).

CpMoCl 2 (4.50 g, 19.4 mmol) was suspended in 100 mL of THF. Excess butadiene was condensed into the mixture (the volume of the liquid increased by ca. 20 mL), followed by heating to 70-80˚C for 4 h with magnetic stirring. The yellow brown solid gradually dissolved to yield a red brown suspension. After cooling to room temperature, the solvent was reduced to ca. 10 mL by evaporation under reduced pressure and n-heptane (100 mL) was added to complete the precipitation of the product. The supernatant was decanted off and the crystalline red-brown solid was washed with several portions of nheptane (total ca. 100 mL) and dried under vacuum overnight (yield 5.13 g, 90 %). EPR (THF): g = 2.012, a Mo = 38 G. The physical and spectroscopic properties of this material match those previously reported. 14 Table 1. 1 H-and 13 C-NMR Data a compd 1 H-NMR (δ) 13 C-NMR (δ) anti-[CpMo(η-C4H6)(µ-Br)]2 (2a) b 7. 54 7.67 (ddddd, 2H, Ha, 3 JHHc = 8.9 Hz, [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHb = 3 JHHa' = 6.9 Hz, [START_REF] Erker | [END_REF] JHHb' = [START_REF] Erker | [END_REF] JHHc' = 1.2 Hz); 7.37 (ddddd, 2H, Ha', [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHc' = 8.4 Hz, [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHb' = 3 JHHa = 7.9 Hz, [START_REF] Erker | [END_REF] JHHb = 4 JHHc = 1.2 Hz); 4.53 (s, 10H, Cp); 2.37 (ddd, 2H, Hb, 3 JHHa = 6.9 Hz, 2 JHHc = 4

JHHa' = 1.5 Hz); 2.35 (ddd, 2H, Hb', [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHa' = 6.9 Hz, 2 JHHc' = 4 JHHa = 1.2 Hz); 0.08 (overlap, Hc); 0.00 (overlap, Hc') syn-Cp2Mo2(η-C4H6)2(µ-Br)(µ-Cl) (3b) b 7.24 (ddddd, 2H, Ha, 3 JHHc = 8.9 Hz, [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHb = 3 JHHa' = 7.4 Hz, [START_REF] Erker | [END_REF] JHHb' = 4 JHHc' = 1.2 Hz); 6.87 (ddddd, 2H, Ha', [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHc' = 8.9 Hz, [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHb' = 3 JHHa = 7.4 Hz, [START_REF] Erker | [END_REF] JHHb = 4 JHHc = 1.5 Hz); 4.33 (s, 10H, Cp); 2.24 (ddd, 2H, Hb, 3 JHHa = 7.9 Hz, 2 JHHc = 4

JHHa' = 1.5 Hz); 2.24 (ddd, 2H, Hb', [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHa' = 7.9 Hz, [START_REF] Jolly | The Organic Chemistry of Nickel[END_REF] JHHc' = 4 JHHa = 1.5 Hz); 0.04 (overlap, Hc); -0.08 (dddd, 2H, Hc', [START_REF] Birch | Transition Metal Organometallics in Organic synthesis[END_REF] JHHa' = 8.9 Hz, [START_REF] Jolly | The Organic Chemistry of Nickel[END_REF] JHHb' = 1.6 Hz, [START_REF] Erker | [END_REF] JHHa = 1.4 Hz, 5 This product does not decompose in the solid state upon brief exposure to air, but it is significantly deteriorated upon exposure for several hours. Crystals suitable for the X-ray crystallographic analysis were obtained from recrystallization from diethyl ether at -80˚C. The Schlenk was immersed in a water bath, which served to maintain the temperature below (2.5 mg, 0.013 mmol) was added. All the solid rapidly dissolved to yield a red solution. The 1 H-NMR spectrum of this solution shows the presence of 4b and no significant amount of 4a.

Preparation of

Continued 1 H-NMR monitoring of this solution revealed the isomerization of 4b to 4a with t 1/2 = 6.5 h at room temperature.

Oxidation of compound 4c. Formation of compounds 5c and 5d. Compound 4c

(1.5 mg, 5.9 µmol) and Cp 2 FePF 6 (2.0 mg, 6.0 µmol) were placed in a 3 mm Pyrex EPR tube.

The addition of THF (150 µL) to the tube produced a suspension. The reaction was monitored by EPR spectroscopy, indicating the formation of two new isomers of compound 5 (g = 2.000, a H = 6.0 G for one isomer; g = 1.986, a H = 6.0 G for the other isomer) (see Results). It was clearly asymmetric between these two atoms and believed to be a portion of a partial occupancy C 4 H 6 group that overlapped with the major group in nearly the same location. Two 2). A final difference-Fourier map was essentially featureless with the largest peaks, ||  1.095 eÅ -3 , in the vicinity of the Mo(1) or Br(1) atoms. Selected bond distances and angles are listed in Table 3. 2, while selected bond distances and angles are listed in Table 4.

Table 4. Selected bond distances (Å) and angles (°) for the structure of 4a.

centrosymmetric choice is believed to be the correct one. The molecule sits on a special position, creating symmetry-related disorder, in all space groups (more severely so in the centrosymmetric one), thus a complex set of constraints was required to allow proper convergence of the structure. Full details of the structure refinement in the various groups are available in the Supporting Information. A final difference-Fourier map was essentially featureless with the largest peak having ||  1.13 e . Å -3 within the PF 6 counterion and ||  0.44 e . Å -3 within the cation. Crystal and refinement parameters are collected in Table 2. No table of distances and angles are reported because the severe disorder and the extensive use of restraints makes these data void of chemical significance. These data are, however, available in the Supporting Information. CCF 3 ) 2 with butadiene, 14,15 this methodology being limited to a maximum theoretical yield of 50%. The availability of CpMoCl 2 as starting material 13 has allowed us to directly prepare complex 1 according to eq 1 in yields up to 90%.

CpMoCl 2 + C 4 H 6 ² /THF CpMoCl 2 (C 4 H 6 ) (1)
Treatment of a THF suspension of compound 1 with one equiv of allylmagnesiumbromide at -78˚C produces compound [CpMo(-C 4 H 6 )(µ-Br)] 2 (2), accompanied by a small amount of Cp 2 Mo 2 (-C 4 H 6 ) 2 (µ-Br)(µ-Cl) (3), see eq. 2. The chemical and structural nature of the isolated material is confirmed by 1 H-NMR spectroscopy and X-ray crystallography (vide infra). In addition, the nature of the major component as a bromo instead of chloro derivative is established by a low-resolution anionic mass spectrum, which clearly exhibits the bromide mass peaks at M = 79 and 81 with 1:1 isotopic pattern as the most intense peaks in the spectrum.

The chloride anion was not detected in the mass spectrum, presumably due to low concentration (Br:Cl = 93:7 mole/mole as derived from 1 H-NMR, vide infra).

CpMoCl 2 (C 4 H 6 ) C 3 H 5 MgBr [CpMo(µ-Br)(C 4 H 6 )] 2 + Cp 2 Mo 2 (µ-Br)(µ-Cl)(C 4 H 6 ) 2 2, major (86%) 3, minor (14%) (2)
In a C 6 D 6 solution, both compounds 2 and 3 give rise to two isomers in a 1:1 ratio.

The assignment of the resonances of species 2a to the anti isomer and those of species 2b to the syn isomer, rather than the opposite, is based on the dissolution and immediate NMR investigation of crystals of the compound, which is shown by X-ray crystallography to have the anti configuration, in toluene-d 8 at 0°C. The spectrum shows the dominant signals of species 2a (2a:2b = 2 : 1), but the ratio rapidly evolves to the equilibrium ratio of 1 : 1 at room temperature, indicating that (i) the two isomers have essentially the same energy, and (ii) the interconversion between the two isomers is quite facile. The assignment of the 3a and 3b sets of resonances to the anti and syn isomers, respectively, follows that of species 2a and 2b, with the most downfield shifted set of resonances assigned to the anti isomer and the most upfield shifted one assigned to the syn isomer. There is a greater dependence of the chemical shifts on the configuration (anti vs. syn) than on the nature of the halogen bridges (2 vs. 3), see Table 1.

Because of extensive overlap between the resonances of the various compounds, a complete assignment was only possible following a 2D-COSY NMR experiment 16 (Supplemental Figure 1). The nomenclature of the butadiene H and C atoms in Table 1 follows I. All butadiene resonances are first order, well resolved doublets of doublets of doublets for the isomers of 3, while all are unresolved multiplets for the isomers of 2.

Direct reaction of 1 with two equiv of allylmagnesium bromide (eq. 4), or reaction of 2/3 with one equiv of the same reagent (eq. 5), yields the allyl-butadiene complex [CpMo(- -MgClBr

C 3 H 5 )(-C 4 H 6 )] (4
2 C 3 H 5 MgBr 2 CpMo(-C 3 H 5 )(-C 4 H 6 ) [CpMo(µ-X)(-C 4 H 6 )] 2 (5)

-MgXBr

The assignment of the 13 C NMR signals was possible by way of a 1 H- 13 C-HMQC-NMR experiment (Supplemental Figure 2). 17 All the 13 C chemical shifts appear to be in the expected region. [START_REF] Mann | 13-C NMR data for organometallic compounds[END_REF] The coupling constant for the butadiene terminal carbon atom ( 1 J CH = 154 Hz) indicates an sp 2 hybridization [START_REF] Friebolin | Basic One-and Two-Dimensional NMR Spectrocopy[END_REF] and suggests a conventional butadiene ligand.

The assignment of the 1 H-NMR resonances for isomer 4b has been confirmed by both homonuclear decoupling experiments and integration, but the orientation of the ligands (supine The major effect is on proton H d because this experiences a greater difference of ring current upon rearrangement from the prone to the supine configuration. In 4b, the resonance of H d is close to that of the central butadiene proton (H a ). The reliable assignment of the allyl conformation on the basis of chemical shifts has been illustrated previously. [START_REF] Faller | [END_REF]21 An additional interesting observation is the presence of discernible allyl geminal coupling (i.e. H e H f ) in 4a

but not in 4b. The same phenomenon has previously been reported for the prone and supine isomers of CpM( 3 -C 3 H 5 )(CO) 2 (M = Mo, W) and indenyl analogues, [START_REF] Faller | [END_REF] and for CpM( 3 -C 3 H 5 )(CO) and (indenyl)M( 3 -C 3 H 5 )(CO) (M = Fe, Ru). 21,22 The supine-diene-supine-allyl conformation has been observed in a structurally related 16-electron complex, [CpZr(- 23 So long as solutions of 4a/b remain at room temperature, no changes in the spectroscopic properties are observed over long periods of time. However, warming a C 6 D 6 solution to 100°C in a flame sealed NMR tube, or subjecting the same solution to UV irradiation at room temperature, leads to equilibration with a third isomer, 4c. The photolytic treatment also increases significantly the relative amount of isomer 4b (see Experimental).

C 3 H 5 )(-C 4 H 6 )].
Compound 4c can be isolated as a pure crystalline substance because the isomerization process is frozen at room temperature and only 4c is sufficiently stable to withstand a chromatographic treatment on a silica gel column with n-heptane elution. Treatment of pure 4c under thermal or photolytic conditions produces mixtures of the three isomers with the same composition as obtained from the corresponding treatment of the 4a/b mixture. Unlike isomers 4a and 4b, isomer 4c has no symmetry element, as shown by the number of resonances in the 1 H-and 13 C{ 1 H}-NMR spectra (see Table 1; for the nomenclature, refer to IV). Homonuclear decoupling experiments and 2D-COSY (Supplemental Figure 3), 13 C-DEPT, 24 and 1 H- 13 C HMQC 17 (Supplemental Figure 4) NMR experiments allow the complete assignment of all resonances and the formulation as CpMo(supine--C 3 H 5 )(s-trans--C 4 H 6 ), which is confirmed by an X-ray structural investigation (vide infra).

(b) Electrochemistry studies and syntheses and EPR characterization of Mo(III)

compounds. In a cyclic voltammetric investigation, the equilibrium 98:2 mixture of 4a and 4b shows a quasi-reversible oxidation process in CH 2 Cl 2 at E 1/2 = -0.46 V with respect to ferrocene (process A in Figure 1). The return wave of this process has a diminished intensity, while a second reduction wave appears at more negative potential (process B in Figure 1). A second scan shows reversibility for process B (E 1/2 = -0.87 V). A negative scan starting from -0.73 V shows that wave B is initially absent. These observations suggest that oxidation of 4a yields the one-electron oxidation product, [CpMo(prone--C 3 H 5 )(supine--C 4 H 6 )] + , 5a, but that this subsequently evolves relatively rapidly to another redox-active species with a lower reduction potential. A completely analogous behavior is observed in THF, the E 1/2 values being -0.45 V for A and -0.85 V for B. According to these results, the green solid corresponds to 5a, whereas red-violet 5b is an isomer in which 5a transforms rapidly and quantitatively. By generating a dilute solution of green 5a in situ in THF and monitoring the EPR spectrum (vide infra), a complete conversion to 5b was observed within 1 minute at room temperature, leading to an estimation of the half life for the isomerization of t

H b H c H a H a' H b' H c' H d H e' H f' H f H e IV Mo
The EPR spectrum of solutions of 5b displays a binomial septet (a H = 7 G) at g = 2.028, in addition to the characteristic Mo satellites (I=5/2, 25%). The room temperature spectrum is not sufficiently sharp to observe all seven lines, but the resolution improved sufficiently upon cooling to -80°C (see Figure 2). There may be a question concerning the possible attribution of the two outer lines of the central septet feature to the Mo satellites. To resolve this question, a simulation was performed for both the septet and the pentet models (both shown in Figure 2). Clearly, the experimental spectrum can be satisfactorily simulated only as a septet. respectively. Although the spectral resolution under these conditions is not optimal, the simulation in Figure 3(c) appears closer to the experimental spectrum. Thus, it seems that the unpaired electron in this compound is strongly coupled to only four H nuclei (a H = 8.5 G, a Mo = 28 G, linewidth = 4 G). It is also to be noted that the 17-electron compound CpMoCl 2 (-C 4 H 6 ) (1) exhibits coupling to the four terminal hydrogen atoms of the butadiene ligand [a H = 6.64 G (syn) and 3.94 G (anti)]. 14 On the basis of the above results, all four terminal hydrogen nuclei of the butadiene group appear to be responsible for the hyperfine splitting in both compounds 5a and 5b, while the two additional H nuclei that are coupled with the electron in 5b are two of the four terminal H atoms of the allyl ligand. The EPR spectra of substituted allyl derivatives, which will be described in a later contribution, are also in agreement with these assignments. The hyperfine coupling to the different types of nuclei in 5a and 5b (Figures 2 and3) is degenerate within the limits of the instrumental resolution. Compound 4c also undergoes a one-electron oxidation process. In the cyclic voltammogram, this is observed as a reversible wave at E 1/2 = -0.34 V vs. ferrocene in CH 2 Cl 2 or -0.34 V in THF, e.g. slightly less negative than the oxidation of 4a. Of the three isomers, therefore, 4c is the most difficult to oxidize, whereas 4b is the easiest. The chemical oxidation of 4c by ferrocenium in THF initially generates solutions containing three forms of compound 5, as shown by EPR spectroscopy (Figure 4). One compound is 5b, whereas the other two compounds are characterized by resonances at lower g values (2.000 and 1.986, respectively)

showing the same hyperfine pattern as 5b (a H = 6.0 for both). These two species slowly disappear, leaving only 5b as the final EPR active compound, see Figure 4. We presume that one of these two species corresponds to the same geometry of the neutral precursor, i.e.

[CpMo(supine--C 3 H 5 )(s-trans--C 4 H 6 )] + , 5c. The geometry of the other species is likely to differ from that of 5c by the orientation of the allyl ligand, as was found for isomers 5a and 5b.

Thus, we assign to this final isomer the configuration [CpMo(prone--C 3 H 5 )(s-trans-- These decomposition reactions were not further pursued.

X-ray structures. The solid state molecular structure of the 2/3 mixture is shown in Figure 5. The presence of both compounds as a solid solution in the crystal is suggested by the refinement of the X-ray data. The freely refined relative occupancy of 89% Br and 11% Cl is in good agreement with the occupancies independently established in solution by 1 H-NMR.

Obviously, such an agreement could be coincidental, since the crystal chosen for the X-ray analysis is not necessarily representative of the composition of the bulk material. The compound crystallizes in the monoclinic P2 1 /c space group, the dinuclear molecule adopting the anti arrangement. The molecule is located on an inversion center, but the ideal molecular symmetry is C 2h . The dihedral angle subtended by the Mo-C(6)-C( 9) and C(6)-C(7)-C(8)-C( 9) planes () is 87.6(2)°, indicating a conventional  4 -s-cis-1,3-diene coordination mode. 5 However, the shorter Mo-C bond length to the two lateral butadiene carbon atoms relative to two internal Mo-C distances (the average difference ∆d is -0.081), and especially the longer butadiene lateral C-C bond lengths relative to the central bond, suggest significant π back bonding from the metal. [START_REF] Yamamoto | Organotransition Metal Chemistry-Fundamental Concepts and Applications[END_REF] The  and ∆d data fit rather well the previously established correlation. 5 The two metals are at 4.08 Å from each other, in agreement with the absence of a direct interaction as expected from the 18-electron configuration. The analogous dimer [CpNb(C 4 H 6 )(-Cl)] 2 , on the other hand, exhibits a syn rather than anti geometry. [START_REF] Okamoto | [END_REF] This difference may be related to the presence of a direct Nb-Nb bond since this bond must be formed by overlap of the two metal d z 2 orbitals, the local z axis for each metal pointing to the center of the Cp ligand. The supine butadiene conformation observed in 2/3 is ubiquitous, as precedented for the crystallographically characterized CpZr( 3 -C 3 H 5 )( 4 -C 4 H 6 ), 23 Cp*HfCl(py)( 4 -2,3-Me 2 C 4 H 4 ), 23,27 CpTaX 2 ( 4 -C 4 H 6 ) (X = alkyl or halide), 28,29 CpMoCl 2 ( 4 -C 4 H 6 ), 14 CpMoI[P(OMe) 3 ]( 4 -CH 2 =CHCH=CHCH 2 Bu t ), 30 12) is similar to that found for 2/3, and the same is true for the Mo-C bond lengths to the carbon atoms of the butadiene ligand (average ∆d = -0.065), and for the trend of butadiene C-C bond distances. There is no apparent interligand steric hindrance, as can be seen in Figure 6. The supine orientation of the diene ligand is the ubiquitous one, as discussed above. The prone conformation of the allyl ligand is, however, less documented. A few complexes that exist in both isomeric forms in solution have been described. [START_REF] Faller | [END_REF][21][22] In the solid state, the supine orientation is observed for CpMI(NO)( 3 -C 3 H 5 ) (M = Mo, W), 31,32 CpMo( 3 -allyl){P(OMe) 3 } 2 , 30 and ( 5 -C 9 H 7 )Mo( 3 -C 9 H 7 )(dppe), 33 whereas the prone conformation was found for CpMo( 3 -C 3 H 5 )(CO) 2 . 32 Figure 6. Molecular geometry of compound 4a with the numbering scheme employed. The ellipsoids are drawn at the 30 % probability level.

For the X-ray determination of compound 5b, disorder between the allyl and butadiene ligands and a space group ambiguity (see Experimental) do not allow the establishment of reliable metric parameters for the Mo(C 3 H 4 )(C 4 H 6 ) moiety. The nature of the disorder, however, can only and unambiguously be modeled by the supine orientation for both allyl and diene ligands. A view of the structural model used for the refinement is shown in Figure 7.

This structure is reported here only as proof of stereochemistry. Precedents for structures containing allyl or diene ligands for half sandwich Mo(III) are CpMoCl 2 (supine- 4 -C 4 H 6 ) 14 and a variety of (Ring)Mo(supine- 3 -C 3 H 5 ) 2 (Ring = Cp, 2-methylindenyl, 2-metoxyindenyl, and 5,6-dimetoxyindenyl). 34,35 The same situation was found for the Cp 2 Zr(s-trans-diene) derivatives (diene = 1,3-butadiene, (E,E)-1,4-diphenyl-1,3-butadiene), 36,37 whereas the three bonds have similar lengths in CpMo(NO)(s-trans-diene) (diene = (E)-1,3-pentadiene, 2,5-dimethyl-2,4-hexadiene) derivatives. 38,39 Future efforts will be directed at the synthesis and crystallographic characterization of derivatives with substituents on the diene and allyl ligands, with the hope of obtaining ordered crystal packings.

Scheme 1

The relatively rapid syn/anti isomerization is likely to proceed via formation of 16electron mononuclear intermediates, CpMoX( 4 -C 4 H 6 ) (X = Cl, Br). We have recently demonstrated that 16-electron phosphine-substituted half-sandwich Mo(II) derivatives, e.g.

CpMoCl(PMe 2 Ph) 2 , Cp*MoCl(PMe 3 ) 2 and Cp*MoCl(dppe), can be generated and spectroscopically observed, and some are sufficiently stable to be isolated. 40 The 16-electron diene-containing intermediates, however, do not accumulate at sufficiently high levels for direct observation, and their formation is not sufficiently fast to coalesce the individual resonances of compounds 2a, 2b, 3a, and 3b. Only a slight broadening is observed upon heating a solution of 2/3 in C 6 D 6 to 120°C in a sealed NMR tube.

The 1:1 ratio of syn and anti isomers in solution for both dinuclear compounds 2 and 3 indicates little or no steric interaction between the ligands bonded to the two different metals.

The previously reported isoelectronic complexes [( 6 -C 6 H 6 )Mo( 3 -allyl)(µ-Cl)] 2 could not be investigated by NMR because of low solubility, thus their relative syn/anti proportion is not known, 41 although an X-ray structure of anti-[( 6 -C 6 H 6 )Mo( 3 -C 3 H 5 )(µ-Cl)] 2 has been determined. 42 For the related compound [( 6 -2,6-lutidine)Mo( 3 -C 3 H 5 )(µ-Cl)] 2 on the other hand, only one isomer has been reported in solution and the X-ray analysis shows the anti arrangement in the solid state. 43 This phenomenon may be attributed to the large steric bulk of the 2,6-lutidine ligand. The CpMo( 3 -allyl)( 4 -butadiene) system. Compound 4 is the first example of a CpMo(II) allyl-diene complex where the two open π-ligands are not linked with each other in a macrocyclic ligand, and it is the simplest possible example of this class. In previously reported cyclopentadienyl-allyl-diene-molybdenum(II) complexes, the rigid nature of the organic ligand does not allow any conformational flexibility. Examples are compounds V,

where the allyl and diene moieties are forced to adopt the prone and supine conformations, 44 and VI, where both moieties are forced in the prone conformation. 45 Compound 4 has been observed in three different isomeric forms, which differ in the relative orientation of the allyl and diene ligands: the prone-allyl-supine-diene (4a), the supineallyl-supine-diene (4b), and the supine-allyl-s-trans-diene (4c). It is interesting to note that compound 4 is isolobally related to the previously reported [( 6 -arene)Mo( 3 -C 3 H 5 )( 4 -C 4 H 6 )] + , for which, however, different isomeric forms have apparently not been observed. 41 Isomers 4a and 4b thermally interconvert at room temperature (t 1/2 = 6.5 h), leading to a 98:2 equilibrium mixture. The major conformation, thus, corresponds to that of the previously reported compound V, whereas the rigid conformation of VI is not observed for compound 4.

Isomer 4c is not accessible from 4a or 4b at room temperature, but can be obtained either thermally or photochemically, leading to a 1:1 equilibrium mixture of 4a and 4c.

The most typical coordination mode of the butadiene ligand is s-cis. The s-trans coordination mode has been documented in few examples. [START_REF] Erker | [END_REF][START_REF] Okamoto | [END_REF][46][47][48] The factors favoring one diene coordination mode versus the other are not well understood. For instance, the s-cis-diene complex was shown to be the kinetic product and to convert irreversibly to the more stable s-

Me Me H CF 3 F 3 C Me CF 3 CF 3
Mo VI V Mo trans-diene derivatives for the previously reported CpMo(NO)(diene) systems. 49,50 On the other hand, the behavior is exactly opposite for the zirconocene Cp 2 Zr(diene) derivatives, 51 the cationic Ru(II) derivatives [CpRu(CO)(diene)] + , 46 and for the cationic Mo(II) carbonyl complexes [Cp*Mo(CO) 2 (diene)] + , 47 the kinetically controlled product being the s-trans and the thermodynamically more stable one being the s-cis.

The [CpMo( 3 -allyl)( 4 -butadiene)] + system.

The only previously known organometallic Mo(III) compounds containing only Mo-C bonds are CpMo( 3 -allyl) 2 and analogues with substituted Cp rings, 34,35 and the one-electron oxidization product of system V. 44 The potentials for the oxidation of 4 to 5 are in the range observed for Cp*MoCl(CO)L 2 derivatives (e.g. E 1/2 = -0.48 V for L = PMe 3 and -0.44 V for L 2 = dppe), 52 whereas compound 1 is more stable in the Mo(III) neutral form relative to the reduced Mo(II) anion (E 1/2 = -1.03 V). 14 Dihalobis(phosphine) derivatives of Mo(III) are electronically even richer and cannot be reduced to the 18-electron Mo(II) anions (they are rather oxidized to the paramagnetic 16electron Mo(IV) cations). 53 The oxidized compound 5 has been observed in four isomeric forms, three of which correspond to those of the neutral precursor, 5a, 5b, 5c, and 5d. In this case, the isomeric preference is different, isomer 5a transforming rapidly (t 1/2 < 20 s) to isomer 5b. Compound V has also been reported 44 to undergo a one-electron oxidation to the corresponding cation.

However, the nature of the macrocyclic ligand locks [V] + (a green compound like 5a) in the same prone-allyl-supine-diene conformation observed for V. Isomer 5c, obtained by 1electron oxidation of 4c, is in fast equilibrium with 5d, but both of these slowly convert to the most stable isomer 5b. Because of solubility problems in THF and stability problems in CH 2 Cl 2 , rate information could not be gathered on this conversion process. The overall transformations are summarized in Scheme 2.

Scheme 2

Thermodynamics and kinetics. From the equilibrium distributions, rate data, and electrochemical data an almost complete energetic picture can be derived. Compounds 4a and 4c (1:1 at equilibrium) have approximately the same free energy, whereas 4b is 2.3 kcal/mol higher. The half-life of 6.5 h for the transformation of 4b to the 4a/4b equilibrium yields (k 1 +k -1 ) ≈ 3.0 . 10 -5 s -1 which, with the independent knowledge of the equilibrium constant (k 1 /k -1 = 98:2) yields k 1 ≈ 2.9 . 10 -5 s -1 and ∆G 1 ‡ = 23.6 kcal/mol at 298 K. Analogously, from the rate of interconversion of 4a to the 1:1 equilibrium mixture of 4c and 4a at 100°C (3 halflives < 2.5 h), the activation free energy for this process is calculated as ∆G ‡ < 30.2 kcal/mol.

If we consider, as a first approximation, that reaction and activation entropies for these isomerization processes are small, these values can be directly taken as enthlapies and compared on the same temperature independent diagram (see Figure 9). The relative free energies (and thus, enthalpies under the same assumption of negligible entropic term) in the oxidized manifold can be related to the relative free energies of the neutral Mo precursors by the oxidation potentials of the three half-reactions. This calculation confirms that compounds 5a and 5c are thermodynamically unstable with respect to the complete isomerization to 5b, as experimentally observed. The energy of 5d relative to 5c cannot be established with certainty, since these two complexes are observed in a rapid equilibrium only in solutions that are saturated in both. The activation parameters for the interconversion of 5a to 5b is derived as illustrated above for the isomerization in the Mo(II) manifold from the half life of the conversion (< 20 s at room temperature). No significant rate information could be obtained for the other transformations, although the quantitative conversion (> 4 half lives) of both 5c and 5d to 5b in less than 12 h (t 1/2 < 3h) establishes an upper limit of 23 kcal/mol for complexes. It is well established that nondissociative isomerization processes in 18-electron organometallics are significantly accelerated by one-electron oxidation. [START_REF] Trogler | Organometallic radical processes[END_REF] A dissociative isomerization (i.e. via a  3 to  1 allyl rearrangement) would seem inconsistent in principle with the relative rates, because the allyl ligand might reasonably be expected to bind more strongly to the higher oxidation state metal center. In addition, preliminary results of ligand additions processes, which will be reported in a future contribution, show substitution of the butadiene ligand rather than addition with rearrangement of the allyl ligand. The latter argument, however, only proves a thermodynamic preference for the diene substitution reaction and does not necessarily disprove an allyl rearrangement mechanism for the isomerization process.

Compounds 4/5c differ from 4/5b (and correspondingly 5d from 5a) only in the stereochemistry of the diene ligand (s-trans in the former, supine-s-cis in the latter). It is therefore likely that the isomerization of 4/5a to 4/5c and viceversa occurs via compounds 4/5b and, likewise, that the isomerization of 5d to 5b occurs via compound 5c. The cis-trans isomerization of the coordinated diene must necessarily involve dissociation of at least one of the two ene functions and rotation around the diene C-C single bond (see Scheme 3). The barrier to this interconversion is the highest amongst those shown in Figure 9, which would seem to support the idea that the above discussed interconversion of prone and supine allyl involves a nondissociative rotation.

if the 15-electron Mo(III) complex is more stable in a S = 3/2 state. Calculations at both the MP2 and DFT level have shown 56

Conclusions

The synthesis of the allyl-butadiene complex CpMo( 3 -C 3 H 5 )( 4 -C 4 H 6 ) and its 1electron oxidation product has revealed a complex stereochemical behavior, with relative energies and rates of isomer interconversion that depend on the metal oxidation state. The preference for the relative orientation of the allyl and butadiene in the coordination sphere of the same metal may be relevant for the stereocontrol of diene polymerization processes. It has been established that, for active nickel catalysts, the control of the mode of diene coordination is crucial for the stereocontrol of the polybutadiene product, with the [Ni( 3 -allyl)( 4 -s-cisdiene)] + intermediates leading to a 1,4-cis polymer and the [NiL( 3 -allyl)( 2 -s-trans-diene)] + intermediates leading to a 1,4-trans polymer. 11 Since preliminary observations indicate catalytic activity of 4 for butadiene polymerization, a thorough study of this catalytic process might add useful knowledge to the details of the stereocontrol mechanism. It will also be interesting to investigate the cationic Mo(III) systems 5 as polymerization catalysts, in view of the demonstrated greater lability of the Mo-ene bonds for this system.

JHHc = 1 . 4

 14 Hz) [CpMo(prone-η-C3H5)(supine-η-C4H6)] (4a) c 4.26 (s, 5H, Cp); 4.18 (m, 2H, Ha); 2.89 (m, 2H, Hb); 2.35 (tt, 1H, Hd, 3 JHHf = 11 Hz, 3 JHHe = 8.9 Hz); 1.70 (dm, 2H, He, 3 JHHd = 9 Hz); 0.72 (dm, 2H, Hf, 3 JHHd = 11 Hz); 0.58 (m, 2H, Hc) 108.6 (dm, Ca, 1 JCH = 162 Hz); 89.3 (dm, Cp, 1 JCH = 177 Hz); 69.6 (d, Cd, 1 JCH = 157 Hz); 39.4 (td, Cbc, 1 JCH = 154 Hz, 2 JCH = 7 Hz); 31.8 (td, Cef, 1 JCH = 153 Hz, 2 JCH = 7 Hz) [CpMo(supine-η-C3H5)(supine-η-C4H6)] (4b) c 4.10 (s, 5H, Cp); 3.95 (m, 2H, Ha); 3.77 (tt, 1H, Hd, 3 JHHf = 14 Hz, 3 JHHe = 8.5 Hz); 2.73 (m, 2He, 3 JHHd = 8.5 Hz); 2.41 (m, 2H, Hb); 1.22 (dm, 2H, Hf, 3 JHHd = 14 Hz); 0.33 (m, 2Hc) [CpMo(supine-η-C3H5)(s-trans-C4H6)] (4c) d 4.41 (s, 5H, Cp); 2.56 (dd, 1H, He, 3 JHHd = 5.7 Hz, 4 JHHe' = 3.3 Hz); 2.48 (d, 1H, Hb, 3 JHHa = 10.7 Hz); ~2.3 (m, 1H, Hd); ~2.3 (m, 1H, Ha'); 2.21 (d, 1H, Hf, 3 JHHd = 10.0 Hz); 2.06 (d, 2H, Hc and Hc', 3 JHcHa = 3 JHc'Ha' = 5.6 Hz); 1.77 (ddd, 1H, Ha, 3 JHHb = 10.7 Hz; 3 JHHc = 5.6 Hz, 3 JHHa' = 5.0 Hz); 1.37 (d, 1H, Hf', 3 JHHd = 9.6 Hz); 0.97 (dd, 1H, He', 3 JHHd = 6.0 Hz, 3 JHHe = 3.3 Hz); 0.43 (dd, 1H, Hb', 3 JHHa' = 9.6 Hz, JHHc' = 1.7 Hz) 91.5 (Cp); 87.5 (Ca'); 84.5 (Ca); 69.

  CpMo(supine--C 3 H 5 )(s-trans--C 4 H 6 ) (4c). (a) By UV irradiation. A C 6 D 6 solution of the equilibrium mixture of 4a/b (800 mg, 3.13 mmol in 30 mL) was irradiated overnight by a UV Xe lamp, resulting in a darkening of the deep red-brown solution.

Final 2 X-ray Crystallography. (a) Compounds 2 / 3 .

 223 ∑||F o |-|F c ||/∑|F o |. b wR2 = [∑w(|F o |-|F c |) 2 /∑w|F o | 2 ] 1/Crystal parameters, data collection, and structure refinement details are in Table 2 and in the Supporting Information. No decay correction was applied. Data were corrected for Lorentz and polarization factors and for absorption on the basis of nine -scan reflections. Intensity statistics and systematic absences clearly determined the centrosymmetric monoclinic space group P2 1 /c (no. 14). The heavy atoms (Mo, Br) were located by direct methods and all the other non-hydrogen atoms by alternate full-matrix least-squares cycles and difference-Fourier maps. After anisotropic refinement of all non-hydrogen atoms, all of the hydrogen atoms bonded to carbon atoms were placed in calculated positions. Only the hydrogen atoms of the C 4 H 6 group were allowed to refine freely. At this point, one residual peak in the difference Fourier was well above background (1.24 e . Å -3 ) and lay approximately 1.44 Å from C(9) and nearly 1.80 Å from C(6).

C 4 H

 4 6 groups were now input with instructions to refine their respective occupancies: atoms C(6)-C(7)-C(8)-C(9) of the major orientation and atoms C(7A)-C(8A)-C(9A)-C(10A) of the second orientation, arranged in such a way that the C(7)/C(7A), C(8)/C(8A), and C(9)/C(9A) pairs were on the same site. Refinement with the proper occupancy constraints (see Supporting information), yielded a 0.8851:0.1149 ratio for the two orientations and R(F) = 4.05%, wR(F 2 ) = 10.90% and GOF = 1.149 for all data. Since NMR evidence gives conclusive indication of a mixture of dibromo 2 (major) and chlorobromo 3 (minor) mixture, a Br/Cl disorder model was also introduced for the two bridging atoms and the Br:Cl ratio was found to converge to 0.888:0.112, with improvement of the final agreement figures (see Table

a 2 / 3 .

 23 -Mo(1)-Br(1) #a 79.74(2) C(2)-C(1)-C(5) 105.2(5) Br(1)-Mo(1)-CNT a,b 111.9(2) C(1)-C(2)-C(3) 110.5(5) Br(1) # -Mo(1)-CNT a,b 112.4(2) C(2)-C(3)-C(Disordered Br/Cl site (0.888:0.112). b CNT = centroid of C(1) through C(5) atoms. X(nm) = center point of the C(n)-C(m) bond. (b) Compound 4a. All operations were conducted as detailed above for compound No decay correction was necessary. Intensity statistics and systematic absences clearly determined the centrosymmetric monoclinic space group P2 1 /c (no. 14). The structure was determined by direct methods with the successful location of the heavy atom (Mo) and several carbon atoms. Refinement was conducted as described in the previous paragraph, including the treatment of hydrogen atoms. A final difference-Fourier map was essentially featureless with the largest peaks, ||  0.257 e . Å -3 . Crystal and refinement parameters are collected in Table

  (d) Compound 4c. All operations were conducted as detailed above for compound 2/3. No decay correction was necessary. Systematic absences indicated the centrosymmetric space group Pnma (no. 62) or the non-centrosymmetric space group Pna2 1 (no.33), and intensity statistics clearly favored the centric case. The structure was solved by direct methods with the successful location of the Mo and several C atoms. Refinement was conducted as described in the previous paragraph, including the treatment of hydrogen atoms. Since the molecule lies on a mirror plane with the allyl and diene ligands occupying general positions, these ligands were found to be interpenetrating. The s-transbutadiene atoms C(6), C(7), C(8) and C(9) and the allyl atoms C(10), C(11) and C(12) were all refined at half occupancy without restraints on the atomic positions or bond lengths, but the pairs of atoms from different ligands in proximity of each other [i.e. C(9) and C(12); C(8) and C(11); and C(7) and C(10)] were refined with a common thermal parameter. The final sorting of which atom belongs to which ligand was arbitrarily made following the criterion of having similar distances for chemically analogous bonds [i.e. C(6)-C(7) and C(8)-C(9); C(10)-C(11) and C(11)-C(12)] and angles [i.e. C(6)-C(7)-C(8) and C(7)-C(8)-C(9)]. A final difference-Fourier map was essentially featureless with the largest peak near the heavy atom, within 1Å, with ||  1.38 eA -3 ; the largest other peak had ||  0.78 eA -3 . Crystal and refinement parameters are collected in

  (4) C(1)-C(2)-C(3) 107.1(3) CNT-Mo(1)-X(89) a 124.2(5) C(2)-C(3)-C(3) # 108.7(2) CNT-Mo(1)-C(10) a 113.5(3) C(7)-C(6)-C(6) # 134.6(11) CNT-Mo(1)-C(11) a 141.1(6) C(6)-C(7)-C(8) 125(2) CNT-Mo(1)-C(12) a 115.6(4) C(7)-C(8)-C(9) 120(2) C(2)-C(1)-C(2) # 108.5(4) C(10)-C(11)-C(12) 108.6(13) a CNT = centroid of C(1) through C(3) and symmetry related atoms. X(nm) = center point of the C(n)-C(m) vector. Results and Discussion (a) Syntheses and NMR characterization of Mo(II) compounds. Compound CpMoCl 2 (-C 4 H 6 ) (1) has previously been prepared by the interaction of CpMoCl(CF 3 C

  and butadiene) could not be probed by NOE experiments. It is based on the comparison of the chemical shifts in the 1 H-NMR spectra of 4a and 4b. In particular, the protons of the butadiene ligand have approximately the same chemical shift in both isomers (∆ is 0.23 for H a , 0.48 for H b , and 0.25 for H c ), whereas the protons of the allyl ligand resonate in more distinct regions for the two isomers (∆ of 1.42 for H d , 1.03 for H e , and 0.50 for H f ).

Figure 1 .

 1 Figure 1. Room temperature cyclic voltammogram for the oxidation of 4a/b. Potentials are reported relative to the ferrocene standard. Solvent = CH 2 Cl 2 .

Figure 2 .

 2 Figure 2. (a) Experimental EPR spectrum of complex 5b in THF at T = -80°C. (b) Simulation for a system of six equivalent H nuclei (a H = 7.0 G, linewidth = 3.2 G). (c) Simulation for a system of four equivalent H nuclei (same conditions as in (b)).

Figure 3 .

 3 Figure 3. (a) EPR spectrum of a mixture of 5a and 5b, obtained by oxidation of the 98:2 equilibrium mixture of 4a and 4b in THF at -80°C (after 5 min). (b) As (a), after

C 4 H 6 )

 46 ] + , 5d. It cannot be established which isomer gives rise to which resonance in Figure4(a): the ratio of the two resonances after immediate recording of the EPR spectrum is ca. 1:1, and does not further change during the time scale of the interconversion to 5b.

Figure 4 .

 4 Figure 4. EPR monitoring of the oxidation of 5c with Cp 2 Fe + in THF. (a) After 10 min. (b) After 12 h.

  and the dinuclear niobium compound [CpNb(C 4 H 6 )(-Cl)] 2 .[START_REF] Okamoto | [END_REF]

Figure 5 .

 5 Figure 5. A view of the major component for the structure of 2/3, showing the numbering scheme employed. The ellipsoids are drawn at the 30 % probability level.

Figure 7 .

 7 Figure 7. Molecular geometry of the cation 5b with the numbering scheme employed. Only one of the two possible symmetry-related orientations of the diene and allyl ligands is shown for clarity. The ellipsoids are drawn at the 30 % probability level.

Figure 8 . 3 ,

 83 Figure 8. Molecular geometry of compound 4c with the numbering scheme employed. Only one of the two possible symmetry-related orientations of the diene and allyl ligands is shown for clarity. The ellipsoids are drawn at the 30 % probability level.

Figure 9 .

 9 Figure 9. Thermodynamic and kinetic parameters related to the processes in Scheme 2.

  ∆G ‡ of this process. The real value, however, may be considerably lower since most of the reagents are in the solid state during the experiment.Mechanistic considerations.The isomerization of either 4b to 4a, 5a to 5b, or 5d to 5c involves a formal 180° rotation of the allyl ligand around the Mo-allyl axis. The actual mechanism of these isomerization processes, however, does not necessarily involve the rotation of the ligand. An alternative is the rearrangement of the allyl ligand from  3 to  1 , followed by rotation around the Mo-CH 2 or the CH 2 -CH bond and recoordination. The high barriers to these processes are consistent with significant loss of bonding in either case. It is interesting to observe that the barriers are significantly smaller for the 17-electron Mo(III)

Preparation of [CpMo(-C 4 H 6 )(-Br)] 2 (2) and Cp 2 Mo 2 (-C 4 H 6 ) 2 (µ-Br)(µ-Cl) (3).

  CpMoCl 2 (-C 4 H 6 ) (0.850 g, 2.97 mmol) was suspended in 30 mL of THF, the resulting mixture was cooled to -78°C, and 3.3 mL of a Et 2 O solution of C 3 H 5 MgBr (1 M, 3.3 mmol) was added. The red-brown suspension turned green within 1 h. The mixture was stirred for an additional hour, yielding a yellow-green precipitate. The supernatant liquid was decanted off at -78°C and the solid was dissolved in 300 mL of toluene at room temperature, followed by filtration through Celite. The solution was evaporated under reduced pressure to dryness, yielding the product as a yellow-green solid in 92% yield (0.806 g). Since the product is a

mixture, an elemental analysis was not attempted. Low-resolution anionic mass spectrum (FAB -): m/z 81 [ 81 Br] -98%; 79 [ 79 Br] -100%. A 1 H-NMR spectrum indicates the presence of 86 % of 2 and 14 % of 3, following the peak assignments shown in Table

1

(see Results).

A single crystal for the X-ray analysis was obtained by recrystallization from slow diffusion of a n-heptane layer into a THF solution.

Reaction of [CpMo(C 4 H 6 )(µ-Br)]

2 (2) and Cp 2 Mo 2 (C 4 H 6 ) 2

(µ-Br)(µ-Cl)CpMo(C 4 H 6 ) (3) with PPN + Cl -. A sample of crystals the 2/3 mixture (ca. 2 mg) and PPN + Cl -(ca. 2 mg) were placed in an NMR tube and ca. 0.5 mL of C 6 D 6 was added. The suspension was heated at the reflux temperature for 1 min. The 1 H-NMR spectrum of the resulting mixture indicated a 2/3 ratio of 61:39.

Preparation of CpMo(-C 3 H 5 )(-C 4 H 6 ) (4a and

4b). To a suspension of CpMoCl 2 (-C 4 H 6 ) (1.000 g, 3.50 mmol) in 30 mL of THF was added 7.7 mL of a Et 2 O solution of C 3 H 5 MgBr (1 M, 7.7 mmol) at -78˚C. The red-brown suspension gradually turned green within 0.5 h. After further stirring for 2 h at -78˚C, the temperature was gradually elevated to -20˚C, at which point the green suspension quickly turned red-brown. The solvent was removed under reduced pressure at -20˚C, followed by extraction of the residue with n-heptane at room temperature (ca. 50 mL). The supernatant liquid was decanted into another Schlenk and concentrated to ca. 1/2 of its original volume, followed by cooling to -80˚C overnight, to yield the product as a red-brown crystalline solid. The supernatant liquid was decanted off and the solid was dried under vacuum (yield 666 mg, 74%). Anal. Calcd. for C 12 H 16 Mo: C, 56.25; H, 6.25. Found: C, 56.0; H, 6.4. The ratio of 4a and 4b as determined by 1 H NMR was 98 : 2.

(b) By thermolysis. 10 mg of the equilibrium 4a/b mixture was dissolved in

	recrystallization of
	the solid in heptane at -80 ˚C. Anal. Calcd. for C 12 H 16 Mo: C, 56.25; H, 6.25. Found: C, 56.16;
	H, 6.36.
	0.5 mL
	of C 6 D 6 in an NMR tube, which was then flame-sealed and heated to 100˚C for 2.5 h. NMR
	monitoring showed the formation of a mixture of 4c and 4a in ca. 1:1 ratio. No 4b was detected
	in this experiment. Prolonged heating resulted in decomposition.
	Photolysis of compound 4c. A C 6 D 6 solution of 4c (10 mg in 0.5 mL) in an NMR
	tube was placed in water bath and irradiated for 1.5 h. The temperature of the bath remained
	below 40°C during photolysis. NMR monitoring showed the formation of a mixture of 4c (62
	%), 4a (20 %) and 4b (18%), accompanying unidentified decomposition products.
	Thermolysis of compound 4c. 10 mg of 4c was dissolved in 0.5 mL of C 6 D 6 in an
	NMR tube, which was then flame sealed and heated to 100˚C for 1 h. NMR monitoring showed
	the formation of a mixture of 4c and 4a in ca. 1:1 ratio. No 4b was detected in this experiment.

40 ˚C. From a separate experiment with NMR monitoring, a mixture of 4c (43%), 4a (43%) and 4b (14%) was observed after irradiation for 1.5 h. Further irradiation beyond this point does not significantly change the ratio of the three species. The solution was eluted with nheptane through a silica gel column (2x15 cm, prepared from heptane). A yellow fraction of 4c was collected, while the other Mo species present in solution remained trapped by the column. The solvent was removed under reduced pressure, giving a yellow-brown solid (yield 239 mg, 30 %). Crystals suitable for the X-ray analysis were obtained by

Preparation of [CpMo(supine--C 3 H 5 )(supine--C 4 H 6 )][PF 6 ] (5b). To

	a solution
	of CpMo(  -C 3 H 5 )(  -C 4 H 6 ) (4a and 4b) (486 mg, 1.90 mmol) in 10 mL of CH 2 Cl 2 was added
	a solution of FcPF 6 (629 mg, 1.90 mmol) in CH 2 Cl 2 (230 mL), resulting in the precipitation of
	a green crystalline solid. After 2 h, the solid was filtered off, washed with 5 mL heptane, and
	dried under vacuum (yield 455 mg, 60 %). This crude material is a mixture of two isomers (5a
	and 5b, see Results Section). The solubility of the green solid is very poor in THF, chlorinated
	solvents (dichloromethane or 1,1,2-trichloroethane) or aromatic solvent (benzene or toluene).
	The compound has a higher solubility in acetone, giving red-violet solution of 5b. Dark violet
	crystals of 5b were obtained from the concentrated acetone solutions by cooling to -20°C.
	Anal. Calcd for C 12 H 16 MoPF 6 : C, 35.91; H, 3.99. Found: C, 35.69; H, 4.31. EPR
	(CH 2 ClCHCl 2 ): g = 2.028 (septet with Mo satellites, a Mo = 33 G, a H = 7 G) IR (Nujol mull):
	842 cm -1 , PF 6 . This compound appears to be indefinitely air stable in the solid state (the
	crystals did not deteriorate in air for over 1 month).

Reaction of compound 5 with Cp 2 Co. (a) From the crude 5a/b mixture. The crude

	5a/b mixture of the above preparation (10 mg, 0.025 mmol) was suspended in 20 mL of THF
	and Cp 2 Co (30 mg, 0.16 mmol) was added. The mixture was stirred for 2 h, during which time
	the green solid slowly disappeared to yield a red-brown solution. The suspension was
	evaporated to dryness under reduced pressure and the residue was extracted into 30 mL of n-
	heptane. The mixture was filtered and the solution was again evaporated to dryness. The red-
	brown residue was dissolved in 0.5 mL of C 6 D 6 . A 1 H NMR spectrum indicates the presence
	of 80 % of 4a and 20 % of 4b (see Table 1 and Results section). The latter compound gradually
	transformed into the former.
	(b)

From crystallized 5b. Selective formation of 4b. Red

	-violet crystalline 5b (5.0
	mg, 0.012 mmol) was suspended in C 6 D 6 (0.5 mL) in a thin-walled 5 mm NMR tube. Cp 2 Co

Table 2

 2

	. Crystal Data for All Compounds
	compound	2/3	4a
	formula	C 18 H 22 Br 1.78 Cl 0.22 Mo 2	C 12 H 16 Mo
	fw	580.10	256.19
	space group	P2 1 /c	P2 1 /c
	a, Å	10.652(2)	12.4437(5)
	b, Å	7.5072(6)	7.3498(4)
	c, Å	11.9581(14)	12.0043(5)
	, deg	110.698(8)	111.927(3)
	V, Å 3	894.5(2)	1018.48(8)
	Z	2	4
	d calc , g/cm 3	2.154	1.671
	µ(Mo K), mm -1	5.396	1.236
	radiation (monochromated	Mo K ( = 0.71073 Å)
	in incident beam)
	temp, °C	153(2) K	153(2))
	transmissn factors: max, min	0.1694, 0.0698	0.7363, 0.6428
	Final R indices [I>2(I)] a,b	R1 = 0.0373	R1 = 0.0163
		wR2 = 0.0988	wR2 = 0.0410
	no. of data [I>2(I)]	1533	1510
	R indices (all data) a,b	R1 = 0.0379	R1 = 0.0211
		wR2 = 0.0994	wR2 = 0.0438
	a R1 = ∑||F o |-|F c ||/∑|F o |. b wR2 = [∑w(|F o |-|F c |) 2 /∑w|F o | 2 ] 1/2

Table 2 .

 2 Crystal Data for All Compounds (continued)

	compound	5b	4c
	formula	C 12 H 16 F 6 MoP	C 12 H 16 Mo
	fw	401.16	256.19
	space group	Cmcm	Pnma
	a, Å	7.6228(5)	11.9689(6)
	b, Å	23.336(2)	11.6175(11)
	c, Å	7.7887(4)	7.4177(6)
	, deg
	V, Å 3	1385.5(2)	1031.42(14)
	Z	4	4
	d calc , g/cm 3	1.923	1.650
	µ(Mo K), mm -1	1.116	1.221
	radiation (monochromated	Mo K ( = 0.71073 Å)
	in incident beam)
	temp, °C	153(2)	153(2)
	transmissn factors: max, min

Table 3 .

 3 Selected bond distances (Å) and angles (°) for the structure of 2/3.

	Mo(1)-C(2)	2.361(5)	C(1)-C(5)	1.419(8)
	Mo(1)-C(3)	2.301(5)	C(2)-C(3)	1.393(8)
	Mo(1)-C(4)	2.213

Mo(1)-Br(1) a 2.6613(6) Mo(1)-C(9) 2.215(10) Mo(1)-Br(1) #a 2.6570(6) Mo(1)-C(10A) 2.53(8) Mo(1)-C(1) 2.308(5) C(1)-C(2) 1.396(8)

Table 2 ,

 2 while selected bond distances and angles are listed in Table5.

Table 5 .

 5 Selected bond distances (Å) and angles (°) for the structure of 4c.

	Mo(1)-CNT a	2.009(5)	C(1)-C(2)	1.411(4)
	Mo(1)-C(1)	2.318(5)	C(2)-C(3)	1.395(5)
	Mo(1)-C(2)	2.340(3)	C(3)-C(3) #	1.397(6)
	Mo(1)-C(3)	2.341(3)	C(6)-C(6) #	1.52(2)
	Mo(1)-C(6)	2.264(7)	C(6)-C(7)	1.37(2)
	Mo(1)-C(7)	2.20(3)	C(7)-C(8)	1.40(2)
	Mo(1)-C(8)	2.30(4)	C(8)-C(9)	1.29(3)
	Mo(1)-C(9)	2.29(4)	C(10)-C(11)	1.46(2)
	Mo(1)-C(10)	2.28(3)	C(11)-C(12)	1.44(3)
	Mo(1)-C(11)	2.23(4)
	CNT-Mo(1)-X(67) a	120.2

  to cyclopentadienyl ring (vide infra). A procedure for the selective generation of 4b will be shown later. The assignment of the 1 H-NMR resonances and the ligand orientations for the major isomer 4a (prone for the allyl ligand; supine for the butadiene ligand; see Table1and refer to III for the nomenclature) have been confirmed by homonuclear decoupling experiments and NOE difference NMR experiments. In particular, an NOE effect was detected upon irradiation of H a on H b and H f , of H b on H(Cp) and H c , of H c on H(Cp) and H b , of H d on H(Cp) and H e , of H e on H d and H f , and of H f on H d and H e . Irradiation of the Cp protons does not significantly affect the other resonances.

	H c H c' H b'	H a' Mo	X X	Mo	Mo	X X	Mo
	H b	H a
		I				II
	CpMoCl 2 (-C 4 H 6 )	2 C 3 H 5 MgBr -C 3 H 5 .	CpMo(-C 3 H 5 )(-C 4 H 6 )	(4)

). Solutions of complex 4 show two isomers, 4a and 4b, in an equilibrium 98:2 ratio by 1 H-NMR. These isomers differ in the relative orientation of the allyl ligand with respect

  CpMoBr(C 4 H 6 )" and "CpMoCl(C 4 H 6 )" would lead to 85.9% of 2, 13.6% of 3, and only 0.5% of the unobserved compound [CpMo(C 4 H 6 )(µ-Cl)] 2 .

	of species "
	on the 16-electron "CpMoX(-C 4 H 6 )" (see Scheme 1) followed by combination of two 16-
	electron CpMoX(C 4 H 6 ) fragments. Theoretically, a dichloro-bridged compound should also
	be present, but its concentration was not sufficiently high for NMR detection. Under halide-
	independent enthalpic and entropic conditions for the dimerization process, a 0.927:0.073 ratio

  that the related 15-electron CpMoCl 2 (PH 3 ) compound (a model of the proven dissociative intermediate of the phosphine exchange on CpMoCl 2 L 2 ) 57 has a spin quartet ground state, stabilized by several kcal/mol over the alternative doublet state. Differences in pairing energy stabilization could also, in principle, rationalize the faster isomerization for the [Cp*Mo(CO) 2 ( 4 -diene)] + system, in case the 16-electron [Cp*Mo(CO) 2 ( 2 -diene)] + intermediate adopts a spin triplet state, because the expected higher effective positive charge for the cationic dicarbonyl derivative relative to the neutral allyl derivative should give rise to greater pairing energies and, consequently, greater stabilization of an unsaturated spin triplet state. Examples of stable 16-electron organometallic Mo(II) complexes with a spin triplet ground state have been found recently, i.e. Cp*MoClL 2 (L = PMe 3 , PMe 2 Ph or L 2 = dppe) 40,58 and [HB(3,5-Me 2 Pz) 3 ]MoI(CO) 2 . 59

Acknowledgment. We are grateful to the National Science Foundation (CHE-9508521) for support of this work. The EPR upgrade was made possible in part by a NSF shared equipment grant (CHE-9225064).

Scheme 3

There are a few previously published observations of rates of interconversion of s-cis and s-trans diene ligands, but the factors affecting these rates do not seem obvious. For instance, at one extreme we find the niobium complexes CpNb(supine-s-cis-C 4 H 6 )(prone-scis-C 4 H 6 ) and CpNb(s-cis-C 4 H 6 )(s-trans-C 4 H 6 ) which do not interconvert at 80°C, while at the other we have a variety of [Cp*Mo(CO) 2 (exo(or endo)-s-trans-

) which can only be generated under kinetically controlled conditions at low temperatures, but rearrange rapidly to the more stable exo(or endo)-s-cis analogues. 47 Our molecules have an intermediate behavior, being inert but isomerizing upon warming to 100°C. The difference in rates between the two isoelectronic dicarbonyl and allyl Mo(II) systems is particularly striking. This difference indicates that the Mo-ene bond is stronger in compound 4 than in the cationic dicarbonyl analogues. Since the Cp*Mo(CO) 2 + unit is reasonably expected to be more electron deficient than the CpMo(allyl) unit, the obvious conclusion seems that the major contribution to the Mo-ene bond stabilization comes from the π back-bonding component. Particularly striking is also the faster conversion of 5c to 5b relative to that of 4b to 4c. This finding may be rationalized in the same fashion: the weaker Mo-ene bond for Mo(III) than for Mo(II) may be related to the expected weakening of the π back-bonding component, implying a greater importance of the π relative to the  component for the Mo-ene bond stabilization.

There is, however, a second and more subtle way to rationalize a faster s-cis/s-trans isomerization for 5 relative to 4. A factor that may provide greater stabilization of the unsaturated intermediate for the higher oxidation state system is the regain of pairing energy [START_REF] Poli | [END_REF] This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, can be ordered from the ACS, and can be downloaded from the Internet; see any current masthead page for ordering information and Internet access instructions.