James C Fettinger

Heinz-Bernhard Kraatz

Rinaldo Poli
email: poli@u-bourgogne.fr

E Alessandra Quadrelli

Prof Rinaldo Poli

Formation of organometallic hydroxo and oxo complexes by oxidation of transition metal hydrides in the presence of water. X-Ray structures of [CpMo(OH)(PMe3)3][BF4] and [CpMo(O)(PMe3)2][BF4]

Introduction

The oxidation of transition metal hydride complexes and the reactivity of the resulting oxidation products has recently been the subject of intensive and detailed investigations. Oxidation of typical 18-electron neutral hydride complexes, M-H, dramatically increases the acidity of the hydride ligand in the resulting 17-electron products [M-H] + , resulting in its rapid decomposition by deprotonation (see Scheme 1). 16 The cationic hydride can be deprotonated either by an external base or by the unoxidized starting compound, resulting in different oxidation stoichiometries [equations (1)-( 3

In some cases, the oxidation stoichiometry for the same hydride material depends upon whether the complex is oxidized chemically or electrochemically. 16,26 This phenomenon has been attributed to the presence of adventitious water as an external base in the electrochemical cell. We have recently studied in detail the influence of water on the oxidation and protonation of compounds CpMoH(CO)2L (L = PMe3, PPh3) and described the general phenomenon of the action of an external base as a "proton shuttle". 37 In essence, when B is a stronger base than M-H, equation ( 1) is thermodynamically favored over equations ( 2) and ( 3), but if the less favored protonation of M-H is followed by irreversible H2 loss, then the BH + product delivers its proton to unoxidized M-H, changing the stoichiometry to that of equation (3).

However, deprotonation is not the only decomposition pathway available to oxidized hydride complexes. When good donor and/or sterically encumbering ligands are coordinated to the metal center in the precursor hydride complex M-H, the acidity (either kinetic or thermodynamic) of [M-H] + is not sufficiently high, allowing further oxidation with concomitant solvent (S) coordination, to yield [M-H(S)] 2+ as an intermediate of a decomposition mechanism by disproportionation (Scheme 2). 30 This dicationic hydride can then be deprotonated, leading to the same possibilities of oxidation stoichiometries encountered for the alternative decomposition mechanism of Scheme 1.

Scheme 2

While, in most cases, the [M-H] 1 and [M(S)H] 2+ intermediates are too unstable to be spectroscopically observed, the oxidation of the electron-rich and sterically crowded CpMoH(PMe3)3 complex, 1, has allowed the formation and spectroscopic observation of the relatively stable one-electron oxidation product, 1 + , and the isolation and single crystal X-ray analysis of [CpMoH(PMe3)3(MeCN)] 2+ . 38 The steric bulk and donor power of the ancillary ligands in this system are sufficiently great to slow down the deprotonation process of both 1 + and [1(MeCN)] 2+ by either 1 or NEt3. In this contribution, we examine the intervention of water as a coordinative molecule (S in Scheme 2) during the oxidation of compound 1. Since free water is not sufficiently basic to engage in proton transfer reactivity for this system (i.e. as B in Schemes 1 and 2), new reactivity pathways are made possible by the presence of acidic protons on the coordinated water ligand. An unprecedented paramagnetic organometallic hydroxo derivative of Mo(III), its conjugate base, and a previously reported organometallic oxo derivative of Mo(IV) are identified as the products of this process. The geometry and energies of model compounds for the Mo(III) and Mo(IV) complexes have been calculated through geometry optimizations and have been used to draw the energetic profile of the transformations.

Part of this work has previously been communicated. 39 Experimental General All operations were carried out under an atmosphere of dinitrogen using standard glove-box and Schlenk-line techniques. Solvents were dehydrated by standard methods (THF from Na/K/benzophenone, heptane from Na and acetone from MgSO4), deoxygenated, and distilled directly from the dehydrating agent under dinitrogen prior to use. The amount of residual water in the solvents was assessed by titration with a coulomatic Karl-Fischer titrimeter (Fisher Scientific). Complex CpMoCl(PMe3)3 was prepared as previously described. 40

Spectroscopic analyses

Samples for 1 H and 31 P NMR in thin-walled 5 mm glass tubes were measured on a Bruker WP200 spectrometer. The 1 H NMR spectra were calibrated against the residual proton signal of the deuterated solvents. The 31 P NMR were calibrated against 85% H3PO4 in a capillary tube which was placed in a different 5 mm glass tube, containing the same deuterated solvent used for the measurement. All NMR spectra are collected in 41 b Splits further into a doublet (JPH = 12 Hz) in the 31 P{selective-1 H} NMR. c The hydride and water resonances are not visible in the 1 H NMR. d The PMe3 resonance could not be identified because of extensive overlap with the other reaction products (see Experimental section). e Splits further into a doublet (JPH = 14 Hz) in the 31 P{selective-1 H} NMR.

Oxidation of 1 by Fc + PF6 -in CD3CN in presence of water

The CD3CN solution (0.753 mL) resulting from the addition of a solution (0.2 mL) of FcPF6 (18.2 mg, 55 mmol) to a solution (553 mL) containing 1 (56 mmol) and H2O (3 mL, 160 mmol) was monitored over time and compared to the analogous reaction performed without the addition of water. 38 The 1 H and 31 P NMR spectra confirmed the previously reported formation 38 In addition, however, the spectra also revealed Cp resonances of two additional diamagnetic products whose formation does not occur under dry conditions. These were attributed to Complex 1 + was generated in situ according to the literature: 38,40 to a stirred solution of CpMoCl(PMe3)3 (149 mg, 0.351 mmol) in THF (10 mL) was added via microsyringe a 1 M solution of LiBHEt3 (351 mL, 0.351 mmol). After 30 min, the reaction mixture was evaporated to dryness and extracted with n-heptane (2 × 10 mL), yielding a spectroscopically pure solution of CpMoH(PMe3)3. After filtration and evaporation to dryness, the residue was dissolved in THF and cooled to 278 °C. Solid AgBF4 (68 mg, 0.350 mmol) was added, resulting in the immediate formation of a dark brown precipitate. An aliquot of this reaction mixture was transferred into an EPR tube via cannula and frozen in liquid nitrogen until the introduction into the EPR probe, which was preset at 278 °C. The resulting EPR spectrum (doublet of quartets [g = 2.017; a(P) = 28.5 G; a(H) = 13 G]) confirmed the formation of complex 1 1 , by comparison with the previously reported spectrum. 38 The reaction mixture was allowed to warm to room temperature, filtered and evaporated to dryness under reduced pressure. This solid was extracted with acetone (3 × 10 mL) and the resulting orange solution was filtered, reduced in volume to about 10 mL, and the product was precipitated out by the addition of heptane (20 mL). Recrystallization of the dark solid from acetone-heptane led to the deposition of three types of crystals: red prisms, yellow blocks and yellow needles, which were all used for single crystal X-ray analyses. The yellow blocks and yellow needles correspond to [CpMo(PMe3)3(OH)][BF4], 2, and to [CpMo(PMe3)3(OH)1-xClx]- [BF4], respectively, whereas the red prisms correspond to [CpMoO(PMe3)2][BF4], 3. A few red crystals were separated by hand-picking and investigated by NMR spectroscopy. The 1 H NMR spectrum in CD3COCD3 is identical with that previously reported for compound 3. 4 (b) Spectroscopic study of the reaction mixture. A reaction identical to that described in the previous section was carried out from 1 (0.86 mmol) and AgBF4 (164 mg, 0.84 mmol).

of [CpMoH(CD3CN)(PMe3)3] 2+ , [CpMoH2(PMe3)3] + and [CpMo(CD3CN)(PMe3)3] 1 .
After filtration and evaporation to dryness, the brown solid was washed with Et2O and dried under vacuum. Yield 332 mg. This solid was used for NMR and EPR investigations. EPR (CD3COCD3, -50 °C): dq (g = 2.032, ad(H) = 14.1 G; aq(P) = 28.8 G, aMo = 29 G). This spectrum is attributed to complex 1 1 (cf. EPR spectrum in THF). 38 The 1 H NMR spectrum (CD3COCD3, δ) showed four different products in an approximate relative ratio of 10 : 60 : 15 : 15, as follows.

[CpMoO(PMe3)2] + (relative intensity 10), 41 [CpMoH2(PMe3)3] + (relative intensity 60): 41 [CpMoH(PMe3)3(CD3COCD3)] 2+ (relative intensity 15), [CpMoH(PMe3)3(H2O)] 2+ (relative intensity 15). The CD3COCD3 solution was evaporated to dryness and redissolved in CD3CN. To a CD3COCD3 solution (0.5 mL) of CpMoH(PMe3)3 (17 µmol, generated in situ as above) was added water (1 mL, 56 mmol) and AgBPh4 (4.3 mg, 10 µmol, Mo:Ag = 1 : 0.6). The 1 H NMR monitoring showed a slow reaction that eventually consumed all of the starting hydride.

After 24 h, an aliquot of the solution was investigated by EPR spectroscopy: binomial triplet with Mo satellites, g = 2.000, aP = 20 G, aMo = 30 G. This resonance is assigned to CpMoO(PMe3)2, 4. At the same time, the 31 P NMR spectrum revealed a broad (w1/2 = 360 Hz) resonance at δ -36 and a single organometallic diamagnetic product (δ 7.9), identified as [CpMoH2(PMe3)3] + by comparison with an authentic sample. 38,41 An analogous experiment was carried out with a CD3COCD3 solution (1.0 mL) of CpMoH(PMe3)3 (100 mmol, generated in situ as above), containing 5 µL (0.25 mmol) of water.

In a separate Schlenk, Ag + BF4 -(65 mg, 0.333 mmol) was dissolved in CD3COCD3 (200 µL).

A first aliquot (66 µL, 0.11 mmol, ca. 1 equiv.) was added to the hydride solution, generating the EPR spectrum assigned to 4 (see previous section). A second aliquot of silver solution (50 µL, 83 mmol) was added to the residual solution (Mo :Ag = ca. 1 : 2). The resonances of complex [CpMoO(PMe3)2] + were present in the 1 H NMR spectrum, while a subsequent EPR investigation showed no residual EPR activity.

Protonation of 1 in the presence of PMe3

Upon addition of HBF4•Et2O (10 µL, 70 mmol) to a CD3COCD3 solution (0.5 mL) of 1 (70 µmol) and PMe3 (10 µmol), no changes in the 1 H and 31 P NMR resonances of the trimethylphosphine were detected, while the resonances of 1 were replaced by those of [1-H] + .

Protonation of 1 in the presence of H2O

Upon addition of HBF4•Et2O (10 µL, 70 mmol) to a CD3COCD3 solution (0.6 mL) of 1 (79 µmol) and H2O (2 mL, 110 µmol), no major change in the 1 H resonance of the water peak was detected (δ 2.2), while the resonances of 1 were replaced by those of [1-H] + .

X-Ray crystallography (a) [CpMo(OH)(PMe3)3][BF4]

. Data collection and reduction and structure solution were routine. The structure was refined with an initial Flack parameter refining to 0.87 (10) indicating that the structure needed to be inverted. Following inversion and further refinement, one of the three PMe3 groups was found to be disordered with major : minor contributors of 0.631 : 0.369.

The hydroxide hydrogen was initially located in the difference Fourier map at a distance of 0.88 Å from the O atom and a Mo-O-H angle of 140°, but free refinement led to instability. Thus, it was eventually refined with DFIX (restrained O-H distance and free Mo-O-H angle). Crystal data and refinement parameters are collected in Table 2, while selected bond distances and angles are listed in Table 3.

, Z = 4, T = 293(2) K. b R1 = Σ||Fo|-|Fc||/Σ|Fo|. c wR2 = [Σw(|Fo|-|Fc|) 2 /Σw|Fo| 2 ] 1/2 .

(b) [CpMo(OH)0.79Cl0.21(PMe3)3][BF4].

Routine structure solution revealed two independent molecules in the asymmetric unit. During the refinement, it became apparent that one of the two molecules had disorder in the hydroxo position. Further analysis revealed that this position was composed of two different ligands, OH and Cl. The ratio was determined to be OH : Cl = 0.58 : 0.42 by refinement of the X-ray data. The overlap on this site required the use of restraints. The Mo-O-H moiety was imposed to be identical with that in the other independent molecule. This allowed for both the hydroxyl oxygen and chlorine atoms to refine to acceptable positions. Crystal data and refinement parameters are collected in Table 2, while selected bond distances and angles are listed in Table 4.

P(3)-Mo(1)-CNT(1) 108.3(5) P(5)-Mo(2)-O(2) 78(2) O(1)-Mo(1)-CNT(1) 112.1(6) P(5)-Mo(2)-CNT(2) 112.3(5) Cl(2)-Mo(2)-P(4)
76.8( 11)

P(6)-Mo(2)-O(2) 139(2) Cl(2)-Mo(3)-P(5)
70.9( 10)

P(6)-Mo(2)-CNT(2) 107.6(5) O(2)-Mo(2)-CNT(2) 113 (2)
(c) [CpMoO(PMe3)2][BF4]
. Two of the PMe3 carbon atoms, C4 and C5, were found to suffer from disorder that was modeled resulting in a final ratio C4:C4A = C5:C5A = 0.584:0.416. The BF4 molecule was also directly located and displayed large librational motion.

The final model included two orientations for the fluorine atoms with equal occupancy (50:50).

Crystal data and refinement parameters are collected in Table 2, while selected bond distances and angles are listed in Table 5.

CCDC reference number 186/1270.

See http://www.rsc.org/suppdata/dt/1999/497/ for crystallographic files in .cif format.

Theoretical calculations

All calculations were performed using Gaussian-9442 on a SGI Origin 200 at the Université de Bourgogne and on a DEC/Alphastation 250 at the University of Maryland. The LanL2DZ set was employed to perform complete geometry optimizations with a Density Functional Theory (DFT) approach. The three-parameter form of the Becke, Lee, Yang and Parr functional (B3LYP), [START_REF] Becke | [END_REF] was employed. The LanL2DZ basis set includes both Dunning and Hay's D95 sets for H and C, [START_REF] Dunning | Modern Theoretical Chemistry[END_REF] and the relativistic Electron Core Potential (ECP) sets of Hay and Wadt for the heavy atoms. [START_REF] Dunning | Modern Theoretical Chemistry[END_REF][START_REF] Hay | [END_REF][46][47] Electrons outside the core were all those of H, C and O atoms, the 3s and 3p electrons in P, and the 4s, 4p, 4d and 5s electrons in Mo. The mean value of the spin of the first order electronic wavefunction, which is not an exact eigenstate of S 2 for unrestricted calculations on open-shell systems, was considered suitable to unambiguously identify the spin state. Spin contamination was carefully monitored and the energies shown in the Results section correspond to unrestricted B3LYP (UB3LYP) calculations.

Results

Chemical oxidation of CpMoH(PMe3)3 in the presence of water

The oxidation of 1 in dry CD3CN has been previously investigated. 38 We have now reexamined this process in the presence of water. The NMR monitoring shows the formation of the same products that are obtained when the solvent is dry, i.e. [1(CD3CN)] 2+ , [1-H] + and [CpMo(PMe3)3(CD3CN)] + . In addition, small amounts of the previously described 41 complex [CpMoO(PMe3)2] + and another species that has very similar NMR properties to those of [1(MeCN)] 2+ , were also present. The latter species could not be isolated, but its assignment to The oxidation was also carried out in less coordinating solvents such as THF and acetone.

The solid that could be isolated when the oxidation was carried out with one equivalent of oxidizing agent (Ag + ) in THF shows, after dissolution in CD3COCD3, the presence of 1 + by EPR spectroscopy 38 and complexes [CpMoO(PMe3)2] + , [1-H] + , [1(H2O)] 2+ (vide supra), and a new complex assigned as [1(CD3COCD3)] 2+ by 1 H and 31 P NMR spectroscopy. Complex [1(CD3COCD3)] 2+ shows 1 H and 31 P NMR patterns similar to those of the related CD3CN and H2O derivatives (see Table 1). The corresponding complex [1(THF)] 2+ was previously obtained by oxidation of 1 with two equivalents of oxidant in THF, 38 but its sparing solubility in THF did not permit its NMR characterization. When the isolated solid was dissolved in CD3CN, the smooth transformation of [1(CD3COCD3)] 2+ to [1(CD3CN)] 2+ could be witnessed over a few hours by NMR spectroscopy, while the resonances attributed to [1(H2O)] 2+ remained unchanged over the same time scale.

A spectroscopic monitoring of the oxidation reaction of 1 in CD3COCD3 carried out in the presence of small amounts of water and with a substoichiometric amount of oxidant shows the formation of an EPR active species characterized by a triplet resonance (aP = 20 G) flanked by Mo satellites (aMo = 28 G) in the EPR spectrum (see Fig. 1). This resonance disappears after the addition of an excess of oxidant. This EPR active intermediate is formulated as CpMo(O)(PMe3)2, 4, see Discussion. This EPR triplet must be due to an odd-electron Mo complex that contains two PMe3 ligands and no hydride ligand. Hydride ligands on Mo typically lead to large, observable coupling patterns in the EPR spectra, for instance aH = 12.9 G in 1 + . 38 A simultaneous NMR investigation shows the formation of the dihydride cation [1-H] + and a very broad resonance at δ -36 in the 31 P NMR spectrum. The latter is assigned to free PMe3, the broadness and shift relative to the usual 31 48 The presence of the chloride impurity in this material can be rationalized on the basis of the preparative chemistry, as follows. Compound 1 is very soluble in all nonpolar solvents and quite difficult to crystallize. 40 For that reason, it was generated in situ by the action of LiBEt3H on CpMoCl(PMe3)3, this being a facile and selective reaction 40 so long as care is taken to assure a complete conversion. In the present case, a residual chloride impurity probably contaminated the isolated product 1, and its subsequent oxidation generated, as previously established, 40 the stable [CpMoCl(PMe3)3] + cation which cocrystallized with compound 2. Fig. 1 Room temperature EPR spectrum of a solution of 4 in CD3COCD3.

Compound 2 has also been recently obtained in our laboratory by an alternative procedure, namely one-electron oxidation of the 16-electron Mo(II) hydroxo complex, CpMo(OH)(PMe3)2, in the presence of PMe3. 39,49 A unit cell determination on single crystals confirms that the two products are identical. This compound does not lead to detectable EPR or NMR resonances. Compound 3, in turn, has previously been obtained by protonation of CpMoH(PMe)3 with aqueous HBF4. 41 Other similar CpMo(IV) oxo cations, i.e.

[CpMo(O)(dppe)] + PF6 -and [CH3C(CH2-η 5 -C5H4)(CH2PPh2)2MoO] + , 50,51 have also been previously described in the literature.

X-Ray structures

The previously communicated 39 X-ray structure of compound 2 is reported here in full. A top view of the cation is shown in Fig. 2. The geometry of the complex is the typical four-legged piano stool, the stool being the Cp ligand and the four legs being identified by the three PMe3 ligands and the OH group. The hydroxo hydrogen atom could not be freely refined and restraints were applied to the O-H distance, but the Mo-O-H angle was refined freely and converged to 133(4)8. The Mo-CNT and Mo-P distances are the same, within experimental error, to the corresponding distances in the isostructural complex [CpMoCl(PMe3)3] + . 48 The shorter Mo(1)-P(1) distance relative to the Mo(1)-P(2) and Mo(1)-P(3) distances reflects the weaker trans influence of the OH group relative to PMe3. The Cp ligand is somewhat asymmetrically disposed on top of the metal center, the difference between the longest and the shortest Mo-C distance being 0.130(6) Å. As is clear from Fig. 2, the two relative trans PMe3 ligands are distorted toward the OH ligand and away from the third PMe3 ligand. This is probably the result of the steric repulsion betweeen the bulkier PMe3 ligands. The same effect, although less pronounced, was observed for the chloro analogue. 48 Compound 2 is, to the best of our knowledge, the first known compound with a terminal Mo III -OH bond. The Mo-O distance, therefore, can only be compared with those found in hydroxide-bridged complexes. These comprise (averaged Mo-O distances in parentheses) [Mo2(µ-OH)2(µ-O2CMe)(edta)] -(H4edta = ethylenediaminetetraacetic acid) [2.04(1) Å], 52 [Mo2L2Cl2(M-OH)2] 2+ [2.13(1) Å], 53 [Mo2L2(µ-O2CMe)(µ-OH)2] 3+ [2.08(1) Å], 53 and [Mo2L2Br2(µ-OH)(µ-Br)] 2+ (L = 1,4,7-triazacyclononane) [1.84(2) Å]. 5 The distance in 2 compares relatively well with some of those reported above. A compensation of different effects might be responsible for this similarity: compound 2 is electronically more saturated than the above edge-bridged bioctahedral examples (less oxygen π bonding is allowed), and the OH ligand in 2 experiences a stronger trans influence from the PMe3 ligand. Both these effects should lengthen the Mo-O bond in 2.

On the other hand, the terminal bonding mode in 2 will in part counter those effects. The angular distortions from the Cp ring are similar for all four monodentate ligands [CNT-Mo-X angles = 108.5(1), 110.7(1), 110.8(1), 112.2(2)8 for P(1), P(2), P(3) and O(1), respectively], as found for the analogous chloro derivative. 48 These angular parameters have been shown to be diagnostic of the electronic configuration at the metal and the σ/π bonding capabilities of the ligands. 55,56 A second crystal from the batch that contains crystals of 2 turned out to correspond to a solid solution of 2 and the corresponding chloro complex. The structure contains two molecules in the asymmetric unit, the first one corresponding to a fully ordered hydroxide derivative (Fig. 3) and the second one to a compositional disorder of the hydroxide and the chloride in a 58 : 42 ratio (Fig. 4). It is somewhat curious that the OH/Cl disorder is only observed in one of the two crystallographically independent molecules. The metric parameters of both molecules compare very well with those of the crystal structure of pure 2 described above (cf. Tables 3 and4). The closeness of all bonding and angular parameters between the two complexes rationalizes the ease with which they form a solid solution. However, the molecular packing is not quite identical for the two complexes, since the pure chloride forms monoclinic crystals (space group P21/c), whereas the pure hydroxide and the solid solution crystallize in different orthorhombic space groups (see Table 2). 57 1.676(7) and 1.801(9) Å for cis,mer-MoOCl2(L)3 (L = PMe2Ph and PEt2Ph, respectively), 58,59 and 1.8184(8) Å in trans-MoO2(dppe)2. 60 Some of the previously reported values might be artificially long because of what has been initially described as "bondstretch isomerism" and later recognized as a disorder-related artifact. 61 The Mo-O distance found for 3, however, is close to the shortest distances ever reported, thus we strongly believe that there are no disorder problems (e.g. O/Cl compositional disorder) in this case. The Mo-P distance is shorter than in compound 2, as expected for the smaller ionic radius associated with the higher oxidation state. The Cp ring is highly distorted for this compound, with the carbon atoms trans to the oxo ligand being the furthest away from the metal center. This phenomenon has also been observed for the isoelectronic Cp*ReOCl2 molecule. 62 For the purpose of estimating reaction energies (see Discussion), optimized geometries and energies were also obtained for the redox Fe(II/III) couple Cp2Fe n+ (n = 0, 1; singlet and doublet, respectively) and for the inorganic base PH3 and its conjugated acid PH4 + . All calculations with mirror-imposed symmetry were carried out only with eclipsed Cp and X ligands (i.e. with the unique Cp carbon atom eclipsed with a ligand). Calculations on both eclipsed and staggered geometries for [CpMoO(PH3)2] + show a difference of only 0.9 kcal mol -1 (1 cal = 4.184 J), and previous studies on other half-sandwich systems, namely CpMoCl2(PH3) (M = Cr, Mo) 63 and CpMo(PH2)(PH3)2 49 have shown analogous small differences in energy between the two possible Cs conformations. All energies of relevant systems are collected in Table 6. The optimized bonding parameters for the [CpMo(OH)(PH3)3] + and [CpMoO(PH3)2] + species are compared with the X-ray results in Tables 3 and5, respectively, while those of the other Mo complexes are collected in Table 7.

The comparison between experimental and calculated geometries is excellent. The main differences consist in the well known overestimation of the bond distances by the DFT method, [START_REF]Recent developments and Applications of Modern density Functional Theory[END_REF] while the angular parameters are very similar. Other minor differences are attributable to the different steric encumbrance of the PMe3 ligand relative to the PH3 model.

Views of all optimized Mo complexes are shown in Fig. 6. This geometry is indicated for the previously reported 41 [CpMoH2(PMe3)3] + complex by low-temperature 1 H and 31 P NMR studies 38 and corresponds to that observed by X-ray crystallography for the isoelectronic complex CpMoH3(PMe2Ph)2. [START_REF] Abugideiri | [END_REF] A similar geometry is also adopted by the isoelectronic complex [(C6H5Me)WH2(PMe3)3]2 + . 66 The input geometry for . 38 The metric parameters correspond rather closely to those of the dihydride cation, the major difference being that the axial hydride ligand is now pushed more strongly by the hydroxo ligand, thus bending toward the Ptrans atom. This is an indication that the hydrogen atom has indeed hydridic character, resulting in a repulsive effect of the oxygen lone pairs. Complex [CpMo(PH3)3(H2O)] + was generated by adding a proton to CpMo(OH)(PH3)3 49 in C1 symmetry, leading, however, to a pseudo-C2v final geometry as shown in Fig. 6(f). In view of the fact that the metal is formally saturated in this complex, it is somewhat surprising to find a nearly planar environment at oxygen (the sum of bond angles at oxygen is 359.50°).

The geometry of optimized [CpMoH(PH3)3] + [Fig. 6(g)] is very similar with that of the neutral precursor. A comparison of the bond distances and bond angles of CpMoH(PH3)3 + with those previously obtained 49 for CpMoH(PH3)3 shows that, upon oxidation, the Mo-Cp and Mo-H bonds shorten, while the Mo-P bonds lengthen. The same effects have been noted for the [CpWH3(H2PCH2CH2PH2)] n+ (n = 0, 1) pair. 67 In addition, there is an overall increase in the umbrella compression of the "legs" of the four legged piano stool structure. In comparison with the [CpMo(PH3)3(OH)] + and [CpMo(PH3)3(H2O)] + structures, the steric repulsion between two adjacent PH3 ligands results in a wider P-Mo-P angle, because the hydride ligand opposes a weaker repulsive effect relative to the OH and H2O ligands. The optimized ferrocene (eclipsed configuration) has distances (Fe-C 2.120 Å, C-C 1.443 Å) in good agreement with those determined by X-ray 68 and neutron diffraction. 69 The ferrocenium cation (S = 1/2) gave a slightly distorted eclipsed geometry (13° distortion, Fe-C 2.164 Å, C-C 1.443 Å).

Discussion

As stated in the Introduction, the decomposition of 1 + in dry MeCN has been the subject of a recent report from our Laboratory. 38 The new results reported herein show that water can participate in the decomposition scheme of oxidized hydride complexes by acting as a ligand and deviate the decomposition path toward the formation of higher oxidation state organometallic hydroxo and oxo derivatives.

Water as a ligand and not as a base

Water is not expected to act as a deprotonating agent toward 1 + in the presence of 1. This is because it has been shown previously that compound 1 can be protonated by strong acids in water 41 (that is, 1 is a stronger base than water). As a matter of fact, compound 1 is even a stronger base than PMe3. In a control experiment, we have shown that the protonation of a 1 :

1 mixture of compounds 1 and PMe3 with a substoichiometric amount of HBF4 gives rise to the NMR resonances ( 1 H and 31 P) of complex [1-H] 1 , while those of free PMe3 remain unchanged.

In addition, no evidence for the formation of PHMe3 + was ever obtained during the oxidation studies of 1, while the formation of [1-H] + has been verified by NMR spectroscopy.

The computational work on the PH3 model systems also indicates that the Mo complex is the better base: from the data in Table 6, we calculate that equation ( 4) is highly exothermic.

The quantitative result of the energetic calculation should be taken cautiously, because the solvent might significantly alter the energetic situation relative to the gas phase calculation via solvation effects. The solvation energies on the two sides of the equation, especially those related to the cationic species, might not balance off exactly. In addition, the replacement of PH3 with PMe3 should render both the free phosphine and the neutral hydride complex more basic. The inductive effect of a single H/Me replacement will influence the basicity of the P lone pair (only one bond away) in a stronger way relative to the Mo lone pair (two bonds away).

However, the Mo hydride complex has 9 H/Me substitutions as opposed to only three for the free phosphine. Once again, these effects might not balance off completely. An additional, probably smaller effect, is related to entropy changes. Considerations such as these should be kept in mind also when analyzing the other computed reaction energies below. On the basis of the above considerations, we conclude that the presence of water (and even PMe3) as a base is irrelevant when compound 1 is present (for instance when part of 1 is left unoxidized by use of a substoichiometric amount of ferrocenium). Therefore, the different outcome of the oxidation under wet conditions (formation of 2, 3 and 4) relative to the oxidation in dry MeCN or THF may be attributed to the coordination of water in place of a solvent molecule and to the follow-up reactivity that results from this event.

The deliberate addition of water to a MeCN solution does not change the course of the oxidation reaction of compound 1 in a substantial way, relative to the reported procedure in the dry solvent. 38 The major products of this reaction are the same ones obtained under dry conditions. However, the formation of some oxo complex [CpMoO(PMe3 has also been spectroscopically observed. Exchange studies carried out in CD3CN show that the acetone adduct is more susceptible than the water adduct toward exchange of the solvent donor.

Mechanistic proposal

A possible mechanism for the formation of the observed products is shown in Scheme 3, where all boxed species have been observed spectroscopically or analyzed by single crystal X-ray diffraction. This mechanism is consistent with the basic understanding that we have gained for this system by the studies in dry acetonitrile, 38 with other related previous observations, and with the results of our computational studies. As the computational work has been used as an energetic guide for the rationalization of mechanistic pathways that also involve a few unobserved intermediates, a number of control calculations were also carried out. The first one involves the oxidation reaction itself. Reaction (5), a model of the oxidation process of 1, is strongly exothermic. Equalizing the calculated energy difference with the free energy change (both PV and entropy contributions are probably negligible), we arrive at a calculated reduction potential of 21.15 V for complex CpMoH(PH3)3 relative to the ferrocene standard. This compares with the experimentally determined value of 21.46 V for compound 1. Given that the PMe3 should be a better donor than PH3 and should render the hydride complex easier to oxidize, we consider the agreement of our calculations with the experimental data as satisfactory.

Scheme 3

CpMoH(PH3)3 + Fc Water becomes more acidic when coordinated to a Lewis-acidic metal center. It is thus reasonable to suppose that the water ligand in the latter complex may be easily deprotonated, leading to complex [1-OH] + , which is still an 18-electron complex of Mo(IV). The alternative proton transfer decomposition pathway, according to the well established mechanism, 16,18,19,26 would lead to the same product, viz. [CpMo(PMe3)3(H2O)] + or to the hydrido-hydroxo tautomer, according to Scheme 1. Up to this point, the oxidation of 1 with 1 equivalent of Fc + should proceed according to the stoichiometry of equation ( 6), which corresponds to that established for the decomposition of the 1 + radical in dry MeCN. However, there is no evidence in the NMR monitoring for the accumulation of an intermediate that could be interpreted as [CpMo(PMe3)3(H2O)] + . Therefore, if this intermediate forms, it must be further transformed rapidly under the reaction conditions.

The transformation of [CpMo(PMe3)3(H2O)] + to the cation of 2 requires loss of one proton and one electron. The order of these events shown in Scheme 3 is suggested by the existence of CpMo(OH)(PMe3)2, 39,49 and by the energetics of Table 6. CpMo(OH)(PMe3)3 is unstable relative to PMe3 dissociation and formation of the 16-electron CpMo(OH)(PMe3)2 complex, a spin triplet complex. 49 The deprotonation process [equation (7)] is predicted as too unfavorable when using PMe3 or H2O as a base. Thus, this pathway is possible only in the presence of unoxidized 1.

[ The formation of 2 from 1 requires the consumption of 3 oxidizing equivalents and leads to the release of 2 equivalents of protons. The resulting stoichiometry is that of equation (9).

CpMo(PH3)3(H2O)] + + CpMoH(PH3)3  CpMo(OH)(PH3)3 + [CpMoH2(PH3)3] + (7)
Note that this equation involves an ox/1 ratio of 1 : 1, just like equation ( 6). 10) and (11), respectively. Equation (11) involves again an ox/1 ratio of 1 : 1, whereas the formation of 4 in equation (10) requires less than 1 equivalent of oxidant. The observation of the EPR triplet resonance when using a small amount of oxidant (see It is interesting to observe that each oxidation step is followed by the release of a proton, and that each molecule of the oxidizing 1 + generates a molecule of 1 that can capture the proton.

Thus, all 1 is ultimately consumed by the oxidation reaction with 1 equivalent of ferrocenium, to yield the oxidation products and the protonation product [1-H] + . According to the stoichiometry of equations ( 9), ( 10) and (11), compound 4 should only be observed when using a substoichiometric amount of oxidant (experimentally verified), while the use of 1 or more equivalents of oxidant should yield only 2 and 3.

In conclusion, the formation of oxo and hydroxo products probably starts with coordination of a water molecule to an unsaturated metal center which is generated by an oxidation process, and continues with tandem oxidation/deprotonation steps.

Conclusions

With this contribution we have shown that water can intervene in the decomposition of paramagnetic hydride complexes not only as a proton acceptor, as previously established for less electron-rich hydride complexes, but also as a ligand. This event leads to higher oxidation state hydroxo and oxo complexes via a cascade of energetically favorable deprotonation/oxidation steps. The existence of sequential electron and proton transfer reactions is well documented in coordination chemistry. In particular, these tandem reactions on molybdenum derivatives containing a coordinated water residue have been proven biologically relevant. [START_REF] Stiefel | Proc. Natl. Acad. Sci[END_REF][START_REF] Stiefel | [END_REF] When the parent neutral hydride is loaded with electron-releasing ligands, it becomes more basic than water and is the preferred proton acceptor. However, the need for a donor solvent to electronically saturate the products of decomposition of the paramagnetic hydride may make water the preferred substrate when this process is carried out in solvents of limited coordinating ability. The follow-up processes are then a consequence of the increased acidity of water upon coordination to the Lewis-acidic metal center.

1 M

 1 Scheme 1

[

  CpMoO(PMe3)2] + (ca. 15% of all diamagnetic products by integration of the Cp resonances), and [CpMoH(PMe3)3(H2O)] 2+ (ca. 20%). Decomposition of 1 + in THF (a) Isolation of [CpMo(PMe3)3(OH)][BF4], 2, and [CpMoO(PMe3)2][BF4], 3.

The 1 H

 1 NMR spectrum showed five Cp resonances, whose relative intensities changed with time. The initial relative intensities and assignments are 23 ([CpMoO(PMe3)2] 1 ) : 55 (δ 4.80, overlap of [CpMoH2(PMe3)3] + and [CpMo(CD3CN)(PMe3)3] + ) 38 : 7 (δ 5.31, [CpMoH(PMe3)3-(CD3CN)] 2+ ) : 7 ([CpMoH(PMe3)3(CD3COCD3)] 2+ ) : 8 ([CpMoH(PMe3)3(H2O)] 2+ ). Over 24 h, there is no significant change in the relative intensities of the [CpMoO(PMe3)2] + , [CpMoH2(PMe3)3] + , [CpMo(CD3CN)(PMe3)3] + and [CpMoH(PMe3)3(H2O)] 2+ resonances, while the [CpMoH(PMe3)3(CD3COCD3)] 2+ resonance disappeared and that of [CpMoH-(PMe3)3(CD3CN)] 2+ correspondingly increased. NMR and EPR monitoring of the decomposition of 1 + in wet CD3COCD3. Formation of CpMoO(PMe3)2, 4

[ 1 (

 1 H2O)] 2+ appears most reasonable. The two species [1(MeCN)] 2+ and [1(H2O)] 2+ have the same pattern of 1 H and 31 P NMR resonances (one Cp resonance and two PMe3 resonances in a 2 : 1 ratio with similar chemical shifts and coupling constants), and both show coupling of the 31 P NMR resonances to a single hydride ligand upon selective decoupling (JHP = 10 Hz for the CD3CN adduct 38 and 14 Hz for the H2O adduct, see Table 1). Complex [1(H2O)] 2+ does not decompose readily in CD3CN (24 h), nor does the ratio of complexes [1(MeCN)] 2+ and [1(H2O)] 2+ change with time. On a longer time scale, however (4 days), only the oxo and dihydrido Mo(IV) complexes are observed in solution.

  P NMR resonance (δ ca. -60) being attributed to chemical exchange with the paramagnetic compound 4. This is confirmed by the upfield shift and sharpening of this resonance upon addition of free PMe3 to this solution. After further oxidation, the 1 H and 31 P NMR resonances of [CpMoO(PMe3)2] + become observable, in concomitance with the disappearance of the signal of 4 from the EPR spectrum. The recrystallization of 1 + BF4 -from wet acetone yields three different types of crystals. These were shown by X-ray crystallography (vide infra) to correspond to the BF4 -salt of the [CpMo(O)(PMe3)2] + complex, compound 3, the Mo(III) hydroxo species [CpMo(OH)-(PMe3)3]BF4, 2, and a solid solution of the latter with [CpMoCl(PMe3)3]BF4, corresponding to the composition [CpMo(OH)0.79Cl0.21(PMe3)3]BF4. The X-ray structure of a pure sample of the chloro species was reported previously.

Fig. 2 A

 2 Fig. 2 A top view of the [CpMo(OH)(PMe3)3] + cation in compound 2. Only the major orientation of the disordered PMe3 ligand (P1 donor atom) is shown for clarity.

Fig. 3 A

 3 Fig. 3 A top view of the [CpMo(OH)(PMe3)3] + cation in the structure of [CpMo(OH)0.79Cl0.21-(PMe3)3] + BF4 -. Ellipsoids are drawn at the 30% probability level.

Fig. 4 A

 4 Fig. 4 A top view of the disordered [CpMo(OH)0.58Cl0.42(PMe3)3] + cation in the structure of [CpMo(OH)0.79Cl0.21(PMe3)3] + BF4 -. Ellipsoids are drawn at the 30% probability level.

Fig. 5 A

 5 Fig. 5 A view of the [CpMo(O)(PMe3)2] + ion in compound 3. Ellipsoids are drawn at the 30% probability level.

aFig. 6

 6 Fig. 6 Optimized geometries of (a) [CpMoO(PH3)2] + ; (b) [CpMo(OH)(PH3)3] + ; (c) CpMo(OH)-(PH3)2 (S = 1/2); (d) [CpMoH2(PH3)3] + ; (e) [CpMoH(OH)(PH3)3] + ; (f) [CpMo(PH3)3(H2O)] + ; (g) [CpMoH(PH3)3] + .

  The geometry of CpMoO(PH3)2 converged to a pseudotetrahedral coordination environment with one position occupied by a η3 -Cp ligand [Fig. 6(c)], similar to the corresponding one-electron oxidation product, [CpMoO(PH3)2] + [Fig. 6(a)]. The shortening of all the bonds to the Mo atom (especially the Mo-P bonds) upon oxidation is perfectly consistent with the increased effective charge of the oxidized Mo center. The derivative [CpMoH2(PH3)3] + [Fig. 6(d)] was optimized starting from a pseudo-octahedral geometry with the Cp ring and one hydride occupying the axial positions and the three phosphine ligands adopting a mer arrangement in the equatorial plane.

[

  CpMoH(OH)(PH3)3] + was derived from the optimized [CpMoH2(PH3)3] + geometry, by replacement of the equatorial H ligand with OH. Two different optimizations were carried out, with an initial dihedral CNT-Mo-O-H angle of 45° and 135°, respectively, leading to the same final result [Fig. 6(e)]. The equatorial placement of the OH ligand and the axial placement of the hydrido ligand are suggested by the identical stereochemistry of the isoelectronic [CpMoH(PMe3)3(MeCN)] 2+

PH4 + + CpMoH(PH3) 3 

 3 PH3 + [CpMoH2(PH3)3] + (4) ΔE = -66.8 kcal mol -1

) 2 ]

 2 + and the water adduct [CpMoH(PMe3)3(H2O)] 2+ indicates that water is able to compete with the MeCN coordination and partially deviate the course of the decomposition of the 1 + intermediate. In the less coordinating solvents THF or acetone, the coordination of water is seen to compete even more favorably with the solvent, although the acetone adduct [CpMoH(PMe3)3(CD3COCD3)] 2+

  Hydridohydroxo complexes analogous to [1-OH] + are amply precedented in the literature, for instance Cp*2W(OH)H70 and [IrH(OH)(PMe3)4] + .71 In addition, Mo(IV) complexes that are isoelectronic with [1-OH] + such as CpMoH(OH)(C6D5)(PMe3)2 and CpMoH(OH)(η 2 -CH2PMe2)(PMe3) have been implicated as intermediates in C-H activation processes.49 However, an alternative possibility involves deprotonation of [1(H2O)] 2+ at the metal, to afford the tautomer complex [CpMo(PMe3)3(H2O)] + . This corresponds to the pathway established in dry MeCN, transforming [1(MeCN)] 2+ to the final product [CpMo(PMe3)3(MeCN)] + . Either way, the deprotonating agent must be 1, which is generated locally and stoichiometrically by the preceding electron transfer step. Indeed, complex [1-H] + is one of the observed products of the oxidation of 1 in wet acetone or THF.The calculations on the PH3 model systems indicate that the Mo(II) aquo complex is 4.0 kcal mol -1 more stable than the Mo(IV) hydrido-hydroxo tautomer, implying a stronger thermodynamic acidity at the hydride position for [CpMoH(PH3)3(H2O)] 2+ . The tautomerization reaction corresponds to an oxidative addition/reductive elimination of the water O-H bond and could well be a very rapid process. It is relevant to mention here that a fast Mo(H2O)/MoH(OH) tautomerization process has been invoked for a very similar CpMoH(OH)(η 2 -CH2PMe2)(PMe3) system to rationalize the rapid H/D scrambling between the hydroxo and PMe3 positions in compound CpMo(OH)(PMe3)2.49

2 1 -

 1 2 e -+ H2O  [1-H] + + [CpMo(PMe3)3(H2O)] +

ΔE = +1.8 kcal mol - 1 Complex

 1 CpMo(OH)(PMe3)2 is not spectroscopically observed during the oxidation of 1, thus electron transfer must rapidly follow the proton loss. It is also worth noting here that, as we have previously shown,39,49 the FcBF4 oxidation of isolated CpMo(OH)(PMe3)2 in the presence of PMe3 affords a mixture of 2 and 3. The oxidation of CpMo(OH)(PMe3)3 (in equilibrium with CpMo(OH)(PMe3)2 and free PMe3) may be accomplished again by the longlived 1 + radical as indicated in Scheme 3. The model PH3 system confirms the feasibility of this electron transfer process [equation (8)]. CpMo(OH)(PH3)3 + [CpMoH(PH3)3] +  [CpMo(OH)(PH3)3] + + CpMoH(

3 1

 1 -3 e -+ H2O  2 [1-H] + + [CpMo(OH)(PMe3)3] + (9) Compound 3 is obtained by further oxidation of 2, probably via deprotonation and loss of PMe3 to afford the neutral Mo(III) oxo intermediate CpMo(O)(PMe3)2, 4, which can subsequently be oxidized to 3. The formation of 4 and 3 requires the stoichiometries of equations (

4 1 -

 1 3 e -+ H2O  3 [1-H] + + 4 + PMe3 (10) 4 1 -4 e -+ H2O  3 [1-H] + + [CpMoO(PMe3)2] + + PMe3(11)

Fig. 1 )

 1 Fig.1), is in agreement with the above argument. No other species involved in Scheme 3 could give rise to the observed EPR resonance. Our attempts to optimize a neutral CpMoO(PH3)3 model led to direct expulsion of a PH3 ligand and converged to two separate CpMoO(PH3)2 and PH3 units. The experimental evidence of a fast exchange between free PMe3 and the ligand coordinated to compound 4 (see Results) indicates, however, that a CpMoO(PMe3)3 complex may be easily accessible as a transient. The thermodynamic feasibility of the overall deprotonation process is shown in equation(12). Although the process is calculated as slightly endothermic, the reaction is entropically favored by the release of one molecule of free PH3. A spontaneous deprotonation of [CpMo(OH)(PMe3)3] + with proton capture by the released PMe3 ligand does not seem consistent with the calculations on the model system [equation(13)]. The subsequent oxidation of 4 by 1 + is also a favorable process, as indicated in the model system by equation(14).

Table

Table 1 .

 1 NMR data

	. EPR measurements were

Table 2 .

 2 Crystal data for all compounds a

	Compound	2	0.79(2)•0.21[CpMoCl(PMe3)3][BF4] 3
	Formula	C14H33BF4MoOP3 C28H65.58B2Cl0.42F8Mo2O1.58P6	C11H23BF4MoOP2
	Mw	493.06	993.78	415.98
	Space group	Pna21	P212121	Ama2
	a/Å	20.525(5)	8.7140(9)	17.29(2)
	b/Å	13.380(4)	13.300(4)	15.201(2)
	c/Å	7.9740(7)	37.939(6)	6.6459(6)
	V/Å 3	2189.9(9)	4397(2)	1747(2)
	Reflections collected, unique	3712, 3712	3488, 3488	969, 692
	Rint	-	-	0.0336
	µ(Mo-Ka)/mm -1	0.851	0.872	0.963
	R indices [I > 2σ(I)]
	R1 b	0.0267	0.0509	0.0391
	wR2 c	0.0567	0.1139	0.0715
	R indices (all data)
	R1	0.0353	0.0807	0.0782
	wR2	0.0604	0.1292	0.0801

a Details in common: orthorhombic

Table 3 .

 3 Selected bond lengths (Å) and angles (°) for [CpMo(OH)(PMe3)3][BF4] (2) and comparison with B3LYP-optimized [CpMo(OH)(PH3)3] + a

		X-ray	B3LYP		X-ray	B3LYP
	Mo-CNT	1.980(2)	2.054	O(1)-Mo(1)-CNT	112.40(13)	121.35
	Mo(1)-C(1)	2.302(4)	2.025	P(1)-Mo(1)-CNT	108.52(10)	112.22
	Mo(1)-C(2)	2.241(5)	2.302	P(2)-Mo(1)-CNT	110.63(10)	110.31
	Mo(1)-C(3)	2.271(4)	2.474	P(3)-Mo(1)-CNT	110.81(10)	114.38
	Mo(1)-C(4)	2.348(4)		P(1)-Mo(1)-P(2)	94.22(5)	85.97
	Mo(1)-C(5)	2.369(4)		P(1)-Mo(1)-P(3)	93.21(4)	85.20
	Mo(1)-P(1)	2.4533(12)	2.572	P(2)-Mo(1)-P(3)	132.83(4)	136.40
	Mo(1)-P(2)	2.4989(12)	2.632	P(1)-Mo(1)-O(1)	139.07(10)	123.70
	Mo(1)-P(3)	2.4749(11)	2.557	P(2)-Mo(1)-O(1)	71.03(8)	68.64
	Mo(1)-O(1)	2.080(3)	2.050	P(3)-Mo(1)-O(1)	73.15(8)	81.40
	O(1)-H(1A)	0.831(10)	0.979	Mo(1)-O(1)-H(1A)	133(4)	126.60
				CNT-Mo-O-H	-20(5)	83.00

a CNT = centroid of cyclopentadienyl ring.

Table 4 .

 4 Selected bond lengths (Å) and angles (°) for [CpMo(OH)0.79Cl0.21(PMe3)3][BF4]

	Mo(1)-P(1)	2.484(4)	Mo(2)-Cl(2)	2.33(3)
	Mo(1)-P(2)	2.481(4)	Mo(2)-P(4)	2.488(4)
	Mo(1)-P(3)	2.456(4)	Mo(2)-P(5)	2.499(4)
	Mo(1)-O(1)	2.078(9)	Mo(2)-P(6)	2.464(4)
	Mo(1)-C(1)	2.25(2)	Mo(2)-O(2)	2.09(3)
	Mo(1)-C(2)	2.25(2)	Mo(2)-C(15)	2.36(2)
	Mo(1)-C(3)	2.31(2)	Mo(2)-C(16)	2.348(13)
	Mo(1)-C(4)	2.36(2)	Mo(2)-C(17)	2.265(14)
	Mo(1)-C(5)	2.31(2)	Mo(2)-C(18)	2.22(2)
	Mo(1)-CNT(1)	1.98(2)	Mo(2)-C(19)	2.30(2)
			Mo(2)-CNT(2)	1.98(2)
	P(1)-Mo(1)-P(2)	135.4(2)	Cl(2)-Mo(2)-P(6)	138.4(9)
	P(1)-Mo(1)-P(3)	92.6(2)	Cl(2)-Mo(2)-O(2)	7(3)
	P(1)-Mo(1)-O(1)	75.6(2)	Cl(2)-Mo(2)-CNT(2) 113.8(11)
	P(1)-Mo(1)-CNT(1)	108.9(5)	P(4)-Mo(2)-P(5)	135.70(13)
	P(2)-Mo(1)-P(3)	92.0(2)	P(4)-Mo(2)-P(6)	93.7(2)
	P(2)-Mo(1)-O(1)	72.5(2)	P(4)-Mo(2)-O(2)	70(2)
	P(2)-Mo(1)-CNT(1)	111.5(2)	P(4)-Mo(2)-CNT(2)	108.1(5)
	P(3)-Mo(1)-O(1)	139.6(3)	P(5)-Mo(2)-P(6)	91.0(2)

Table 5 .

 5 Selected

		bond lengths (Å) and angles (°) for [CpMo(O)(PMe3)2][BF4] 3 and
	comparison with B3LYP-optimized [CpMo(OH)(PH3)2] + a
		X-ray	B3LYP		X-ray	B3LYP
	Mo-CNT	2.05(2)	2.122	O(1)-Mo(1)-CNT	149.1(6)	147.41
	Mo(1)-C(1)	2.20(2)	2.265	P(1)-Mo(1)-CNT	106.9(3)	107.44
	Mo(1)-C(2)	2.298(13)	2.363	P(1)-Mo(1)-P(1A)	94.4(2)	93.25
	Mo(1)-C(3)	2.521(12)	2.620	P(1)-Mo(1)-O(1)	93.7(3)	94.60
	Mo(1)-P(1)	2.445(4)	2.547	P(1)-Mo(1)-CNT-P(1A)	100.01	99.26
	Mo(1)-O(1)	1.674(13)	1.714

a CNT = centroid of cyclopentadienyl ring.

Table 6

 6 Spin state, U3BLYP total energies, and UB3LYP S(S+1) for various systems

	System	Spin state	Total energy/Eh	S(S+1)
	CpMoO(PH3)2] +	Singlet	-352.6380	0
	[CpMo(OH)(PH3)3] +	Doublet	-361.5012	0.7556
	CpMoO(PH3)2	Doublet	-352.8176	0.7556
	CpMoH(PH3)3 a	Singlet	-286.4484
	CpMo(OH)(PH3)3 a	Singlet	-361.6866
	CpMo(OH)(PH3)2 a	Triplet	-353.4096
	[CpMoH2(PH3)3] +	Singlet	-286.8546	0
	[CpMoH(OH)(PH3)3] +	Singlet	-362.0893	0
	[CpMo(PH3)3(H2O)] +	Singlet	-362.0957	0
	[CpMo(H)(PH3)3] +	Doublet	-286.2541	0.7560
	PH3 a	Singlet	-8.2699
	[PH4] +	Singlet	-8.5697	0
	Cp2Fe	Singlet	-510.4397	0
	[Cp2Fe] +	Doublet	-510.2030	0.8135

a Data from ref.

49.

Table 7

 7

	B3LYP-optimized selected geometric parameters for CpMoO(PH3)2,
	CpMoH2(PH3)3 + , CpMoH(OH)(PH3)3 + , CpMo(H2O)(PH3)3 + and [CpMoH(PH3)3] 1 a
	Compound	Distance/Å		Angle/°		Dihedral angle/°
	CpMoO(PH3)2	Mo-CNT a	2.100	CNT-Mo-O	157.95	P-Mo-CNT-P	108.53
		Mo-C1	2.262	CNT-Mo-P	109.50	O-Mo-CNT-P	122.73
		Mo-C2	2.372	P-Mo-P	99.79
		Mo-C3	2.57	O-Mo-P	84.03
		Mo-P	2.701
		Mo-O	1.746
	[CpMoH2(PH3)3] +	Mo-CNT	2.030	CNT-Mo-Heq	104.03
		Mo-Hax	1.691	CNT-Mo-Hax	170.49
		Mo-Heq	1.718	CNT-Mo-Ptrans	141.10
		Mo-Ptrans	2.583	CNT-Mo-Pcis	111.26
		Mo-Pcis	2.583	Heq-Mo-Pt	141.10
				Heq-Mo-Pc	78.01
				Heq-Mo-Hax	66.46
				Pcis-Mo-Pcis	134.95
				Ptrans-Mo-Pcis	87.68
	[CpMoH(OH)(PH3)3] +	Mo-CNT	2.084	CNT-Mo-H	173.50
		Mo-H	1.716	CNT-Mo-O	106.58
		Mo-O	2.034	CNT-Mo-Ptrans	107.29
		O-H	0.980	CNT-Mo-Pcis	110.57
					111.17

Acknowledgments

We are grateful to the DOE (grant no. DEFG059ER14230) for support of this work and to Dr Raymund C. Torralba for some preliminary observations.


Elsje Alessandra Quadrelli

Rinaldo Poli

Electrophilic Addition vs Electron Transfer for the Interaction of Ag + with Molybdenum(II) Hydrides. 2. Reaction with CpMoH(CO) 2 (PMe 3 )

The interaction of CpMoH(CO) 2 (PMe 3 ), 1, with AgBF 4 in acetonitrile establishes an equilibrium (K ) 9 ( 1) with the hydride-bridged adduct [Cp(CO) 2 (PMe 3 )Mo(µ-H)Ag(MeCN) x ]-BF 4 , 4, which is in a fast chemical exchange on the NMR time scale with compound 1. The interaction of 1 and AgBF 4 in THF leads to the formation of the trinuclear complex {[CpMoH-(CO) 2 (PMe 3 )] 2 Ag}BF 4 , 5, as an insoluble white solid in 73% yield when the Mo/Ag ratio is 2:1. The nature of the precipitate does not depend on the relative stoichiometry of the two reagents. When redissolved in acetonitrile, compound 5 dissociates, generating a mixture of 1 and 4. Compound 4 slowly evolves to the solvent adduct, [CpMo(CO) 2 (PMe 3 )(MeCN)]-BF 4 , 3, with elimination of hydrogen gas and metallic silver. The decomposition of 4 could be interpreted as either an inner-sphere electron transfer or a substitution of unstable "AgH".

Introduction

In part 1 of this series, 1 we have examined the interaction between Ag + (as well as other oxidizing agents including ferrocenium and a platinum anode) and the electron-rich 18-electron Mo(II) complex Cp-MoH(PMe 3 ) 3 . Contrary to the interaction of Ag + with several other hydride complexes, for which products of electrophilic addition to the hydride ligand were obtained, [2][3][4][5][6][7][8][9][10][11][12][13][14][15] the interaction of silver with CpMoH-(PMe 3 ) 3 (whose electrochemically reversible oxidation occurs at E 1/2 ) -1.46 V vs the ferrocene standard in MeCN) proceeds directly to the products of electron transfer without detection of an electrophilic addition intermediate. This paper presents the results of our investigations of the reactivity of complex CpMoH(CO) 2 -(PMe 3 ), 1, with the silver ion. The interest of this investigation is that 1, while analogous with CpMoH-(PMe 3 ) 3 , 2, shows an irreversible oxidation process at E p,a ) 0.19 V vs ferrocene in MeCN, i.e., 1.65 V more positive relative to the tris-PMe 3 complex, 16 rendering the formation of a stable addition product competitive with the oxidation process. It is therefore possible for the first time to make a comparison of the reactivity between Ag + and two hydride complexes belonging to the same class, i.e., 1 and 2, and possessing quite disparate electrochemical properties.

Experimental Section

General Procedures. All operations were carried out under an atmosphere of dinitrogen with standard Schlenkline techniques. Solvents were purified by conventional methods and distilled under argon prior to use (THF and Et2O from Na/benzophenone, heptane from Na, acetone from CaSO4, CH2Cl2 from P4O10, and MeCN form CaH2). Deuterated solvents (C6D6, CD3CN, THF-d8, and CD3COCD3) were degassed by three freeze-pump-thaw cycles and then stored over 4 Å molecular sieves under dinitrogen. NMR spectra were obtained with Bruker WP200 and AF400 spectrometers. The peak positions are reported with positive shifts downfield of TMS as calculated from the residual solvent peaks ( 1 H) or downfield of external 85% H3PO4 ( 31 P). For each 31 P NMR spectrum, a sealed capillary containing H3PO4 was immersed in the same NMR solvent used for the measurement, and this was used as reference. FT-IR spectra were recorded on a Perkin-Elmer 1880 spectrometer with KBr cells. EPR spectra were recorded on a Brucker ER200 spectrometer upgraded to ESP 300, equipped with a cylindrical ER/4103 TM 110 cavity and an X-band microwave generator. Elemental (C, H) analyses were performed by MHW Laboratories (Phoenix, AZ) and the analytical service of LSEO, Universite ´de Bourgogne. The Ag analyses were carried out by atomic absorption with a Perkin-Elmer 5000 flame spectrophotometer. Compound 1 17 was prepared as previously published. AgBF4 was purchased from Aldrich and used as received.

Low-Temperature 1 H NMR Studies of CpMoH(CO)2-(PMe3). In an NMR tube was prepared a solution of compound 1 (76 mg, 258 µmol) in CD3CN (345 µL). 1 H NMR (RT, δ): 5.22 (s, 5H, Cp), 1.50 (d, JHP ) 10 Hz, 9H, PMe3), -6.35 (d, JHP ) 47 Hz, 1H, Mo-H). At room temperature the hydride resonance appears as a highly distorted doublet, with a sharp higher field signal and a very broad lower field signal. Upon cooling signals decoalesce and lead to distinguishable resonances for the cis and trans isomers. 1 H NMR (CD3CN, 240 K, δ): trans isomer, 5.13 (s, 5H, Cp), 1.49 (d, JHP ) 9 Hz, 9H, PMe3), -6.4 (br, 1H, Mo-H); cis isomer, 5.28 (s, 5H, Cp), 1.45 (d, JHP ) 9 Hz, 9H, PMe3), -6.5 (d, JHP ) 66 Hz, 1H, Mo-H). The approximate trans/cis ratio given by integration is 1:1. The assignment of the resonances to the different isomers is based on the comparison with the previously published study in toluene-d8. 17 A similar solution in acetone-d6 cooled at 204 K shows better resolved resonances for the two geometric isomers. 1 H NMR (δ): trans isomer, 5.62 (s, 5H, Cp), 1.77 (d, JHP ) 9 Hz, 9H, PMe3), -6.07 (d, JHP ) 23 Hz, 1H, Mo-H); cis isomer, 5.45 (s, 5H, Cp), 1.72 (d, JHP ) 9 Hz, 9H, PMe3), -6.31 (d, JHP ) 66 Hz, 1H, Mo-H). 31 P NMR: 30.6 (s) and 25. 2 (s).

Reaction between CpMoH(CO)2(PMe3) and Ag + in MeCN. (a) NMR Monitoring in CD3CN. A solution of AgBF4 (286 mg; 1.47 mmol) in CD3CN (855 µL) was prepared. Fifteen aliquots of 50 µL each of this solution (86 µmol of Ag + each) were added stepwise at room temperature to the solution of 1 in CD3CN described in the previous section (∆[Ag + ]/[Mo] ) 1/3 for each addition). A 1 H NMR spectrum was recorded after each addition. While the number of resonances remained constant during the experiment, their chemical shifts changed after each addition (see Results). Toward the end of the experiment ([Ag + ]/[Mo] > 2), some gray precipitate started to form and minor amounts of [CpMo(CO)2(PMe3)(MeCN)] + , 3, appeared in the 1 H NMR spectrum (<5%): cis-3, δ 5.62 (s, 5H, Cp), 1.63 (d, J HP ) 5 Hz, 9H, PMe3); trans-3, 5.43 (s, 5H, Cp), 1.62 (d, J HP ) 10 Hz, 9H, PMe3). These resonances match those previously reported 16 for complex 3. The cis-3/trans-3 ratio is ca. 10 and does not change with time.

(b) IR Study. Compound 1 (268 mg, 0.91 mmol) was dissolved in CH 3CN (15 mL). IR: 1927 and 1841 cm -1 . AgPF6 (210 mg, 0.83 mmol) was added, immediately, turning the solution darker orange. An aliquot was taken for an IR spectrum. New bands at 1943 and 1855 cm -1 , attributed to the silver adduct [Cp(CO) 2(PMe3)Mo(µ-H)Ag(MeCN)x] + , 4, alongside less intense absorbances corresponding to those of the unreacted starting material, were observed. After 1 h, new carbonyl bands at 1985 and 1897 cm -1 , attributed to cis-[CpMo(CO) 2(PMe3)(MeCN)]PF6, 3 (cf. lit. 1987 and 1898 cm -1 ), 16 were also present (ca. 20% transformation). Attempts to recover a stable solid were unsuccessful. Cooling the acetonitrile solutions yielded white solids that proved too lightand heat-sensitive to be handled.

Interaction between CpMoH(CO)2(PMe3) and Ag + in THF. (a) In a 2:1 Ratio. Synthesis of {[CpMoH(CO)2-(PMe3)]2Ag}BF4, 5. To a cooled (-80 °C) THF solution (10 mL) of compound 1 (324 mg, 1.2 mmol) was added dropwise a similarly cooled solution of AgBF4 (107 mg, 0.55 mmol) in THF (5 mL). The immediate precipitation of the product 5 as a white solid ensued. The colorless supernatant was removed by filter-cannula, and the white powder was dried under vacuum in the dark (302 mg, 72% yield). Anal. Found (calcd for C20H30AgBF4Mo2O4P2) %: C, 30.4 (30.7); H, 4.08 (3.86); Ag, 15 (14). IR (Nujol mull, cm -1 ): 2920 (vs), 2725 (m), 1945 (s), 1835 (s) 1295 (m), 1154 (w), 1056 (m), 962 (s). The powder is light-and heat-sensitive and turns yellow upon decomposition. (b) In a 1:1 Ratio. Compound 1 was generated in situ by the addition of 1 equiv of PMe3 (102 µL) to a solution of CpMoH(CO)3 (242 mg, 0.93 mmol) in THF (5 mL) according to the literature. 17 After cooling to -20 °C AgBF4 (189 mg, 0.97 mmol) was added, inducing the precipitation of a light yellow solid. The orange superantant was filtered off, and the solid was dried under vacuum (237 mg, 65% yield relative to Ag + ). The 1 H NMR and IR properties of this solid correspond with those reported above for compound 5.

(c) NMR Investigation in THF-d8. A solution of AgBF4 (132 mg, 677 µmol) in THF-d8 (390 µL) was prepared. A 10 µL aliquot of this solution (17 µmol of Ag + ) was added to an NMR tube containing compound 1 (20.0 mg, 68 µmol) in THF-d8 (425 µL). No change in the chemical shifts of the resonances of 1 was observed, while a yellow solid immediately precipitated. 1 H NMR: δ 5.19 (s, 5H, Cp), 1.50 (d, JHP ) 9 Hz, 9H, PMe3), -6.12 (br d, 1H, Mo-H). The intensity change that occurred upon the addition of silver was quantitatively measured through the comparison of the integrals of the Cp vs THF-d8 before and after the addition. The hydride resonances decreased by a factor of 54% upon reaction with silver, implying that 37 µmol of 1 (i.e., half-equivalent of silver salt per molybdenum center) has been sequestered in the precipitate.

Results

(a) Interaction of CpMoH(CO) 2 (PMe 3 ) with Ag + in Acetonitrile: Formation of [CpMo(CO) 2 (PMe 3 )-(µ-H)Ag(MeCN)

x ] + BF 4 -(4). The addition of silver ions to an acetonitrile solution of 1 induces a slight color deepening and the appereance of new CO stretching bands at 1944 and 1856 cm -1 in the IR solution spectrum. These frequencies are red-shifted with respect to those of 1 (at 1926 and 1841 cm -1 ), consistent with removal of electron density from the complex as could be expected from the addition of the Lewis acidic silver center. Addition of 0.9 equiv of Ag + induced the nearly complete disappereance of 1, consistent with a 1:1 stoichiometry for the reaction.

Additional information on this interaction is provided by the NMR study. Addition of silver ions to a solution of 1 has an indirect effect on the 1 H NMR resonances of the species: instead of creating new resonances, it induces a shift of the already existing ones. The most dramatic change is experienced by the hydridic resonance, which varies from δ ) -6.5 (d, J HP ) 47 Hz) at Ag/Mo ) 0 to -8.1 (d, J HP ) 20 Hz) at Ag/Mo ) 5 (see Figure 1). The drifts of the Cp and PMe 3 resonances are similar in trend but smaller in magnitude (∆ < 0.15 ppm along the titration curve). It might seem surprising that the hydridic resonance in the solution 1 H NMR spectrum of 1 is a very asymmetric doublet. This is due to the fast dynamic exchange at room temperature of the two geometric isomers, cis-and trans-1, on the NMR time scale, resulting in a single set of resonances, as already reported. 17 The distinctive room-temperature asymmetry of the coalesced resonance can be attributed to the large difference of the J HP in the two rapidly exchanging isomers (23 Hz for trans, 66 Hz for cis). This asymmetry is lost when silver cations are introduced in the solution.

Plots of the hydridic chemical shift and of the J HP vs the [Ag + ]/[Mo] ratio are reported in Figure 2a,b, respectively. The dependence of these two parameters on the relative ratio is qualitatively similar. A more detailed analysis was carried out only on the chemical shift data. Three main regions can be defined as the [Ag + ]/[Mo] ratio increases: a first linear region for [Ag + ]/[Mo] < 1, a transition region, and a second linear region for [Ag + ]/ [Mo] > 3, where changes are very small. The two straight lines that fit the linear regions intersect at [Ag + ]/[Mo] ≈ 1, in agreement with a 1:1 stoichiometry, as also suggested by the IR study (see above). Given the results of the interaction between 1 and AuPPh 3 + , 18 the literature precedents mentioned in the Introduction, and the lowering of J HP for the hydride resonance in the 1 H NMR, the product of this interaction is formulated as [Cp(CO) 2 (PMe 3 )Mo(µ-H)Ag(MeCN) x ]BF 4 , 4; see eq 1. This formulation is in accord with the large drift of the chemical shift and with the reduction of the J HP for the hydridic signal as the concentration of silver increases. The latter, in particular, is consistent with a significant weakening of the H-Mo interaction.

The chemical shift trend in Figure 2a can thus be rationalized as the result of a rapid exchange between the 1:1 adduct, 4, and residual 1. This exchange could in principle take place either via dissociation of the Ag + ion or by an associative mechanism (see Scheme 1a). Either way, this chemical exchange prevents the direct observation of the chemical shifts of complex 4 and the H-Ag coupling for the hydride resonance, even at -80 °C. The deviation between the observed chemical shifts and the two straight lines in the transition region shows that eq 1 is not quantitative but rather an equilibrium. [Mo] ratio, as calculated from the spectra reported in Figure 1 (errors on δ < 0.1 ppm, errors on J < 0.2 Hz). In (a) the solid straight lines interpolate the experimental points in the two linear regions.

CpMoH(CO) 2 (PMe 3 )

1

+ AgBF 4 + xMeCN a [CpMo(CO) 2 (PMe 3 )(µ-H)Ag(MeCN) x ] + BF 4 - 4 (1)
The coupling between the hydride and the silver nucleus is not observed even when a large excess of Ag + is administered in order to force equilibrium 1 to the right and slow the exchange between 1 and 4. From the value of K, it can be calculated that 10.5% of 1 remains free in the presence of a 5-fold excess of Ag + under the conditions described in the Experimental Section. A second chemical exchange, namely, beween compound 4 and the excess Ag + , could also take place under these circumstances (see Scheme 1b). Stable complexes of stoichiometry M 2 Ag 6,10,12-15 and MAg 2 , 7 analogous to the proposed intermediates in Scheme 1, have previously been described. In view of the incomplete formation reaction and the extreme light sensitivity, compound 4 could not be isolated in the solid state.

(b) Decomposition of Complex 4. Acetonitrile solutions of 4 slowly decompose with the formation of solid silver, gaseous hydrogen, and [CpMo(CO) 2 (PMe 3 )-(MeCN)] + , 3, as confirmed by IR and NMR spectroscopy (see eq 3). Both the cis and the trans isomers are formed in the equilibrium ratio of 10:1, 16 as confirmed by the NMR spectrum. The bands of the trans isomer in the IR spectrum, expected at 1992 and 1904 cm -1 , 16 are not discernible because they are overshadowed by the more intense bands of the major isomer, at 1987 and 1898 cm -1 . Reaction 3 may take place via an electron-transfer pathway, to generate silver and a 17-electron [CpMo-(CO) 2 (PMe 3 )H] + intermediate. This compound was previously shown to rapidly decompose in MeCN to afford 3 and H 2 , 16 i.e., the observed products of eq 3. Another possibility, however, could involve the substitu-tion of "AgH" by MeCN. Silver hydride 19,20 is unstable and thermodynamically disfavored relative to the elements. [21][22][23] In this respect, reaction 3 would be analogous to the elimination of H 2 upon protonation of 1 in MeCN, to form the same final product 3. 16 The two possibilities are highlighted in Scheme 2.

The available data do not distinguish these two possibilities. The potential for the reduction of Ag + in MeCN (E p,c ) -0.24 V vs Fc/Fc + ) would appear insufficient to carry out the oxidation of compound 1 acetonitrile (E p,a ) +0.19 V vs Fc/Fc + ). 16 However, the irreversibility of both these electrochemical processes does not allow the feasibility of the electron transfer to be assessed from the electrochemical data. In addition, the redox properties of both the Mo and Ag center could be substantially modified by the hydride-bridging interaction in complex 4. Finally, a slightly uphill electrochemical step could still occur if followed by an irreversible chemical process, as would be the case here.

The absence of a stereochemical control on product 3 also does not permit a distinction between the two mechanisms discussed above. Complex 3 is observed by 1 H NMR as a mixture of the cis and trans isomers at thermodynamic equilibrium (cis:trans ) 10). An equilibrium mixture has also been obtained by outersphere one-electron oxidation of 1 and by protonation with HBF 4 , 16 while the reaction with Ph 3 C + generates only the less stable trans isomer, presumably because of the steric control on the Ph 3 C + attack. 16,24

δ ) C Mo -[4] C Mo δ 1 + [4] C Mo δ 4 (2a) K ) [4] (C Mo -[4])(C Ag -[4]) (2b) [CpMo(CO) 2 (PMe 3 )(µ-H)Ag(MeCN) x ] + 4 f [CpMo(MeCN)(CO) 2 (PMe 3 )] + 3
+ 1 / 2 H 2 + Ag + (x -1)MeCN (3) Scheme 2
isomeric ratio obtained for the Ag + is consistent with either an electron transfer as in the ferrocenium reaction or the elimination of a "AgH" unit, in which case Ag + would be sterically nondiscriminating like the proton and unlike the Ph 3 C + reagent.

(c) Interaction of CpMoH(CO) 2 (PMe 3 ) with Ag + in THF: Synthesis of {[CpMo(CO) 2 (PMe 3 )(µ-H)] 2 Ag} + BF 4 -, 5. In an attempt to isolate a silver adduct such as 4, we have carried out the interaction between 1 and Ag + in the less coordinating and less dielectric solvent THF. The reaction leads to the precipitation of a 2:1 adduct, {[CpMo(CO) 2 (PMe 3 )(µ-H)] 2 -Ag} + BF 4 -, 5, even when it is conducted in a different stoichiometric ratio (see eq 4). The reason for the formation of a 2:1 adduct, rather than a 1:1 adduct such as the product formed in MeCN (vide supra), may be attributed to the relative solubility of the different adducts and/or to the relative coordinating properties of the solvents and the [Mo]-H moiety toward the silver ion. In other words, while the Mo-H moiety in 1 coordinates Ag + better than THF, affording the {Ag-(H-[Mo]) 2 } + product, the MeCN solvent coordinates even better, giving at the most a {Ag(H-[Mo])(MeC-N) x } + product, in equilibrium with [Ag(MeCN) y ] + . The product 5 can be recovered in a 73% yield as a white microcrystalline solid, which darkens upon exposure to air and/or light. Its solubility is very slight in THF, but it dissolves well in acetonitrile and A Nujol IR spectrum of the solid displays two carbonyl stretching vibrations at 1945 and 1835 cm -1 . These two bands are blue-and red-shifted, respectively, from those of the starting material (cis-1, 1947 and 1864 cm -1 ; trans-1, 1938 and 1864 cm -1 ), 17 suggesting that the interaction between 1 and the silver center is not a simple electrostatic Mo-H'''Ag + interaction. The same phenomenon was observed for the related trimetallic heteronuclear cluster [CpMo(CO) 2 (PMe 3 )(AuPPh 3 ) 2 ] + . 18 Dissolution of 5 in acetonitrile produces solutions that have the same NMR and IR properties of solutions obtained by adding 0.5 equiv of Ag + to compound 1, namely, equilibrium solutions of compounds 1 and 4 and residual Ag + (eq 1). The IR spectrum, in particular (see Experimental Section), shows that the main components of the solution are complexes 1 and 4, rather than the intact unit 5; see eq 6. Further addition of Ag + to this solution induces the conversion of 1 into 4, as expected. This result indicates, once again, that the MeCN solvent is a better ligand than THF for the silver ion and competes more effectively against coordination of a second unit of compound 1 to the Ag center. This result further indicates that the proposed associative mechanism of silver scrambling (Scheme 1a) is plausible.

A CD 3 CN solution of 5 evolved over time as expected, given the above-described behavior of complex 4 (eq 3). A 1:1 mixture of compounds 1 and 3, plus H 2 and metallic silver, was eventually obtained.

Discussion

Three pathways could be followed in principle for the reaction between a transition metal hydride complex and an electrophilic reagent (Scheme 3): addition to the metal (path a), addition to the hydride ligand (path b), and electron transfer (path c). However, all pathways could potentially derive from the same electrophilic addition to the hydride ligand, subsequently evolving along different directions (paths b.i-iii). There are precedents for all these mechanisms. For certain protonation reactions, it has been shown that the kinetic position of attack is the hydride ligand to afford a nonclassical dihydride intermediate, 25 which evolves either to a classical dihydride product (path b.i) 25 or to a solvent adduct with elimination of H 2 (path b.ii). 16,18 A rare example of path b.iii is the silver oxidation of Cp 2 WH 2 , which allows the identification of the electrophilic addition intermediate {[Cp 2 WH 2 ] 2 Ag} + . 12 We were considering that complexes CpMoH(CO) n -(PMe 3 ) 3-n (n ) 2, 1; n ) 0, 2) could allow us to witness a change of reactivity, because the electronic properties are profoundly influenced by the CO/PMe 3 substitution. This is suggested by previous protonation studies. While the addition of H + to compound 1 affords an unstable [CpMo(H 2 )(CO) 2 (PMe 3 )] + complex which immediately expels H 2 , 16 the protonation of the electron richer 2 affords the stable classical dihydride complex [CpMoH 2 (PMe 3 ) 3 ] + . 1,26 The latter may be occurring via an (unobserved) nonclassical intermediate according to path b.

Concerning the interaction with the silver electrophile, the more easily oxidizable 2 (examined in part 1 of this series) leads directly to the electron-transfer products [2] + and Ag 0 without detection of an electrophilic addition intermediate. 1 The reaction between Ag + and the less easily oxidizable 1 reported in this contribution, on the other hand, permits the observation and

{

[CpMo(CO) 2 (PMe 3 )(µ-H)] 2 Ag} + 5 9' MeCN CpMoH(CO) 2 (PMe 3 ) 1 + [CpMo(CO) 2 (PMe 3 )(µ-H)Ag(MeCN) x ] + 4 (6)
isolation of electrophilic addition products, making a case in favor of the intermediacy of a hydride-bridged MoAg complex during the oxidation of 2. The Ag + adduct obtained from 1 in MeCN (complex 4) slowly decomposes to the expected products of electron transfer and thus qualifies as an intermediate of an inner-sphere electron-transfer process.

Unfortunately, the electron-transfer product [1] + is too unstable and could not be observed during the decomposition of 4. Rather, in addition to Ag, the final products are H 2 and complex 3. These are indeed the same products obtained when [1] + is generated by other methods (e.g., anodic oxidation of 1). However, another pathway (elimination of the unstable silver hydride, followed by its decomposition) would in principle lead to the same observed products (see Results section and Scheme 2).

Conclusions

The reaction between the electrophilic/oxidizing agent Ag + with complex 1 follows the same pattern already reported for the interaction of 1 with a series of electrophiles (H + , Ph 3 C + , AuPPh 3 + ). 16,18 Specifically, the first stage of the interaction leads to an adduct between the substrate and the electrophile through a bridging hydride bond (Scheme 3, path b). However, the stability of the addition product varies in the order E + ) H + , Ph 3 C + , Ag(MeCN) x + < AuPPh 3 + . The decomposition of the silver addition product (complex 4) could involve either a slow intramolecular electron transfer or the elimination of "AgH". The nature of the addition product under different conditions indicates a coordinating ability toward Ag + in the order THF < 1 < MeCN.

The comparison between the results reported in this contribution and those described in part 1 confirms the expected correlation between metal electron density and reactivity toward Ag + . With the electron poorer complex 1, the silver ion can form an electrophilic addition product, whereas the electron richer complex 2 leads directly to the oxidation product. While falling short of representing a new clear-cut example of an innersphere electron transfer, the results suggest that an inner-sphere pathway for electron transfer by Ag + is probably occurring. In other words, the silver oxidation of 2 described in part 1 would occur via the formation of an (unobserved) electrophilic addition intermediate, while the decomposition of the addition of Ag + to 1, namely, complex 4, would take place via electron transfer rather than via hydride abstraction.

1 H

 1 NMR (CD3CN, δ): 5.33 (s, 5H, Cp), 1.61 (d, JHP ) 10 Hz, 9H, PMe3), -7.01 (d, JHP ) 28 Hz, 1H, Mo-H). 31 P{ 1 H} NMR (CD3CN, δ): 20 (s, PMe3). 31 P{ 1 H-selec} NMR (CD3CN, δ): 20 (d, JHP ) 28 Hz, PMe3). IR (CD3CN, cm -1 ): 1947, 1926, 1860 (sh), 1841. The first and third bands are due to compound 4, while the second and fourth bands are due to compound 1. The yellow solutions of 5 slowly turn darker, and a dark precipitate forms. The formation of new resonances attribuable to H2 (at 4.5 ppm) and [CpMo(CO)2(PMe3)(MeCN)]-BF4, 3 (see previous section), is observed by 1 H NMR. During this decomposition, the original resonances attributed to 5 do not disappear but rather continuously shift toward the position of the resonances of 1. At the end of the transformation, the solution contains a 1:1 mixture of 1 and 3, traces of H2, and a gray precipitate.

  Fitting the NMR data to the simultaneous eqs 2a and 2b, where δ 1 and δ 4 are respectively the hydride chemical shifts for the pure compounds 1 and 4, C Mo is the analytical concentration of Mo complexes (C Mo ) [1] + [4]), C Ag is the analytical concentration of added silver (C Ag ) [Ag + ] + [4]), and K is the equilibrium constant of eq 1, gives K ) 9 ( 1 and δ 4 ) -8.4 ( 0.8 ppm. (18) Galassi, R.; Poli, R.; Quadrelli, E. A.; Fettinger, J. C. Inorg. Chem. 1997, 36, 3001-3007.

Figure 1 .

 1 Figure 1. Stack plot of the 1 H NMR spectra of the mixture of 1 and AgBF 4 at different [Ag + ]/[Mo] ratios. Each plot represents an addition of 1/3 units in the ratio, starting from a silver-free solution.

Figure 2 .

 2 Figure 2. Plots of (a) δ (ppm) and (b) J HP vs the [Ag + ]/[Mo] ratio, as calculated from the spectra reported in Figure1(errors on δ < 0.1 ppm, errors on J < 0.2 Hz). In (a) the solid straight lines interpolate the experimental points in the two linear regions.

2CpMoH(CO) 2 ( 1 +

 21 PMe 3 ) AgBF 4 f {[CpMo(CO) 2 (PMe 3 )(µ-H)] 2 Ag} + BF 4 -5

  Thus, the

	Scheme 1

(19) Gmelin Handbook of Inorganic Chemistry; Springer-Verlag: Berlin, 1990; Vol. Ag Suppl. Vol. 1B, p 61. (20) Pietsch, E. Z. Elektrochem. 1933, 39, 577-585. (21) Eaborn, C. J. Chem. Soc. 1955, 2517-2519. (22) Halpern, J.; Czapski, G.; Jortner, J.; Stein, G. Nature 1960, 186, 629-630. (23) Siegel, B.; Libowitz In Metal Hydrides; Mueller, W. M., Ed.; Academic Press: New York, 1968; pp 545-653. (24) Smith, K.-T.; Tilset, M. J. Organomet. Chem. 1992, 431, 55-64.

(17) Kalck, P.; Pince, R.; Poilblanc, R.; Roussel, J. J. Organomet. Chem. 1970, 24, 445-452.

Acknowledgment. We are grateful to the DOE (Grant DEFG059ER14230) for support of this work and Rossana Galassi for preliminary studies. We also wish to thank Dr. Jordan Adelson and Mr. Robert Reuter for assistance with the AA analyses. Scheme 3