Miguel Baya

Jennifer Houghton

Jean-Claude Daran

Rinaldo Poli
email: poli@lcc-toulouse.fr

Louise Male

Alberto Albinati

Matthias Guttman

Synthesis, structure and electrochemical properties of sterically protected molybdenum trihydride redox pairs: A paramagnetic "stretched" dihydrogen complex?

Keywords: Molybdenum, bulky cyclopentadienyl ligands, hydride ligands, paramagnetic hydride complexes, oxidatively induced reductive elimination, neutron diffraction

Complexes Cp # Mo(PMe3)2H3 (Cp # = 1,2,4-C5H2tBu3, 2a; C5HiPr4, 2b) have been synthesized from the corresponding compounds Cp # MoCl4 (1a, 1b) and fully characterized, including by X-ray crystallography and by a neutron diffraction study for 2a. Protonation of

THF and to [(1,2,4-C5H2 t Bu3)Mo(PMe3)2(MeCN)H2] + (4a) in MeCN. Complex 4b analogously derives from protonation of 2b in MeCN, whereas the tetrahydride complex 3b is unstable. One-electron oxidation of 2a and 2b by [Cp2Fe]PF6 produces the EPR active 17-electron complexes [2a] + and [2b] + . The former is thermally more stable than the latter and could be crystallographically characterized as the PF6 -salt by X-ray diffraction, providing evidence for the presence of a stretched dihydrogen ligand (H … H = 1.36(6) Å). Controlled thermal decomposition of [2a] + yielded the product of H2 elimination, the 15-electron monohydride complex [(1,2,4-C5H2tBu3)Mo(PMe3)2H]PF6 (5a) which was characterized by X-ray crystallography and by EPR spectroscopy at liquid He temperature. The compound establishes an equilibrium with the solvent adduct in THF. An electrochemical study by cyclic voltammetry provides further evidence for a rapid H2 elimination process from the 17electron complexes. Contrary to the previously investigated [Cp*Mo(dppe)H3] + system, the decomposition of [2a] + by H2 substitution with a solvent molecule appears to follow a dissociative pathway in MeCN.

Introduction

Hydride complexes have paramount importance in light of their implication in a variety of catalytic processes and as models of a number of biological functions such as hydrogenase and nitrogenase. [1][2][START_REF] Peruzzini | Recent Advances in Hydride Chemistry[END_REF][START_REF] Igarashi | [END_REF][5][6] Because of the strong covalent nature of the M-H  bond and the absence of additional orbital interactions (i.e. of  type), they are usually stable only in a closed-shell configuration. Open-shell versions are reactive, which is the very reason for their involvement as catalytic intermediates. An interesting subclass of open-shell hydride complexes are those with an odd-electron (mostly 17-electron) configuration, characterized by paramagnetism. These complexes have generally been accessed by one-electron oxidation of stable diamagnetic precursors. In most cases, they decompose by deprotonation, [7] disproportionation, [8] dihydrogen reductive elimination (for complexes containing at least two hydride ligands), [9] atom transfer, [10] and other pathways. [START_REF] Poli | Recent Advances in Hydride Chemistry[END_REF] This multitude of available reaction pathways complicates their potential application, for instance in electrocatalysis. It is therefore useful to investigate in greater detail how the various pathways depend on the reaction conditions (e.g. solvent, available substrates) and molecular parameters (e.g. stereoelectronic properties of the ligand environment). For this purpose, it is necessary to develop more stable systems. We have learned from previous investigations [9,[START_REF] Pleune | [END_REF][13][14][15][16] that all decomposition pathways are disfavored by both a stronger electron donating and more sterically protecting ligand environment.

The oxidation of Cp*Mo(dppe)H3 (dppe = Ph2PCH2CH2PPh2) was studied in the greatest detail. [9,[START_REF] Pleune | [END_REF]16] It leads to the paramagnetic complex [Cp*Mo(dppe)H3] + , which is stable at low temperatures and was characterized in situ by EPR spectroscopy. The detailed investigation of its decomposition at room temperature enabled us to quantify the relative rates of deprotonation (by the residual neutral precursor), disproportionation, and H2 elimination in various solvents. [9,16] This was the first reported example where H2 oxidatively induced reductive elimination could be unambiguously demonstrated and distinguished from other decomposition pathways. Oxidation of a polyhydride complex {MHn} is expected to favor its rearrangement to a nonclassical isomer, {MHn-2(H2)} + , [17,18] but the multitude of decomposition pathways, all possibly leading to H2 evolution, [19] have previously made the identification of the H2 elimination pathway uncertain. [17,20] For this specific trihydrido molybdenum complex all three decomposition pathways were shown to occur via the nonclassical intermediate [Cp*Mo(dppe)H(H2)] + , although theoretical calculations and circumstantial evidence indicates that the oxidized complex adopts a classical structure. Since the nonclassical tautomer is energetically less accessible for the related tungsten system, complex [Cp*W(dppe)H3] + turned out to be sufficiently stable to be isolated and crystallographically characterized. [9] In this contribution, we report the synthesis and investigations into new molybdenum systems, isoelectronic with Cp*Mo(dppe)H3, that contain an even more strongly donating and sterically encumbering coordination sphere.

We used the two highly substituted was also structurally characterized. Some aspects of this investigation have been recently communicated. [21] While those preliminary results will be reported again here in fuller details, stronger emphasis will be placed on complementary investigations that have not previously been described. Adaptation of Schrock's Cp*MoCl4 synthetic procedure [22,[START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] to the bulkier Cp # analogues (Cp # = 1,2,4-C5H2tBu3, a, and C5HiPr4, b) yielded the corresponding Cp # MoCl4 derivatives 1a and 1b in good yields, see Scheme 1. Subsequent reaction of these compounds with LiAlH4 in the presence of ≥ 2 equivalents of PMe3, followed by methanolysis and crystallization from ether, yielded the corresponding trihydride derivatives, Cp # Mo(PMe3)2H3, 2a and 2b. It is interesting to compare these results with that previously reported for the related Cp* system, which led to a mixture of Cp*Mo(PMe3)2H3 and Cp*Mo(PMe3)3H. [START_REF] Shin | [END_REF] The bulkier Cp # systems afford the trihydride derivatives 2 selectively and show no tendency to replace H2 in the presence of excess PMe3 under thermolytic conditions.

Results and Discussion

MeOH

Scheme 1

Both compounds gave single crystals suitable for a structural analysis. The crystal of 2a had sufficient quality to allow the location and refinement of the hydride positions from the X-ray data, as shown in the previous communication. [21] We have now completed the structural investigation with a neutron diffraction experiment for 2a (the results of both refinements are compared in Table 1) and an X-ray diffraction experiment for 2b. The latter crystals had poorer quality and the hydride positions could not be located; the observable structural parameters are very close to those of 2a, see Table 1. A view of both geometries is shown in Figure 1. The molecular geometry is unusual for a half sandwich (ring)MoX3L2

compound, but parallels that previously reported for the related Cp*MoH3(dppe)

compound. [25,26] This demonstrates that the unusual structure observed for Cp*MoH3(dppe)

is not enforced by the chelating nature of the dppe ligand. Another example for this structural type has recently been reported for the isoelectronic compound Cp*W(CO)2H2(SiH2Ph). [27] As expected, the X-ray diffraction experiment yields shorter Mo-H distances than the neutron diffraction experiment for compound 2a. Only those afforded by neutron diffraction should be considered reliable. The parameters that do not involve hydrogen atom positions, on the other hand, are more precisely determined by the X-ray diffraction experiment.

The most interesting structural feature is the distance between atoms H2 and H3

[1.69(2) Å from the neutron structure]. This value is too long to envisage an interaction, but short in comparison to most nonbonded distances recorded by neutron diffraction for polyhydride complexes. This is evidence for a certain degree of "compression". [START_REF] Kubas | Metal Dihydrogen and -Bond Complexes[END_REF] Other relatively short nonbonded H•••H separations are 1.67(1) and 1.70(1) Å in [CpIr(PMe3)H3] + , [START_REF] Heinekey | [END_REF] and values ranging from 1.637(4) to 1.668(4) Å between adjacent H atoms in compound Os(PiPr2Ph)2H6, [30] whereas there are no shorter intramolecular H•••H separations than 1.77 Å in compound Re(dppe)H7. [31] The longest H-H separations in compounds that have been defined as "stretched H2 complexes" are 1.357(7) Å in ReH7(P(C6H4-p-CH3)3)2 [32] and 1.34 (2) in [Os(H2)(en)2(O2CMe)] + PF6 -, [33] all these values resulting again from neutron diffraction experiments. Finally, complex OsH5(PMe2Ph)3 + exhibits an even longer separation (1.49(4) Å) and was described as falling in a "gray" region where there may or may not be a direct H/H attractive interaction. [34] The Cambridge Crystallographic Structural Database does not reveal H-H contacts between 1.0 and 1.7 Å for Mo complexes, but a distance of ca. 1.18 Å has recently been estimated from T1 and JHD measurements for complex [Mo(NPh)(PMe3)2(H2)(o-(Me3SiN)2C6H4)]. [35] Interestingly, if the H2 and H3 atoms are considered as defining a stretched H2 ligand, therefore occupying a single coordination position in a Mo II complex, then the coordination geometry would be described as a "four-legged piano stool", which is indeed a quite common geometrical arrangement for Mo II , [36] as exemplified by CpMo(PMe2Ph)3Cl [37] and CpMo(dppe)(CO)H. [38] <Table 1> The NMR properties of 2a and 2b confirm the presence of three hydride ligands. The single 31 P{ 1 H} resonance observed at room temperature is transformed into a binomial quartet in a 31 P{sel. 1 H} NMR experiment, showing that the P nuclei are coupled to three equivalent protons. In addition, the 1 H spectrum shows a single, sharp triplet resonance. A rapid exchange process between the inequivalent hydride positions must therefore be present. For the related Cp*Mo(dppe)H3 compound, the fluxional process could not be frozen out even at the lowest attainable temperatures. [26] For compounds 2a and 2b, on the other hand, cooling results in decoalescence for the 1 behaviour can be rationalized by either the freezing out of a single Mo-(C5HiPr4) rotamer with symmetry inequivalent PMe3 donors (but these ligands also need to be characterized by a small enough P-P coupling since this is not observed at 193 K), or by the presence of two equilibrating rotamers having similar energy, perhaps differing by the relative conformation of the iPr groups in the C5HiPr4 ring. The latter phenomenon would not be expected to lead to the existence of different rotamers for the 1,2,4-C5H2tBu3 substituted complex.

(b) Protonation studies

Compound 2a reacts with HBF4 at -80°C in THF to yield the tetrahydride complex

[(1,2,4-C5H2tBu3)MoH4(PMe3)2] + BF4 -, 3a, see Scheme 2.
This product is related to the previously described [Cp*Mo(dppe)H4] + , [26] but shows a much greater thermal stability.

Scheme 2

The cation geometry is illustrated in Figure 3. The crystal quality allowed the location and refinement of all four hydride ligands. The geometry of the tetrahydride cation can be described as a highly distorted pentagonal bipyramid, when the bulky (1,2,4-C5H2tBu3) ligand is considered to occupy a single coordination position at one of the vertices of the bipyramid.

The second axial position is occupied by the hydride ligand H4, with the 5 equatorial ligands bent toward H4 and away from the cyclopentadienyl ligand (especially the two phosphine ligands, for steric reasons). The two PMe3 ligands occupy 1,3 positions in the pseudopentagonal plane, with atom H1 bisecting the P1-Mo-P2 angle. The related complex [Cp*W(dppe)H4] + was found to adopt an analogous coordination geometry, with the chelating dppe ligand occupying one equatorial and one axial site. [26] Figure 3. ORTEP view of the cation in compound 3a. Hydrogen atoms, except those directly bonded to the Mo atom, are not shown for clarity.

The cationic tetrahydride complex 3a was also formed by protonation of 2a with HBF4

in MeCN, but further rapid evolution took place in this case. This reaction was only carried out on a spectroscopic scale in CD3CN and the resulting solution was monitored with time by

I R n R n = 1,2,4-tBu 3 , iPr 4
The corresponding reaction of 2b, when carried out in THF at -80°C, led to the precipitation of a white solid, presumably corresponding to the [(C5HiPr4)Mo(PMe3)2H4] + complex, 3b. However, this compound decomposes, even in the solid state, when warmed up to room temperature. Therefore, it could not be spectroscopically characterized. When the same protonation reaction was carried out in acetonitrile, the formation of complex [(C5HiPr4)Mo(PMe3)2(MeCN)H2] + , 4b, could be observed. As with 4a, the hydride ligands in complex 4b are involved in a fluxional process at room temperature, appearing as a very broad resonance at -2.5 ppm. On cooling to 200 K in acetone-d6, two doublet of triplet resonances decoalesce, at -0.13 and -5.27 ppm respectively, confirming that each hydride ligand couples to two equivalent phosphorus nuclei and to the other hydride ligand, in agreement with structure I. At this temperature, all four isopropyl groups on the cyclopentadienyl moiety also become inequivalent. In both complexes 4, a resonance integrating to 3 protons, assigned to the coordinated MeCN, can be observed at  2.17 (for 4a in CD3CN) and 2.71 (for 4b in acetone-d6). This behaviour is analogous to that of the related Cp*Mo(dppe)H3 complex. [26] The greater coordinating ability of the acetonitrile solvent induces a more facile H2 elimination by more efficiently trapping the resulting 16-electron fragment.

(c) Oxidation studies: isolation and characterization

Preliminary electrochemical investigations indicated that both complexes 2a,b undergo an electrochemically reversible one-electron oxidation process and suggested that the oxidation product is relatively stable. The electrochemical behavior will be analysed in more detail later in section (e). The stoichiometric oxidation was accomplished by the use of Cp2Fe + PF6 -in THF, see Scheme 3. The product for the 1,2,4-C5H2tBu3 system was sufficiently stable to be isolated and crystallized. Its X-ray structure and its EPR spectrum (Figure 4) demonstrate its chemical identity as the PF6 -salt of the one-electron oxidation product, [2a] + , as discussed previously. [21] Cp The most relevant metric parameters of the [2a] + structure are compared with those of the parent compound 2a in Table 1. The overall geometry of the cation is essentially unchanged relative to that of the neutral precursor (see the previous communication for an ORTEP view). [21] There is no evidence of an interaction between the two ions, notably hydrogen bonding between the hydride ligands and the fluorine atoms of the anion. The Mo-CNT distance is slightly shorter, whereas the Mo-P distances is slightly longer, relative to the neutral precursor. The most notable change is a decrease of the H2 … H3 contact from 1.69(2)

# Mo(PMe 3 ) 2 H 3 [Cp 2 Fe] + PF 6 - [Cp # Mo(PMe 3 ) 2 H 3 ] + PF 6 - Cp # = 1,2,4-C 5 H 2 tBu 3 C 5 HiPr 4 ([2a] + PF 6 -) ([2b] + PF 6 -) Scheme 3 50 G 50 G 10 G 10 G (exp) (simul) (exp) (simul) (simul) [2a] + [2b] + LW = 2.07 G LW = 1.4 G
Å (neutron diffraction) or 1.63(4) Å (X-ray diffraction) in the neutral complex to an average of 1.36(6) Å (from the X-ray data) in the cation. The difference is significant at the 2 level relative to the X-ray structure, at the 4 level relative to the neutron structure. Unfortunately, suitable crystals of compound [2a] + PF6 -for a neutron diffraction analysis could not be grown.

Even when keeping the uncertainty into account, however, the H2 … H3 separation in the cationic complex falls inside the range of compounds that have previously been described as "stretched" or "elongated" dihydrogen complexes, or alternatively as "compressed"

dihydrides. [START_REF] Kubas | Metal Dihydrogen and -Bond Complexes[END_REF]39] Thus, it appears that the oxidation process has increased the interaction between the two hydride ligands H2 and H3.

Compound [2a] + PF6 -appears to be the first reported paramagnetic polyhydride complex showing evidence for a stretched dihydrogen ligand (or compressed MH2 system). It is interesting to compare this structure with that of the previously published isoelectronic [Cp*W(dppe)H3] + complex. The two systems show a very different arrangement of the three hydride ligands, the closest H•••H contact in the tungsten complex being 2.11 Å. [9] Although the H positions in this tungsten complex should again be considered with caution since this structure was also obtained from X-ray diffraction data, the relative arrangement of the heavy atoms that define the coordination sphere (the Cp ring, the metal, and the P donor atoms) is quite different in the two compounds. As we know, the W complex is stable and has no tendency to decompose, notably by H2 elimination.

The oxidation product obtained from 2b is rather short-lived at room temperature and could not be isolated. It was only characterized in situ by EPR spectroscopy, see Figure 4.

Like [2a] + , it exhibits a rather broad spectrum at ambient temperature, which become sufficiently resolved at lower temperatures to allow the identification of the expected quartet of triplet feature, consistent with the presence of two phosphorus donor atoms and three hydride ligands, flanked by the 95 Mo and 98 Mo isotope satellites. This suggests, like for the 1,2,4-tBu3 analogue, that one-electron oxidation has afforded complex [2b] + .

The spectroscopic properties, however are peculiar in many respects. The simulation on the basis of any combination of spin ½ nuclei different than P2H3 (for instance, 2 P and 2 H) failed to provide a spectrum sufficiently resembling the experimental one. However, the simulation for the P2H3 spin system was not nearly as satisfactory as for the [2a] + homologue.

An unrestricted full parameter optimization resulted in too broad lateral features and a too sharp central one, compared to the experimental spectrum (Figure 4), for a broadening factor of 2.07 G. An artificial reduction of the line broadening parameter to 1.4 G allows a perfect match of the two lateral features of the triplet, but the central feature becomes too sharp relative to the experiment. This may indicate a dynamic exchange process on the EPR time scale. Indeed, virtual triplets displaying artificially broadened central features are commonly found in the NMR spectra of diamagnetic compounds featuring suitable site exchange phenomena, such as the X signal for an ABX system where sites A and B are near the fast exchange limit. [START_REF] Sandström | Dynamic NMR Spectroscopy[END_REF][START_REF] Binsch | [END_REF] A rapid hydride scrambling process takes place for [2a] + (symmetric EPR coupling pattern, inequivalent hydride positions by X-ray crystallography), as well as for the neutral precursors (vide supra) and for the related [Cp*Mo(dppe)H3] n+ (n = 0, 1)

complexes. [9] This exchange is possibly slower for the more encumbered (C5HiPr4) derivative, causing the observed lineshape effect. It is interesting to note that the exchange rates are similar for the neutral precursors (slightly smaller for 2a), whereas the exchange process appears faster in 2a + than in 2b + .

The next peculiar feature of the EPR spectrum of [21] and [Cp*Mo(dppe)H3] + (aP = 29.8 G; aH = 11.8 G). [9] The cause of this phenomenon is not quite clear, but the different coupling values suggest that

[2b] + adopts a different geometry relative to that of [2a] + , possibly involving the complete collapse of two hydrides to a dihydrogen ligand. It is easy to imagine how the greater bulk of the substituted cyclopentadienyl ring in [2b] + might force the two H atoms closer together.

This phenomenon may well be related to the slower hydride mutual exchange, as well as to our inability to isolate the compound. Related to this point, we recall that the tetrahydride protonation product is stable in the case of 3a but decomposes in the case of 3b, although the same system 4a,b is obtained in MeCN. Thus, we speculate that the extreme bulk of the C5HiPr4 ligand has the effect of pushing out an H2 ligand from both systems 3b and [2b] + .

(d) Oxidatively induced H2 reductive elimination

Although compound [2a] + PF6 -is quite stable as a crystallized solid and in THF solution at low temperatures, it slowly decomposed at T > 0°C, as indicated by a color change from orange to green. Well formed green crystals were obtained by slow crystallization from THF/pentane at -20°C. This product appeared thermally stable with no noticeable change over time in the solid state and in THF solution at room temperature. X-ray diffraction analysis revealed the identity of the compound as [(1,2,4-C5H2tBu3)MoH(PMe3)2] + PF6 -, 5a.

A view of the structure is presented in our previous communication, [21] while selected bonding parameters are reported in Table 1. Therefore, the compound derives from its precursor [2a] + PF6 -by H2 elimination, see Scheme 4.

[

(1,2,4-C 5 H 2 tBu 3 )Mo(PMe 3 ) 2 H 3 ] + PF 6 - -H 2 [(1,2,4-C 5 H 2 tBu 3 )Mo(PMe 3 ) 2 H] + PF 6 - THF, 0°C

Scheme 4

Although the quality of the data set allowed the identification of a single hydride ligand with a high level of confidence (see Experimental section), the question of the possible presence of additional hydride ligands in the structure of compound 5a has been considered carefully, since hydrogen atoms may be difficult to locate from X-ray diffraction data.

Possibilities include the presence of one, two or three additional hydride ligands, giving a 16electron dihydride, a 17-electron trihydride (a stereoisomer of the precursor [2a] + ), and an 18electron tetrahydride cation (i.e. complex 3a), as well as a dihydrogen ligand, yielding a nonclassical tautomer of [2a] + . The color of 5a relative to [2a] + PF6 -excludes an isomeric form of [2a] + and also the tetrahydride formulation, although the bond distances and angles related to the heavy atoms in the cation of 5a, see Table 1, are not too different from those observed for compound 3a.

The 1 H NMR spectrum of the isolated solid only revealed the resonances of the tetrahydride complex 3a, indicating that this compound is a decomposition by-product (the solid was a mixture of well formed crystals and a powder). Complex 3a certainly arises from the transfer of a proton from acidic [2a] + to residual 2a, similarly to what occurs for the Cp*Mo(dppe)H3 analogue. [16] A solid sample of the isolated compound showed bulk paramagnetism, however a reliable value for the magnetic moment could not be obtained,

given the impure nature of the sample. 1 H NMR monitoring of the decomposition reaction also showed the formation of 3a, in addition to the formation of H2.

Positive identification of the green decomposition product as a 15-electron species comes from EPR spectroscopy. As detailed in the communication, [21] the solid sample shows features consistent with a spin quartet ground state at the liquid He temperature (Figure 5a): gx and gy at 3.74 and 3.45 (±1/2 transition), plus a weak feature (gz for the forbidden ±3/2 transition) at 5.33. The g = 3.74 peak appears to display a fine structure, possibly due to coupling to the two equivalent P nuclei. The gz component of the ±1/2 transition is not visible because it is overshadowed by stronger resonances in the g = 2 region (shown in Figure 5b).

The resonance observed at g = 2.009 for the polycrystalline sample is very close to the position observed at higher temperature for the precursor complex [2a] + and is therefore attributed to a residual amount of this material, which had co-crystallized with the H2 elimination product 5a (see Experimental section). 15-electron, spin quartet monohydride complex and a spin doublet solvent adduct. The compound crystallizes preferentially in the solvent-free form, but the THF adduct also appears to exist in the solid state. Additional evidence for this equilibrium will be provided by the electrochemical analysis (vide infra).

As stated in the Introduction, we previously reported the first unambiguous oxidative induced reductive elimination of H2 from the one-electron oxidation of complex Cp*Mo(dppe)H3. However, the elimination product could not be crystallized and was only characterized in solution as the 17-electron solvent adduct [Cp*Mo(dppe)(solv)H] + by EPR spectroscopy (solv = THF, CH2Cl2) [9] and by electrochemistry (solv = MeCN). [16] The steric bulk of the PMe3 and 1,2,4-C5H2tBu3 ligands, in combination with the electron pairing stabilization provided by the spin quartet state, [42,43] accounts for the absence of solvent coordination to complex 5a. The oxidatively induced reductive elimination of compounds containing two one-electron ligands {M(X)(Y), leading to the elimination of X-Y} has previously been demonstrated for dialkyl complexes {M(R)2} to give the alkane coupling product R-R [18,[44][45][46][47][48][49][50] and for alkyl-hydride complexes {M(R)(H)} to give the corresponding alkane R-H, [18,47] plus products originating from {M} + . To the best of our knowledge, the 15electron {M} + product was not isolated and characterized in any of those studies. For X = Y = H, as stated in the Introduction, oxidation often results in dihydrogen evolution but the multitude of decomposition pathways of the intermediate oxidized polyhydride complexes often obscure the clean identification of the oxidatively induced reductive elimination pathway. Therefore, the present investigation illustrates the first well defined example of an oxidatively induced reductive elimination of H2, through the full characterization of starting and end product of the H2 elimination process.

Since the greater bulk of the C5HiPr4 ring should cause an even more favorable H2 elimination process, the decomposition of the less thermally stable [2b] + would be expected to lead to another 15-electron monohydride species, 5b, analogous to 5a. Upon warming to room temperature, orange solutions of [2b] + change color to blue, but the transformation is accompanied by the development of new EPR signals indicative of other S = ½ species, which replaced the signal of the cationic trihydride complex. Thus, this decomposition is less well behaved than that of [2a] + , which led to an EPR silent solution (at room temperature).

We cannot exclude the presence of species 5b in this solution, but attempts to crystallize one or more of the decomposition products from this solution were unsuccessful.

(e) Electrochemical studies

Both compounds 2 exhibit a reversible one-electron oxidation in both THF and MeCN at the usual scan rates. The measured E1/2 values for 2a and 2b are very similar (2a: -0.93 V in MeCN, -0.89 V in THF; 2b: -0.95 V in MeCN, -0.88 in THF vs. the ferrocene standard).

These potentials are slightly more negative than those measured for the related Cp*Mo(dppe)H3 compound (-0.85 V in MeCN and -0.73 V in THF) [9] in agreement with the greater electron donating power of the coordination sphere. While the process is reversible in THF for scan rates as low as 10 mV s -1 for both compounds, the back reduction wave loses intensity relative to the oxidation wave at slow scan rates in MeCN. A figure is provided in the Supporting Information.

The cyclic voltammetry of compounds 2a and 2b has been investigated in MeCN and THF at variable scan rates and different potential ranges. The observed behavior is closely related to that of complex Cp*Mo(dppe)H3, [9,16] a detailed study of which revealed three simultaneous decomposition pathways for the one-electron oxidation product, and kelim = 2.0(4)•10 -2 s -1 M -1 in MeCN). [9,16] For complexes 2a and 2b, we did not carry out a thorough quantitative study. Rather, we focused only the voltammetric features that could provide additional information about the H2 elimination pathway.

All three decomposition pathways of [Cp*Mo(dppe)H3] + have an associative character:

disproportionation is initiated by solvent coordination to the 17-electron [Cp*Mo(dppe)H(H2)] + isomer; deprotonation needs the involvement of a molecule of neutral, 18-electron Cp*Mo(dppe)H3; and H2 elimination follows initial MeCN coordination. [16] Indeed, the H2 elimination is much slower in THF (kelim = 2.2(2)•10 -5 s -1 M -1 ; measured independently by decay of the EPR signal), [9] with no visible consequence on the cyclic voltammetric behavior in this solvent. Since compounds 2a and 2b have a more crowded coordination sphere than Cp*Mo(dppe)H3, the above three pathways should be slower.

However, whereas both disproportionation and deprotonation pathways demand a rate determining associative step, the H2 elimination may also occur dissociatively, in which case the greater ligand steric pressure should accelerate it. We remind here that compound 5a is the stable product of H2 dissociation from [2a] + , although equilibrium amounts of the solvent adduct are present in THF (see EPR characterization above). This suggests a dissociative H2 elimination process in THF but does not exclude an associative process in MeCN.

A two-scan cyclic voltammogram of 2a at a scan rate of 5 V s -1 is shown in Figure 6.

When the scan is reversed at 0.7 V (vs. the reference Ag/AgCl electron, i.e. 0.27 V vs. [M]H2(MeCN) + . The latter is responsible for the reversible process C/C'. A deeper analysis of these processes and complimentary investigations confirming their assignment were carried out during the previous study. [16] The two anodic waves of peaks B and C accidentally overlap for [M] = (1,2,4-C5H2tBu3)Mo(PMe3)2, whereas they differ only slightly for [M] = Cp*Mo(dppe) (E = 0.14 V). [16] The essentially identical potentials measured for 17-electron

[M]H(MeCN) +• and 18-electron [M]H2(MeCN) + agree with a high covalent nature for the Mo-H bond.

[M]H(H 2 ) +.

[M](S)H(H) 2 [M]H +.

+. +S [M]H 3 [M]H 3 +. +S [M]H 3 2+ [M]H 2 S 2+ [M]H 2 S + [M]H 4 + [M]H 3

Classical

+S -e -

E°B

[M](S)H 2+ -H 2 dissociative associative Scheme 5

The essential point is that peak B is observed even at the highest scan speeds (up to 5 V s -1 ), whereas the same process for system Cp*Mo(dppe)H3 became observable only at v < 0.6 V s -1 in neat MeCN. This illustrates that the H2 elimination process is fast for systems [2a] + and [2b] + , faster than when [M] = Cp*Mo(dppe), thus strongly suggesting that it proceeds dissociatively.

The voltammograms of 2a in THF, see Figure 7, show many similarities but also interesting differences with respect to the behavior of the same compound in MeCN and to that of compound Cp*Mo(dppe)H3 in THF. Following the oxidation at A, the irreversible peak B and the second oxidation process D are observed like in MeCN. Transit over process D does not generate a reversible C/C' couple. Transit over process B, on the other hand, generates a weak and broad reduction peak E at ca. -1.9 V, while peak A loses reversibility.

A full rationalization of all these observations is not possible, as many of the species implicated in Scheme 5 may be unstable and evolve to other unknown products when S = THF. Whereas the potentials of peaks A and D are close to the values observed in MeCN, peak B occurs at Ep,a ca. 0.25 V in THF (vs. -0.18 V in MeCN). The more positive potential in THF may be related to a reduced donating power of THF relative to MeCN and/or to a more extensive solvent dissociation equilibrium established by the THF adduct with the 15electron complex. Indeed, the presence of this equilibrium has been evidenced by the EPR study (see above). The most interesting feature, however, is the fact that compound 2a shows peak B also in THF, whereas compound Cp*Mo(dppe)H3 only shows it in MeCN. [16] This is consistent with a dissociative mechanism for the H2 substitution in [2a] + , in which case the rate should be essentially solvent independent. Indeed, peak B is observable in THF even at the fast scan rates (1 V s -1 ). The behavior of 2b in THF shows similarities to that of 2a (see figure in the Supporting Information). Notably, a peak corresponding to process B is again present (Ep,a = 0.2 V vs. ferrocene). The behavior at higher potential, however, is more complex and not fully understood. It is possible that only a fraction of [2b] + undergoes H2 elimination, in competition with other decomposition processes, in the THF solvent. This seems to be suggested also by the EPR monitoring of this decomposition (vide supra). It seems clear, however, that the first decomposition mode (H2 elimination to afford a 15-electron derivative, possibly in equilibrium with a 17-electron THF adduct) is also established by this system.

Conclusion

We have shown that the combination of greater ligand donor power and greater steric bulk in the coordination sphere of half sandwich Mo IV trihydride complexes stabilizes the 17electron products of one-electron oxidation. Complex [(1,2,4-C5H2tBu3)Mo(PMe3)2H3] + is sufficiently stable to be crystallized and structurally characterized. Its geometry is very close to that of the neutral precursor, except that the separation between two hydride ligands is shorter suggesting the presence of a stretched dihydrogen ligand (or compressed MH2 moiety). To the best of our knowledge, this is the first time that such evidence has been obtained for a paramagnetic polyhydride system. The steric control in this system, however, is rather subtle: on going from the 1,2,4-C5H2tBu system to the more encumbered C5HiPr4 system, the paramagnetic trihydride product becomes less stable and could not be isolated.

The C5HiPr4 ligand might impose such steric pressure to the MoH3 system as to force a stronger interaction between two hydride ligands and a more favorable expulsion of H2. This also appears true for the tetrahydrido protonation product (3a vs. 3b). A peculiar difference between the ground state properties of [2a] + and [2b] + has been evidenced by EPR spectroscopy (Figure 4). The present investigation has also revealed a dissociative pathway for H2 substitution by a solvent molecule in the paramagnetic system. The oxidative behavior of the half-sandwich Mo trihydride system is summarized in Scheme 6: H2 substitution by

MeCN on [Mo]H3 + is associative for [Mo] = Cp*Mo(dppe) and dissociative for Cp # Mo(PMe3)2; an equilibrium has been established by EPR spectroscopy for the latter system between the 15-electron species (complex 5a) and its solvent adduct in THF, whereas no evidence was previously obtained for a 15-electron species in the Cp*Mo(dppe) case. The dissociative product has in fact been isolated and structurally characterized, providing the first well characterized example of an oxidatively induced reductive elimination of H2 from a polyhydride compound.

[M]H(H 2 ) +.

[M](S)H(H) 2 +.

+S

[M]H 3 +.

Classical Non classical

[M](S)H +.

[M]H

Scheme 6

Experimental Section General procedures. All operations were carried out under an atmosphere of argon using standard Schlenk line and glove box techniques. Solvents were dehydrated (CH2Cl2:

CaH2; THF, toluene, sodium benzophenone ketyl) and distilled under dinitrogen prior to use.

Mo(CO)6 and PMe3 (1 M solution in THF) were purchased from Aldrich and used as received. Compounds 1,2,4-tri(tert-butyl)-and tetra(iso-propyl)-cyclopentadiene (as isomer mixtures) were prepared by literature methods [51,52] and converted to their corresponding sodium salts by reaction with NaNH2.

(b) Measurements. NMR measurements were carried out on either a Bruker AC 200 or a Bruker AMX250 spectrometer and calibrated with the residual solvent resonances ( 1 H) or with external 85% H3PO4 ( 31 P). The lineshape analyses for the dynamic processes were carried out by simulation with DNMR3, which is incorporated into the freely available SpinWorks program. [53] EPR spectra were measured on a Elexsys E500 BRUKER spectrometer equipped with both a frequencymeter and gaussmeter. The spectrometer frequency was calibrated with diphenylpicrylhydrazyl (DPPH, g = 2.0037). EPR spectra simulations and fittings were carried out with the freely available WinSim program. [54] Cyclic voltammograms were recorded with an EG&G 362 potentiostat connected to a Macintosh Subsequently, the mixture was heated to reflux for 2 h; following which it was cooled and the solvents evaporated. The residue was then extracted with pentane and the pentane solution evaporated to yield (1,2,4-C5H2tBu3)Mo(CO)3CH3 as a yellow solid. Yield = 3.739 g; 80 %.

The compound was used directly for the synthesis of (C5HiPr4)MoCl4 (see below), without characterisation. [55] and refined by least-squares procedures on F 2 using SHELXL-97. [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] All H atoms attached to carbon were introduced in calculation in idealised positions and treated as riding models. In compound 2a + , there are two cations and anions in the asymmetric unit and all the tBu groups of one of the cations are disordered over two positions. In structure 2a, coordinates and Uiso for the hydrides, were fully refined whereas in 3a, the coordinates of the hydrides were fully refined with an overall isotropic thermal parameter. In 2a + and 5a, the coordinates of the hydrides were fully refined with Uiso= 1.2Ueq[Mo(2a + )] or Uiso=1.5Ueq[Mo(5a)]. The disordered moieties were refined applying the restraints available within SHELXL97. [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] Moreover, some residual electron density was difficult to model and therefore, the SQUEEZE function of PLATON [START_REF] Spek | [END_REF] was used to eliminate the contribution of the electron density in the solvent region from the intensity data, and the solvent-free model was employed for the final refinement. There are four cavities per unit cell and PLATON estimated that each cavity contains 32 electrons which could be attributed to a disordered THF molecules. The data collected for compound 2b were of very low quality and although the structural model is mainly correct, it was not possible to locate any hydride ligand. The drawing of the molecules was realised with the help of ORTEP32. [58] Crystal data and refinement parameters are shown in Table 2.

Synthesis of (

The neutron single-crystal diffraction study was performed using the time-of-flight Laue diffractometer SXD [59] installed at the ISIS pulsed spallation source. SXD uses the white beam Laue technique and a stationary crystal combined with eleven highly pixellated area detectors covering around half a sphere around the sample. Thus, large volumes in reciprocal space can be collected in a single shot. A suitable single crystal of the complex (2a) was fixed to an Aluminum pin with thin strips of adhesive Al tape and mounted on a He closed-cycle refrigerator and cooled slowly to 20K. The space group P21/n was confirmed at 20K. No significant change in the crystal mosaic or splitting of the peak was observed during cooling.

Further crystallographic data and experimental details are given in Table 2 and in the Supporting Information. The unit cell dimensions were precisely calculated, at the end of the data collection, from the positions of 60 reflections per each detector orientation. Data were collected at nine different orientations at 20(1) K for ca. 24 hour per orientation. The range of wavelengths used for the data collection was 0.37 < λ < 8.8 Å, even though the bulk of the diffraction information is obtained from the wavelength range 0.5 < λ < 7.0 Å. Data reduction and a Gaussian absorption correction were performed using the standard SXD procedure implemented in the SXD2001 software [60] resulting in a total of 8686 reflections of which 3203 were unique. The starting structural model for the refinement was based on the atomic co-ordinates for the non hydrogen atoms taken from the X-ray structural determination. The structure was refined by full matrix least squares, minimising the function [w(Fo 2 -

(1/k)Fc 2 ) 2 ] and using all independent data. During the refinement, the difference-Fourier maps clearly showed all H atoms of the ligands and the three hydrides. The final structure model included co-ordinates and anisotropic displacement parameters for all atoms. Upon convergence the final Fourier difference map showed no significant features. The coherent scattering amplitudes used were those tabulated by Rauch and Waschkowski. [START_REF] Rauch | Neutron Data Booklet[END_REF] All calculations were carried out by using the PC version of the programs WINGX, [START_REF] Farrugia | [END_REF] SHELX-97 [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] and ORTEP. [58] Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 631893 -631898. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (+44) 1223-336-033; E-mail: deposit@ccdc.cam.ac.uk).

cyclopentadienyl rings, C5HiPr4 and 1 , 2 , 4 -

 124 C5H2tBu3, in place of Cp* and two PMe3 ligands in place of bidentate dppe. Notable results of this investigation have been the isolation and (PMe3)2H3] + , and the observation of its subsequent H2 elimination process leading to the 15-electron monohydride derivative, [(1,2,4-C5H2tBu3)Mo(PMe3)2H] + , which

Figure 1 .

 1 Figure 1. ORTEP view of compounds 2a (neutron diffraction, left) and 2b (right).Hydrogen atoms, except those directly bonded to the Mo atom in compound 2a, are not shown for clarity.

Figure 2 .

 2 Figure 2. Variable temperature 1 H NMR spectrum of (a) compound 2a and (b) compound 2b in the hydride resonance region. The individual temperatures are shown on each spectrum and the time values shown are the longitudinal relaxation times of the corresponding resonance.

Figure 4 .

 4 Figure 4. EPR spectra of complexes [2] + in THF solution. Above: complex [2a] + (T = 193). Below: complex [2b] + (T = 183).

  [2b] + is a much lower value shown by aP and aH (namely aP = 6.2 G; aH = 1.7 G; aMo = 29.4 G) relative to both [2a] + (aP = 36.2 G, aH = 11.4 G and aMo = 30.8 G)

Figure 5 .

 5 Figure 5. Liquid He EPR spectrum of compound 5a: (a) polycrystalline sample in the g = 4-6 region; (b) polycrystalline sample (solid line) and frozen THF glass (dashed line) in the g = 2 region.

[

  Cp*Mo(dppe)H3] + : deprotonation, disproportionation and H2 elimination. Each pathway could be independently quantified (e.g. kdeprot = 2.8(2)•10 2 s -1 M -1 , kdisp = 3.98(9)•10 3 s -1 M -1

Figure 6 .

 6 Figure 6. Two-scan cyclic voltammograms of compound 2a in MeCN. Scan rate = 5000 mV s -1 .

Figure 7 .

 7 Figure 7. Cyclic voltammograms of compound 2a in THF with different switching potentials. Scan rate = 200 mV s -1 .

  computer through MacLab hardware/software. The electrochemical cell was fitted with an Ag-AgCl reference electrode, a platinum disk working electrode and a platinum wire counterelectrode. [Bu4N]PF6 (ca. 0.1 M) was used as supporting electrolyte. The ferrocene standard had a potential of 0.43 V in MeCN and V in THF under our experimental conditions. Synthesis of (1,2,4-C5H2tBu3)Mo(CO)3CH3. A solution of Na(1,2,4-C5H2tBu3) (1.95 g, 8.3 mmol) in THF (20 mL) was transferred into a suspension of Mo(CO)6 (2.20 g, 8.3 mmol) in THF (15 mL). The mixture was heated to reflux for 15 h, during which time a colour change from pale yellow to dark red was observed. The mixture was then cooled and CH3I (1 mL,16 mmol) was added via syringe, causing an immediate colour change to bright yellow. Subsequently, the mixture was heated to reflux for 2 h; following which it was cooled and the solvents evaporated. The residue was then extracted with pentane (100 mL) and the pentane solution evaporated to yield (1,2,4-C5H2tBu3)Mo(CO)3CH3 as a yellow solid. Yield = 2.642 g; 74 %. IR (CH2Cl2): 2005, 1918 cm -1 (CO).Synthesis of (C5HiPr4)Mo(CO)3CH3.A solution of Na(C5HiPr4) (2.82 g, 11.0 mmol) in THF (25 mL) was transferred into a suspension of Mo(CO)6 (2.88 g, 10.9 mmol) in THF (20 mL). The mixture was heated to reflux for 16 h, during which time a colour change from pale yellow to dark red was observed. The mixture was then cooled and CH3I (1.5 mL, 24 mmol) was added via syringe, causing an immediate colour change to bright yellow.

  H NMR signal to yield two signals in a 1:2 ratio, consistent with the solid state structural investigation, see Figure2. This shows quite clearly the effect of the bulky ring substituents on the dynamics of the hydride exchange process. For

	(a)		710 ms
	273 K
	253 K		506 ms
	233 K		359 ms
	213 K	326 ms	303 ms
	193 K	372 ms		383 ms
	-4.5		-5	-5.5		-6
	compound 2a, the lowest temperature (193 K) spectrum displayed well resolved triplets, due
	to coupling to the two equivalent P donor nuclei, with the unique hydride ligand being
	(b)
	characterized by a greater JHP (64 Hz), relative to the two equivalent ones (44 Hz). No H-H 549 ms
	273	K
	coupling between the inequivalent hydrides can be discerned. For compound 2b, on the other 253 K 378 ms
				293	ms
	233	K
	hand, the spectrum remained rather broad even at 193 K. A lineshape analysis yielded the
			319	ms
	213	K
	activation parameters of the hydride scrambling process as H ‡ = 9.0±0.7 (2a) and 8.5±0.3 193 K 443 ms
	(2b) kcal mol -1 ; S ‡ = 17±3 (2a) and 21±1 (2b) e.u. The similar values of both activation
	-3	-4	-5		-6	-7
	parameters for the two compounds are in line with the similar structure of the two
	compounds. The longitudinal relaxation times (T1) of the various signals are also shown in
	Figure 2. They confirm the classical nature of the compounds. Most notably, for compound
	2a, the T1 value for the equivalent hydride ligands signal is not significantly shorter than that
	of the unique hydride resonance below the decoalescence temperature.

  It displays a singlet 31 P{ 1 H} resonance at 1.85 ppm, which converts into a triplet upon selective decoupling of the PMe3 ligand 1 H resonance. At room temperature a single broad hydride resonance is observed, however on cooling a CD3CN solution to -40 o C, two triplet hydride resonances are resolved at -0.27 and -6.77 ppm respectively. This suggests that the two PMe3 ligands occupy equivalent positions at all temperatures, consistent with the structure shown in I.

		+
	Mo	PMe 3
	Me 3 P MeCN	H
	H

1 H and 31 P NMR spectroscopy. Within minutes at room temperature, resonances corresponding to a second complex began to appear and the conversion was complete in 5 h. This product, [(1,2,4-C5H2 t Bu3)Mo(PMe3)2(MeCN)H2] + , 4a, derives from H2 elimination/MeCN coordination from 3a.

Synthesis of (1,2,4-C5H2tBu3)Mo(PMe3)2H3, 2a. Compound 1a

  . 150 mL) and filtered through Celite 545. The final solution was vacuum-dried, and the residue washed three times with methanol (6, 4 and 4 mL) and dried in vacuo.The product 2a was obtained as a pale yellow solid. Yield: 727 mg (50 %). Anal. Calcd. for C23H50MoP2: C, 57.01; H, 10.40. Found: C, 56.48; H, 10.88.1 H NMR (C6D6):  -5.20 (t, J = The reaction mixture was slowly warmed up to 253 K and then concentrated to ca.1 mL. Addition of cold pentane (253 K, 10 mL) afforded a brown precipitate that was decanted and further washed with cold pentane (253 K, 3x10 mL) and finally vacuum-dried.

	1,2,4-C5H2tBu3)MoCl4, 1a. A solution of PhICl2 (5 g, 38.2 mmol) in
	dichloromethane (20 mL) was transferred slowly into a solution of (1,2,4-C5H2tBu3)Mo(CO)3CH3 (2.64 g, 6.2 mmol) in dichloromethane (15 mL). The mixture was heated to reflux for 3 h during which time a colour change from brown to indigo was observed. Solvents were then concentrated to ca. 3 mL and the resulting suspension was filtered to give a purple which was washed with portions of pentane (5 x 50 mL) and then dried under reduced pressure to give (1,2,4-C5H2tBu3)MoCl4, 1a, as a purple solid. Yield = 2.32 g, 79 %. EPR: g = 1.992, aMo = 38.8 G. Synthesis of (C5HiPr4)MoCl4, 1b. PhICl2 (6.87 g, 52.50 mmol) dissolved in dichloromethane(20 mL) was added dropwise to a solution of (C5HiPr4)MoCO3CH3 (6.85 g, 15.98 mmol) in dichloromethane (20 mL). The solution was heated to reflux for 3 h, during which time a colour change from brown to indigo was observed. Solvents were then concentrated to ca. 4 mL and the resulting suspension was filtered. The solid was washed with portions of pentane (5 x 20 mL) and dried under reduced pressure to give (C5HiPr4)MoCl4, 1b, as an indigo solid. Yield = 5.27 g, 70 %. (1415 mg, 3.00 mmol) was dissolved in tetrahydrofuran (20 mL) and a solution of trimethylphosphine in tetrahydrofuran (1 M, 8 mL, 8 mmol) was added. The mixture was stirred for 30 min, and then a suspension of lithium tetrahydroaluminate (ca. 650 mg) in tetrahydrofuran (40 mL) was carefully added. Gas evolution was observed during the addition. The mixture was stirred for 5 h then methanol (ca. 6 mL) was added dropwise causing vigorous gas evolution. The resulting suspension was stirred for 1 h and vacuum-dried; the residue was then extracted with (s, 2 H, C5H2 t Bu3). 31 P{ 1 H} NMR (C6D6):  17.9 (s). A single crystal for the X-ray analysis was obtained by slow diffusion of a MeOH layer into a pentane solution at 5ºC. Synthesis of (C5HiPr4)Mo(PMe3)2H3, 2b. Compound 1b (890 mg, 1.89 mmol) was dissolved in tetrahydrofuran (20 mL), and a solution of trimethylphosphine in tetrahydrofuran (1 M, 5 mL, 5 mmol) was added. The mixture was stirred for 20 min and a suspension of lithium tetrahydroaluminate (ca. 500 mg) in tetrahydrofuran (40 mL) was carefully added. Gas evolution was observed during the addition. The mixture was stirred for 6 h, after which methanol (ca. 5 mL) was added drop by drop. Vigorous gas evolution was observed at this point. The resulting suspension was stirred for 1 h and then vacuum-dried. The residue was extracted with diethyl ether (ca. 100 mL) and filtered through Celite 545. The final solution was vacuum-dried and the residue washed with portions of methanol (5, 3 and 3 mL) and %). Anal. Calcd. for C23H50MoP2: C, 57.01; H, 10.40. Found: C, 56.91; H, 11.10. 1 H NMR (C6D6):  -5.15 (t, J = 52.9 Hz, 3 H, Mo-H), 1.30 -1.80 (42 H, CH(CH3)2, P(CH3)3), 2.79 (m, 2 H CH(CH3)2), 2.96 (m, 2 H CH(CH3)2), 4.72 (s, 1 H, C5H i Pr4). 31 P{ 1 H} NMR (C6D6):  17.3 (s). A single crystal for the X-ray analysis was obtained by diffusion of a MeCN layer onto a THF solution at room temperature. Synthesis of [(1,2,4-C5H2tBu3)Mo(PMe3)2H3] + PF6 -, 2a + PF6 -. A suspension of [Fe( 5 -C5H5)2]PF6 (32 mg, 0.10 mmol) in tetrahydrofuran (5 mL) was added dropwise to a cold The product 2a + PF6 -was obtained as a pale brown solid. Yield: 48 mg, 70 %. EPR (THF): g = 2.0185, aP = 36.2 G, aH = 11.4 G, aMo = 30.8 G. A single crystal for the X-ray analysis was obtained by diffusion of a pentane layer onto a THF solution at -80°C. Synthesis of [(1,2,4-C5H2tBu3)Mo(PMe3)2H4] + BF4 -, 3a. A solution of ( 5 -C5H2 t Bu3)Mo(PMe3)2H3 (2a, 40 mg, 0.08 mmol) in diethyl ether (4 mL) was cooled to -80°C. HBF4 (54 % solution in diethyl ether, 22μL, 0.16 mmol) was added via syringe. Within minutes, a white precipitate formed. A further portion of diethyl ether (2 mL) was added. 38 (s, 9 H, -t Bu), 1.46 (s, 18 H, 2 x t Bu), 1.78 (br, 18 H, P(CH3)3), 5.08 (s, 2 H, C5H2 t Bu3). A single crystal for the X-ray analysis was obtained by diffusion of a diethyl ether layer onto a THF diethyl ether (ca51.0 Hz, 3 H, Mo-H), 1.37 (s, 9 H, t Bu), 1.49 (br, 18 H, P(CH3)3), 1.58 (s, 18 H, 2 x t Bu), 4.86 dried in vacuo. The product 2b was obtained as an orange-yellow solid. Yield: 449 mg (49 solution at room temperature.

solution (193 K) of compound 2a (53 mg, 0.11 mmol) in tetrahydrofuran (5 mL). The solution color immediately changed from pale yellow to dark blue and, within a few minutes, to orange.

The solvent was then decanted and the solid washed with portions of diethyl ether (5 x 3 mL) and dried under reduced pressure to yield 3a as a white solid. Yield: 39 mg, 85 %.

31

P{ 1 H} NMR (THF-d8):  0.4 (s). 1 H NMR (THF-d8):  -4.20 (t, J = 53.4 Hz, 3 H, Mo-H), 1.

Generation of [(1,2,4-C5H2tBu3)Mo(PMe3)2(MeCN)H2] + BF4 -, 4a.

	[(1,2,4-C5H2tBu3)-Mo(PMe3)2H4] + BF4 -, (3a, 10 mg, 0.018 mmol) was measured into and NMR tube and dissolved in CD3CN. 1 H and 31 P NMR spectra were recorded initially and again after 5 h. At this time, the resonances corresponding to 3a were replaced by a new set of resonances, ascribed to [(1,2,4-C5H2tBu3)Mo(PMe3)2H2MeCN] + BF4 -, 4a. 31 P{ 1 H} NMR (CD3CN):  1.85 (s). 31 P{ 1 H sel. decoupler at 1.61 ppm}  1.88 (t, J = 52.85 Hz) 1 H NMR (CD3CN, 298 K):  1.07 (9H, s, C(CH3)3), 1.42, (18H, s, 2 x C(CH3)3), 1.61 (18H, d, J = 8.7 Hz, P(CH3)3), 2.17 monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (3H, s, CH3CN) 4.59 (m, 2H, C5H2 and 5a or a Stoe IPDS diffractometer for 2a, 2a + . Data were collected using the (SIR97)

t Bu4).

1

H NMR (CD3CN, 233 K):  -6.77 (1H, t, JP-H = 40.0 Hz, Mo-H), -0.27 (1H, t, JP-H = 80 Hz, Mo-H), 1.03 (9H, s, C(CH3)3), 1.38 (18H, s,

Table 1 .

 1 Selected bond distances (Å) and angles (°) for all structurally characterized complexes

		2a		2b	3a	2a +		5a
		X-ray	neutron	X-ray	X-ray	X-ray a	X-ray
	Mo-CNT b	2.0150(2)	2.019(5)	2.013(1)	2.0100(2)	1.9992(4) 2.0036(4)	2.0096(3)
	Mo-P(1)	2.3832(6)	2.385(5)	2.377(4)	2.4656(7)	2.4720(14) 2.4744(15)	2.4801(8)
	Mo-P(2)	2.3801(6)	2.377(5)	2.375(4)	2.4720(7)	2.4738(14) 2.4726(15)	2.4685(9)
	Mo-H(1)	1.58(3)	1.712(8)		1.67(3)	1.65(4)	1.60(4)	1.83(3)
	Mo-H(2)	1.57(3)	1.719(9)		1.58(3)	1.52(4)	1.56(4)
	Mo-H(3)	1.58(3)	1.719(9)		1.61(3)	1.54(4)	1.60(4)
	Mo-H(4)				1.66(3)
	H(2)•••H(3)	1.63(4)	1.69(2)			1.33(6)	1.38(6)

Table 2 .

 2 Crystal data and structure refinement for all structurally characterized complexes

	Identification code	2a (X-ray)	2a (neutron)	2b	3a	2a +	5a
	Empirical formula	C23H50MoP2	C23H50MoP2	C23H50MoP2	C23H51BF4MoP2 C23H50F6MoP3	C23H48F6MoP3
	Formula weight	484.51	484.51	481.49	572.33	629.48	627.46
	Temperature,K	180(2)	20(1)	180(2)	180(2)	180(2)	180(2)
	Wavelength , Å	0.71073	0.37-8.8	0.71073	0.71073	0.71073	0.71073
	Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
	Space group	P 21/n	P 21/n	P 21/c	P 21/c	P 21/c	P 21/c
	a, Å	16.2915(15)	16.238(5)	15.599(4)	9.3986(5)	14.0847(10)	9.6185(5)
	b, Å	17.4655(16)	17.392(5)	10.2114(18)	16.9428(8)	26.216(2)	16.8699(10)
	c, Å	9.2144(8)	9.160(4)	16.939(4)	18.2340(9)	20.1855(15)	18.7489(12)
	,°	90.0	90.0	90.0	90.0	90.0	90.0
	,°	92.651(10)	92.83(1)	103.51(2)	92.719(4)	99.221(8)	90.441(5)
	,°	90.0	90.0	90.0	90.0	90.0	90.0
	V, Å 3	2619.1(4)	2584(2)	2623.5(10)	2900.3(3)	7357.2(10)	3042.2(3)
	Z	4	4	4	4	8	4
	Dcalc, Mg/m 3	1.229	1.246	1.227	1.311	1.269	1.370
	, mm -1	0.629	0.0655+0.0002	0.628	0.597	0.535	0.636
	Crystal size, mm 3	0.27x0.23x0.2	2.2 x 1.6 x 1.4	0.13x0.1x0.1	0.25x0.21x0.13	0.2x0.1x0.1	0.58x0.42x0.24
	°, range	2.49 to 26.18	8.52 to 81.95	3.17 to 23.20	2.76 to 28.22	2.14 to 25.00°	3.25 to 26.37

Acknowledgements

We thank the European Commission through the HYDROCHEM program (contract HPRN-CT-2002-00176) for support of this work. MB thanks the Spanish Ministerio de Educación y Ciencia for a post-doctoral fellowship.

Supporting Information available

Figures showing the structure of 2a from the neutron diffraction experiment, the variable temperature 31

Synopsis

Sterically protecting cyclopentadienyl ligands in combination with PMe3 yield relatively stable paramagnetic half-sandwich trihydride complexes of molybdenum, showing evidence of an incipient H-H interaction. Subsequent H2 elimination yields a stable spin quartet 15electron hydride complex, providing the first well defined example of an oxidatively induced reductive elimination of dihydrogen.

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