Natalia V Belkova

Pavel O Revin

Lina M Epstein

Evgenii V Vorontsov

Vladimir I Bakhmutov

Elena S Shubina

Edmond Collange

Rinaldo Poli
email: poli@u-bourgogne.fr

Kinetics and Mechanism of the Proton Transfer to Cp*Fe(dppe)H: Absence of a Direct Protonation at the Metal Site

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Introduction

Proton transfer processes to and from transition metal centers and hydride ligand sites are key steps in many stoichiometric and catalytic chemical and biochemical processes and have received a great deal of attention over the last two decades. [START_REF] Peruzzini | Recent Advances in Hydride Chemistry[END_REF] While protonation of a metal site affords a new hydride ligand, proton transfer to a hydride ligand site affords a dihydrogen ligand (nonclassical dihydride complex, or -H2 complex), see Scheme 1. A fundamental issue is that of the thermodynamic vs. kinetic acidity, whereby nonclassical hydride complexes (III) are shown to be deprotonated faster than the classical tautomers (VI), even when the latter are stronger acids in the thermodynamic sense. [START_REF] Kristjánsdóttir | Transition Metal Hydrides[END_REF] The reverse process, protonation of a hydride complex, is correspondingly faster at a hydride ligand site relative to the metal lone pair (hydride ligands are, in general, kinetically more basic than transition metal centers). [START_REF] Papish | [END_REF][START_REF] Papish | Recent Advances in Hydride Chemistry[END_REF] Indeed, dihydrogen complexes have often been detected as intermediates along the formation of classical polyhydrides as the thermodynamically stable final products. [START_REF] Parkin | [END_REF][6][7][8][9] Another impetus to this area has been given by the discovery of "nonclassical dihydrogen bonding", i.e. adducts of type I are shown to be intermediates along the proton transfer pathway to and from dihydrogen complexes. The latter may exist as either free complexes, III, or retain the conjugate base in a hydrogen-bonding interaction, II. Systems such as I have also been shown to be intermediates along the pathway to the heterolytic splitting of dihydrogen. [START_REF] Morris | Recent Advances in Hydride Chemistry[END_REF] Depending on the relative strength of the proton donor and acceptor, these can be stable entities in solution, displaying characteristic spectroscopic signatures. [START_REF] Epstein | Recent Advances in Hydride Chemistry[END_REF][START_REF] Epstein | [END_REF] The proton transfer to the metal site can analogously be imagined as proceeding via a hydrogen-bonded intermediate IV, leading to either free (VI) or hydrogenbonded (V) classical hydride product. A number of studies of adducts such as IV are available 13 and the general implication is that they are indeed intermediates in the proton transfer process. However, the interconversion between classical and non-classical tautomers takes place rather easily in many cases and can in principle occur both on the free (III and VI)

or hydrogen-bonded (II and V) complexes, thus opening a second possible pathway for protonation of the metal center (fast protonation of the hydride site followed by isomerization).

Hydrogen-bonded complexes such as IV and V have been located along the pathway of proton transfer to the metal center when no other hydride ligands are present, [14][15][16][17][18][19] but only in few cases these have been proven intermediates of the direct proton transfer to the metal site. [20][21][22][23] Norton has recently shown that the rate of metal protonation in compound CpW(CO)2(PMe3)H is 9.6 times smaller than the rate of hydride protonation. The rate of metal protonation was obtained indirectly from the combination of the measured deprotonation rate of the classical dihydride complex CpW(CO)2(PMe3)H2 + and the measured pKa value. It is quite possible, however, that the deprotonation reaction occurs only after isomerization to the nonclassical intermediate. Thus, the 9.6 factor between these two rates could merely be related to the tautomerization thermodynamics and does not imply the occurrence of a direct metal protonation. More detailed information would only be available from an independent analysis of the tautomerization kinetics and/or equilibria but, unfortunately, the nonclassical tautomer cannot be directly observed for this system.

Previous work by other authors has addressed specific aspects of the kinetics and mechanism of proton transfer to a hydride complex. The work of Basallote et al. mostly concerns the formation of dihydrogen complexes that do not evolve to the classical isomers but rather to H2 substitution products, such as [MH(H2)L4] + (M = Fe, Ru; L = P-donor ligand). 24-27 A single exception is the formation of complexes [CpRuL2(H2)] + (L2 = dppm, dppe and (PPh3)2) which further evolve to the classical isomers, but the kinetics investigation was not extended to this isomerization step. 28 Work by Chinn and Heinekey on the protonation of a variety of CpRuH(L)(L') and related Cp* complexes, 6 and the later extension to related systems by Puerta et al., 29 on the other hand, address the mechanism of the reversible or irreversible conversion of the dihydrogen complexes to the corresponding dihydrides. Although the low-temperature protonation was shown to occur selectively at the hydride site for these systems, the results of those studies can not exclude a competitive protonation of the metal site at ambient temperature.

In order to shine more light onto this basic question, we have decided to carry out detailed investigations on the proton transfer to complex Cp*Fe(dppe)H, prepared several years ago by Hamon et al. 8,9 A selective protonation at the hydride site with formation of the dihydrogen complex as an observable intermediate and the thermodynamic preference for the metal site were established by using the strong acid HBF4. Thus, the protonation at the hydride site and the isomerization can be analyzed independently from the kinetic and thermodynamic points of view at the same temperature. Our study was carried out by using the following proton donors (HA), in order of increasing acid strength: 2-monofluoroethanol (MFE), 2,2,2-trifluoroethanol (TFE), hexafluoroisopropanol (HFIP), perfluoro-tert-butanol (PFTB), and trifluoroacetic acid (TFA). The interaction has been investigated by a combination of NMR, IR and UV-visible spectroscopy at various temperatures in the 200-290 K range. This approach allowed us to study the overall process step by step. The proton transfer and isomerization rates at room temperature have been determined by both classical mixing and rapid mixing (stopped-flow) techniques. This is the first reported study where an independent kinetic analysis is carried out for both the proton transfer and the isomerization steps, and the first study of this kind where proton donors of different acid strength are used and compared.

Experimental Section

All manipulations were carried out under an argon atmosphere by standard Schlenk techniques. The Cp*Fe(dppe)H hydride was synthesized according to the literature. 30

IR and UV-visible investigations

The IR measurements were performed on a "Specord M82" spectrometer (IR) on 0.1-0.15 M (for the (OH) measurements) or 0.02-0.025 M (for the (MH) measurements) hydride solutions in CH2Cl2 (0.12 cm path length) using CaF2 cells. UV measurements were performed on Specord M-40 and Varian Cary 5 spectrophotometers on 0.01 M solutions in CH2Cl2. All IR and UV measurements were carried out by use of a home-modified cryostat (Carl Zeiss Jena) in the 200-290 K temperature range. The cryostat modification allows the transfer of the reagents (premixed either at low or room temperature) directly into the cell under an inert atmosphere and at the desired temperature. The accuracy of the temperature adjustment was 0.5 K. This setup was used both for the variable temperature equilibrium studies and for the kinetics investigations at constant temperature with UV-visible spectroscopic monitoring. An IR study was also attempted in cyclohexane. However, the precipitation of the cationic dihydrogen complex rendered impossible a careful investigation in this solvent.

NMR investigations

The NMR studies were carried out in standard 5 mm-NMR tubes containing solutions of the complexes in CD2Cl2. The 1 H and 31 P NMR data were collected with a Bruker AMX 400 spectrometer operating at 400.13 and 161.98 MHz respectively. The conventional inversion-recovery method (180- -90) was used to determine the variable-temperature longitudinal relaxation time T1. The calculation of the relaxation times was made using the nonlinear three-parameter fitting routine of the spectrometers. In each experiment, the waiting period was 5 times larger than the expected relaxation time and 16-32 variable delays were employed. The duration of the pulses were controlled at every temperature. Low temperature experiments were carried out in the 180-260 K temperature range using a TV-3000 Bruker temperature unit. The accuracy and stability of temperature was ±1 K. All mixings between the alcohols and the hydride complexes were performed at low temperature.

Stopped-flow investigations

The stopped-flow kinetic runs were carried out at 25 °C with a Hitech SF-61-DX2 apparatus coupled to a Hitech diode-array UV-visible spectrophotometer. Given the extreme air-sensitivity of the hydride compound, unacceptable results were obtained at the low concentrations required for work in a suitable absorbance range when using a regular 1 cm cell (ca. 5x10 -4 M). This phenomenon is attributed to oxidation by oxygen diffusion through the instrument transfer lines, as confirmed by the observation of small and irreproducible signal evolutions when shooting the same hydride solution from both sirynges. Switching to a tenfold concentration and to a smaller path-length (1.5 mm) reduced the oxidation problem below acceptable noise levels. Only the data that were collected within the first 50 seconds were analyzed, yielding reproducible results. No oxidation problems were evident, on the other hand, for the experiments carried out by the more traditional long time scale monitoring.

Data analyses were carried out by using the SPECFIT [START_REF] Binstead | Specfit/32[END_REF] global analysis package of programs.

Results and Discussion

a. Interaction with HBF4. Spectral characteristics of the classical and non-classical protonation products.

Half-sandwich Fe hydride complexes are relatively rare. 8,9,[START_REF] Jetz | [END_REF][33][34][35][36][37] Those containing two or more hydride ligands display a preference for the classical Fe(IV) isomer, the Fe(II) dihydrogen isomer being observable only for less donating ligand environments. For the Cp*-dppe derivative, both the dihydrogen complex, [Cp*Fe(dppe)(H2)] + , and the classical dihydride complex, [Cp*Fe(dppe)(H)2] + have been described as the kinetic and thermodynamic protonation products, respectively, resulting from the treatment of the hydride complex with HBF4. 8,9 The faster formation of the dihydrogen complex relative to the dihydride tautomer illustrates that proton transfer to the hydride site is more facile than to the metal center, as is also well established for a number of other complexes. [START_REF] Papish | [END_REF]6,7 Since the spectral characterization of the two tautomeric dihydride cations in solution was previously restricted to NMR, we have reinvestigated the HBF4 protonation by IR and UV-visible techniques, in addition to NMR. A careful reinvestigation of the longitudinal relaxation time as a function of temperature for the starting hydride and protonation products, however, provides useful new information. The 1 H (hydride resonance) and [START_REF] Binstead | Specfit/32[END_REF] P NMR chemical shifts for the three species involved are summarized in Table 1. 8,9 The proton transfer is immediate and quantitative at 200 K in CD2Cl2, as shown by the NMR monitoring, yielding the dihydrogen complex selectively. Subsequent slow warming confirmed the previously reported conversion to the classical dihydride complex at temperatures above 250 K. The classical dihydride product is stable in CD2Cl2 at room temperature for a few minutes.

Longer monitoring (few hours) revealed, however, a previously undetected further evolution with formation of a paramagnetic product, which is shown to correspond to the previously described complex [Cp*Fe(dppe)Cl] + . 30 No equivalent decomposition occurs in THF. This decomposition did not prevent us from studying the details of the proton transfer process in CH2Cl2, which occurs on a much faster time scale. 9 This behavior is not expected because the two complexes have practically the same inertia moment and should therefore exhibit a T1min at approximately the same temperature when working at the same field strength. This discrepancy can only be rationalized by the presence of small amounts of the paramagnetic complex [Cp*Fe(dppe)H] + in the neutral precursor. This complex has been shown to be stable, even at room temperature, 8 which is a relatively rare occurrence for paramagnetic hydrides. [START_REF] Poli | Recent Advances in Hydride Chemistry[END_REF] The two complexes must be involved in a degenerate exchange whose rate increases with temperature, and additional observations made in the presence of fluorinated alcohols (vide infra) confirm this view. It is clear that the concentration of the cationic paramagnetic complex is very small and therefore does not affect the resonance line width, whereas it affects the relaxation times. Its presence does not affect, on the other hand, the T1 time of the dihydrogen complex, [Cp*Fe(dppe)(H2)] + , because a hypothetical exchange between these two complexes would be much slower than the degenerate exchange with Cp*Fe(dppe)H. Unfortunately, the IR spectra (measured in 3000-1500 cm -1 region) do not reveal any absorbance for (H-H) and (MH2) vibration modes. These bands are known to be very weak and often hidden under strong CH vibrations or overtones. [START_REF] Bender | [END_REF] This spectrum remains essentially unchanged upon rising the temperature, until this reaches ca. 250 K. At higher temperatures, conversion to the classical dihydride complex occurs, yielding eventually spectrum (c). A weak band at 1940 cm -1 can clearly be attributed to the cationic dihydride complex, while the higher frequency shoulders of this band are probably overtones of the aryl group vibrations. Such significant high-frequency shift (MH = +100 cm -1 ) of the (MH)

band is typical for transition metal protonation yielding a cationic classical product. 20,40,41 relatively strong and broad metal-ligand charge-transfer band [START_REF] Lever | Inorganic Electronic Spectroscopy[END_REF] with a maximum around 388 nm ( = 2370 L mol -1 cm -1 ), spectrum (a). The dihydrogen complex, spectrum (b), has a much weaker and featureless absorption ( = 640 L mol -1 cm -1 at 388 nm). Evolution to the classical dihydride complex is accompanied by a minor change of the spectrum, which is more accurately determined by the analysis of the stopped-flow kinetic data (vide infra). The dihydride spectrum is given by trace (c) ( = 1093 L mol -1 cm -1 at 388 nm). However, the UV-visible monitoring shows that this product is unstable in dichloromethane at room temperature and decomposes over several hours to afford trace (d), which is characterized by a weak d-d transition with maximum at 466 nm ( = 587 L mol -1 cm -1 ). [START_REF] Lever | Inorganic Electronic Spectroscopy[END_REF] No analogous evolution occurs in THF. This behavior is fully consistent with the results of the NMR investigation described above. The spectral characteristics of the nonclassical and classical hydrides and for the final [Cp*Fe(dppe)Cl] + decomposition product now allow us to study the mechanism of the protonation process in more detail.

b. Interaction with MFE and TFE. Determination of hydrogen bonding enthalpy and basicity factor Ej of the proton accepting site

The interaction of MFE and TFE (on a short time scale, vide infra) with excess Cp*Fe(dppe)H leads to hydrogen bond formation without any complication from proton transfer processes. [START_REF]An NMR investigation also confirmed the absence of any proton transfer at room temperature in the presence of 12 equivalents of MFE[END_REF] This interaction could conveniently be investigated according to well established protocols. [START_REF] Epstein | Recent Advances in Hydride Chemistry[END_REF] The spectral features in the (RFO-H) stretching region (Figure 2)

show the expected stronger interaction with the more fluorinated alcohol, as indicated by the greater  (OH(free)-OH(bonded)). The resulting interaction enthalpies were obtained by Iogansen's empirical correlation (Equation 1) 44 and are reported in Table 3. They show that the hydrogen bonds between complex Cp*Fe(dppe)H and MFE or TFE have medium strength (4-6 kcal/mol). The enthalpy for the interaction with TFE was also determined by van't Hoff's method, which required investigations at variable temperature and furnishes also the interaction entropy. The equilibrium shifts toward the hydrogen bonded adduct upon cooling from 290 to 250 K, see Figure 3, yielding Hº = -5.40.3 kcal mol -1 and Sº = -13.60.9 cal mol -1 K -1 . The enthalpy values obtained by the two different methods agree rather well with each other, confirming once again the applicability of the correlation given by Equation 1.

The equilibrium constant for the hydrogen bond formation is K = 10 (298 K) and 870 (200 K).

It can reasonably be predicted that the more acidic alcohols HFIP and PFTB provide higher equilibrium constants for hydrogen bond formation, leading to the prediction that most of the hydride complex is in the hydrogen-bonded form in the presence of excess alcohol, especially at low temperatures. Use of the empirical relationship in Equation 2 [START_REF] Iogansen | [END_REF] led to the calculation of the Ej basicity factors, see Table 3. As expected, the values obtained for Ej by use of the two different alcohols are essentially identical. This places the Cp*Fe(dppe)H complex amongst the most basic hydrides so far investigated, for example [ReH3( 4 -N(CH2CH2PPh2)3)] (Ej = 1.45) 21 and {(MeC(CH2PPh2)3}Ru(CO)H2 (Ej = 1.39). 46 Note, however, that hydrogen bonding is established with the metal atom in the first case and with the hydride ligand in the second case. Therefore, our next step was the establishment of the hydrogen bonding site for the title Fe hydride complex.

Equation 2

i j P H H E   11    (2)

c. Establishment of the hydrogen bonding site.

It is now well established that hydrogen bonding to a hydride ligand site, also termed "nonclassical or dihydrogen bonding", causes a low-frequency shift of the M-H band, [START_REF] Epstein | Recent Advances in Hydride Chemistry[END_REF] whereas the involvement of the metal lone pair(s) as proton acceptor(s) for hydrogen bonds shifts other M-X stretching vibrations (notably the M-H band) to a higher frequency range. [START_REF] Epstein | [END_REF]20,21 Therefore, we have studied the interaction between the hydride complex and different proton donors by IR spectroscopy in the ν(MH) region. The alcohol addition was first carried out and studied at low temperatures. All alcohols induce the development of a low frequency shift of the ν(MH) band (1836-1828 cm -1 ), which indicates the formation of the dihydrogen-bonded complex Cp*(dppe)FeH•••HORF. The extrapolation of the variable temperature data for hydrogen bonding with TFE (Figure 3) gives a formation constant of 870 at 200K. Thus, essentially all the hydride complex is in the hydrogen-bonded state under these conditions. Consequently, the (MH) band 1930 cm -1 in Figure 4 (trace b) can be unambiguously be attributed to the Fe-H stretching vibration in dihydrogen-bonded complex.

The absence of a high-frequency shoulder shows that no significant amount of hydrogen bond at the metal center, i.e. Cp*(dppe)HFe•••HORF, is present, see Figure 4 In conclusion, the IR study shows the establishment of hydrogen bonding involving the hydride ligand as the first step of the proton transfer leading to the dihydrogen complex. In comparison with previous literature data, [START_REF] Epstein | [END_REF] the rather high Ej value suggests that this hydride can be protonated by fluorinated alcohols such as HFIP and PFTB in CH2Cl2. Indeed, the (MH) band intensities observed in the presence of these alcohols dramatically decrease with a temperature decrease, especially when PFTB is used. This contrasts with the intensity increase in the absence of alcohol, see section a. This change is reversible in the low temperature range and signals a reversible proton transfer process leading to the dihydrogen complex.

d. Interaction with PFTB, HFIP and TFE: NMR investigations. The nature of the alcohol and the alcohol/Fe ratio strongly affect the proton transfer equilibrium position with the nonclassical intermediate and the temperature at which further evolution to the classical product starts to occur (ca. 250 K for PFTB, 270 K for HFIP and room temperature for TFE). There was no evidence for the formation of products other than the classical dihydride complex on a short time scale (few minutes) at room temperature. In particular, there were no resonances that could be assigned to a hypothetical Cp*Fe(dppe)(ORF) product, which may result from dihydrogen evolution. However, a longer NMR monitoring revealed the slow formation (few hours at room temperature) of the same paramagnetic product, [Cp*Fe(dppe)Cl] + , which is observed by using HBF4.

The initial proton transfer to yield the intermediate dihydrogen complex is more shifted toward the right, at constant alcohol/Fe ratio, as the alcohol acidity increases (TFE < HFIP < PFTB) and, for the same alcohol, as the alcohol/Fe ratio increases. For instance, the FeH/Fe(H2) + ratio is approximately 45:55 when using PFTB/Fe = 1 , 75:25 when using HFIP/Fe = 3, and 40:60 at TFE/Fe = 10 (220 K).

The starting hydride resonance (mixture of starting hydride in fast equilibrium with the hydrogen-bonded adducts) broadens significantly as the temperature increases (see Table 4), whereas the resonance of the dihydrogen complex maintains approximately the same width (ca. 25 Hz) independent of temperature, nature of the alcohol and alcohol/Fe ratio. This phenomenon can not result from a rapid exchange between the free and hydrogen-bonded hydride complexes, nor from an exchange between the hydrogen-bonded complex and the proton transfer product. In the first case, the peak should sharpen upon increasing the temperature because coalescence is already achieved, whereas in the second case the broadening phenomenon should be equally observable on both resonances, contrary to the observation. The most likely explanation for the observed phenomenon is the self-exchange process with trace amounts of the 1-electron oxidation product, the paramagnetic [Cp*Fe(dppe)H] + ion, whose presence is also responsible for the unusual temperature dependence of the T1 time for the starting hydride complex (vide supra). Further indication that this is the correct explanation is the measurement of unrealistically small relaxation times (T1) for the broad hydride resonance in the presence of TFE (see Table 2). The longitudinal relaxation time has been frequently used as a criterion for establishing the presence of dihydrogen-bonding interactions. The T1 value slightly decreases relative to that of the starting hydride complex, because of the additional dipolar relaxation induced by the hydrogen-bonded proton. The presence of a dihydrogen-bonding for the Cp*Fe(dppe)H complex, however, could not possibly give such low values for T1 (e.g. 0.67 ms at 240 K, see Table 2), even lower than for the dihydrogen complex product where the H-H distance is certainly shorter. This value can only result from the fast relaxation induced by the unpaired electron in the oxidized 17-electron complex, with which the starting hydride complex is in fast degenerate exchange. Shaking the NMR solutions with Zn/Cu alloy inside the NMR tube did not, unfortunately, eliminate this problem. This occurrence unfortunately renders a more thorough NMR characterization of hydrogen bonding for the Cp*Fe(dppe)H hydride complex impossible.

The reason why the starting hydride band is broader and the T1 shorter in the presence of alcohol may be the result of the partial conversion to the protonation product, increasing the [Cp*Fe(dppe)H] + /Cp*Fe(dppe)H ratio. Additional oxidation upon introduction of the alcohol cannot be excluded. In any event, the extent of this oxidation process remains small.

If a significant amount of hydride complex had been oxidized, the resonance would not only broaden but also shift to an extent that would depend on temperature. In fact, this is not the case, the chemical shift being essentially temperature independent as in the alcohol-free solution.

e. Interaction with PFTB, HFIP and TFE: IR investigations.

Low-temperature IR spectroscopic studies also provide qualitative indications on the interaction between Cp*Fe(dppe)H and fluorinated alcohols, leading to proton transfer. The essential findings from the NMR study were confirmed by the IR investigation: (i) the proton transfer equilibrium between the hydrogen-bonded system and the dihydrogen complex is shifted toward the protonation product to a greater extent for stronger alcohols and for higher alcohol/Fe ratios; (ii) conversion of the overall mixture to the final classical dihydride product started to occur at a higher temperature for the weaker alcohols.

With PFTB, the complete disappearance of the starting material occurs when using a threefold excess at 200 K in CH2Cl2, whereas an equilibrium situation is obtained when the alcohol is used stoichiometrically. Like in the NMR experiment, the isomerization started to occur upon raising the temperature above 250 K. For the HFIP experiment, larger excess amounts of alcohol are required to consume a substantial amount of the hydride precursor at low temperature. For TFE, only ca. 50% of dihydrogen complex was formed when using a fifteen fold excess of TFE at 200 K (cf. 98% for HFIP under otherwise identical conditions).

In addition, the proton transfer is much slower (> 30 min for TFE vs. < 5 min for HFIP at 200 K). No quantitative kinetics information, however, can be obtained using this technique. In all three cases, the reversibility of the IR spectral changes with temperature (below the temperature at which isomerization to the final classical dihydride product starts to occur) qualitatively indicates that the proton transfer equilibrium shifts toward the dihydrogen complex when temperature decreases, but no quantitative thermodynamic information can be derived.

f. Interaction with PFTB, HFIP and TFE: preliminary UV-visible investigations.

Finally, the Cp*Fe(dppe)H-HA interaction was investigated by UV-visible spectroscopy in CH2Cl2, since this technique is more adept to yielding quantitative results. The study confirmed the two-step pathway already established by the IR and NMR techniques, followed by the slow decomposition to the paramagnetic [Cp*Fe(dppe)Cl] + . The evolution of the UVvisible spectrum is qualitatively identical with respect to the corresponding protonation by HBF4 (see Figure 1, bottom part). This evolution appeared best suited to detailed kinetic investigations of the proton transfer process, which will be destribed below in section g.

The interaction of Cp*Fe(dppe)H with PFTB (1:5 ratio) at 200 K immediately gives the dihydrogen complex quantitatively, followed by transformation to the dihydride isomer at temperatures greater than 250 K and finally to [Cp*Fe(dppe)Cl] + over several hours at room temperature. With HFIP, the proton transfer equilibrium could be investigated in a wider temperature range and for several HFIP/Fe ratios. In the 200-270 K temperature range, the UV-visible spectral changes are perfectly reversible and indicate, in agreement with the IR evidence, a temperature dependence of this equilibrium, see Figure 5. The UV-visible properties of the dihydrogen-bonded complexes are essentially indistinguishable from those of the free hydride complex, as confirmed by the fast kinetics studies, vide infra. This is reasonable, because the hydrogen bonding interactions should perturb the electronic structure of the metal center only in a minimal way. The least absorbing species in this equilibrium solution, however, is the dihydrogen complex, see Figure 1. Therefore, the overall changes shown by Figure 5 are consistent with the presence of a greater relative amount of the dihydrogen complex at lower temperatures. The reversibility of this process enabled us to obtain the equilibrium constant for Equation 3 assuming that all hydride is in the dihydrogen-bonded form at 200-270 K (vide supra, section b) and that the equilibrium involves a second alcohol molecule (vide infra, section g). The van't Hoff plot (Figure 6) gives the enthalpy (H = -6.6±0.9 kcal mol -1 ) and the entropy (S = -17.2±1.4 e.u.) of the process shown in Equation 3.

Equation 3

Cp*Fe(dppe For TFE, the slow equilibration rates made it impractical to determine the accurate equilibrium position at low temperatures. In addition, the smaller proton transfer equilibrium constant required the use of greater alcohol concentrations, with consequent crystallization of the excess alcohol at low temperatures in dichloromethane.

)H•••HA + HA [Cp*Fe(dppe)(H2)] + [AHA] - (3)

g. Kinetics investigations of the proton transfer from TFE, HFIP, PFTB and TFA

The collective spectroscopic studies of the hydride-HA interactions described in the previous section suggest Scheme 2, in line with previous knowledge in this field as outlined in the Introduction. In principle, each different hydrogen-bonded complex can be seen as an intermediate leading to a different proton transfer product. The dihydrogen bond (K1) would lead to the nonclassical product (K3=k3/k-3), while the hydrogen-bond to the metal center (K2) would lead to the classical product (k4'). However, the classical dihydride complex may also be obtained directly from the nonclassical tautomer (k4). The second hypothesis appears consistent with the observation by Hamon et al. that the isomerisation also takes place for the isolated dihydrogen complex in the solid state. 9 The results of a more detailed kinetic investigation by UV-visible monitoring allow us to throw more light on this dichotomy.

Scheme 2

Fe H 2 P P (k 4 )

(K 3 = k 3 /k -3 ) Fe H P P H OR F Fe H P P H (K 1 ) Fe H P P (k 4 ') (K 2 ) Fe H P P H R F O + R F OH + + R F OH slow immediate fast slow + (a) (b) immediate + R F OH + R F OH
The reactions with the three fluorinated alcohols TFE, HFIP and PFTB were carried out in CH2Cl2 at 298 K under pseudo-1 st order conditions with HA/Fe ratios in the 30-200 range.

The entire kinetics investigation required the use of both stopped-flow and classical mixing and monitoring methods. In all cases, the spectrum recorded immediately after mixing (ca. 1 ms) is undistinguishable from that of the pure hydride in the absence of alcohol, 48 proving that the hydrogen bonding does not significantly perturb the electronic structure of the metal center. The establishment of hydrogen bonds is a diffusion controlled step. [START_REF] Scheiner | Hydrogen bonding: A Theoretical Perspective[END_REF] The investigations yielded the rate constants for three separate processes, as indicated in Scheme 3 For the PFTB process, the first measurable step was complete within a fraction of a second and could be accurately measured only by stopped-flow kinetics. The second, slower step was also accessible from the stopped-flow data. The two processes occur on quite different times scale and the first process is essentially quantitative even for the lowest

[HA]/[FeH] ratio, rendering the kinetic analysis straightforward. These two processes correspond to the proton transfer equilibrium yielding the dihydrogen intermediate and to the isomerization process, respectively. This is confirmed by comparison of the extrapolated spectra of intermediate and product (as obtained from a global Specfit analysis) with those obtained from the HBF4 protonation (see Figure 1). The first order dependence of the proton transfer rate constant (k3obs) on the alcohol concentration is shown in Figure 7(a). The intercept is zero within experimental error (0.10.7 s -1 ), confirming the assumption of irreversibility for this step. The subsequent rate constants, k4obs and k5obs, on the other hand, do not show any dependence on the alcohol concentration. All rate constants are collected in Table 5. For the HFIP run, the first step required a few seconds and partially overlapped with the second kinetics. In addition, the above determined thermodynamic data for this proton transfer step (extrapolated equilibrium constant of 7.97 at 298 K) suggest that this may be equilibrated, even for the relatively high HFIP/Fe ratios employed in this kinetic study.

However, a good fit was possible for the model A B C, yielding individual observed rate constants for each step. The first step (proton transfer) has a first order dependence on HFIP and the intercept is close to the experimental error (k-3obs = 0.210.13 s -1 ), see Figure 7(b). Thus, this step is almost irreversible under the experimental conditions used for this kinetic experiment. Even though it is affected by a large uncertainty, the estimated k3obs/k-3obs ratio is consistent with the estimated proton transfer equilibrium constant from the van't Hoff analysis (vide supra). The values of k4obs and k5obs are independent from the alcohol concentration. Furthermore, and most notably, they are also independent on the alcohol nature, the values obtained for PFTB and HFIP being very close to each other, see Table 5.

Contrary to the PFTB and HFIP reactions, the proton transfer from TFE led to a single measurable kinetics for the transformation of FeH to the classical dihydride product. The reason of this result is that the second step is faster than the first one. This was too slow for the stopped-flow time scale but it could be accessed by regular mixing and monitoring in an air tight cuvette. A complication in this case was the contamination by the final transformation to the paramagnetic chloro complex. However, the SPECFIT global analysis afforded reasonably precise values for both k3obs and k5obs. The value of k3obs was linearly dependent on the alcohol concentration, with a zero intercept within experimental error like for the previous cases, (1.63)x10 -4 s -1 . The value of k5obs does not show any significant dependence on the alcohol concentration and is very close to those observed with the other alcohols (see Table 5).

In order to further widen the acidity range of the proton donor, we also carried out a kinetic study of the protonation with trifluoroacetic acid (TFA). With this acid, the first step was too fast to measure by the stopped-flow technique, the first recorded spectrum after ca. 1 ms corresponding already to that of the intermediate dihydrogen complex (i.e. spectrum b in Figure 1). Further transformation to the classical tautomer (spectrum c in Figure 1) followed clean first order kinetics, the k4obs being once again independent on the acid concentration and close in value to those observed for the fluorinated alcohols (see Table 5).

In summary, proton donors instantaneously establish hydrogen bonding at the hydride site. This is in agreement with the general knowledge that they are diffusion-controlled, barrierless reactions. [START_REF] Scheiner | Hydrogen bonding: A Theoretical Perspective[END_REF] The subsequent step is proton transfer to the hydride site, whose rate and equilibrium constant increase in the order TFE < HFIP < PFTB < TFA, namely as the acidity of the proton donor increases. This rate has a first order dependence on the alcohol concentration (established experimentally for the three alcohols).

If one neglects the establishment of the hydrogen bonds, the first order dependence on the alcohol concentration would point to a rather simple bimolecular elementary process, whereby one alcohol molecule transfers its proton to the hydride complex in a direct, single step process. This would also explain the rate constant dependence on the proton donor acidity. However, the measured thermodynamic parameters for hydrogen bonding with TFE (Figure 3) allow us to estimate a 6:1 FeH•••HX/FeH ratio under the conditions used for the stopped-flow kinetics study. When using HFIP and PFTB, the constant for dihydrogen bond formation will be even higher. Therefore, the [FeH•••HA] complex predominates in the mixture which transforms farther into non-classical protonation product.

The starting point for the first kinetics process is not the pure hydride precursor but rather the equilibrium mixture with the hydrogen-bonded adducts, Scheme 3. A straightforward manipulation of the rate expression leads to Equation 4, which can be simplified as shown at high alcohol concentrations (quantitative formation of the hydrogen bonded adducts). Therefore, the observed first order dependence on HA implies the intervention of a second molecule in the rate-determining step, as indicated in Scheme 4. If a second alcohol molecule were not involved in the proton transfer step, the k3obs expression would lead to an essentially alcohol independent observed rate constant. This observation suggests that a direct proton transfer leading to a salt which contains a free alkoxide anion does not occur. The involvement of a second alcohol molecule presumably renders the proton transfer process thermodynamically more favorable and faster via the establishment of a hydrogen bonding interaction and the formation of an alcohol/alkoxide homoconjugate pair (see Scheme 4). This homoconjugate pair formation has been noted in other proton transfer processes to hydride complexes, [START_REF] Papish | [END_REF][START_REF] Belkova | [END_REF] and is comforted by theoretical calculations. 51 Because of this homoconjugate pair formation, we consider it unlikely that the alkoxide ion also remains hydrogen-bonded to the dihydrogen ligand in the dichloromethane solvent used. The product, therefore, is probably present in solution in the form of free ions.

Scheme 4

Fe

˜

The question remains as to the intimate mechanism of the tautomerization process, as indicated by the two possible pathways (a) and (b) in Scheme 2. Both pathways would lead to a zero order dependence on the proton donor concentration (see Supporting Information), as experimentally observed. Therefore, they cannot be distinguished on this basis. However, the independence of the rate constant on the nature of the proton donor strongly points toward an internal reorganization mechanism (pathway (b) in Scheme 2). The alternative possibility of reversible deprotonation followed by protonation at the metal center would certainly lead to a kinetics dependent on the nature of the acid.

It is interesting to compare our results with those reported for the related ruthenium systems (Ring)RuH(L)(L') (Ring = Cp, Cp*; L,L' = tertiary phosphine ligands). 6,29 For these, the occurrence of an intramolecular rearrangement mechanism leading from the dihydrogen complex to the dihydride isomer was proposed, in part, on the basis of the observation that the rate constant is invariant with the acid concentration. However, we have shown that this rate should not depend on the acid concentration even for a deprotonation/reprotonation mechanism (see Supporting Information). The observation by Chinn and Heinkey 6 that the isomerization is faster than H/D scrambling in the presence of external acids is much stronger evidence in favor of an intramolecular mechanism. Our observations, based on the comparison of isomerization rates in the presence of acids of different strength, are complementary and lead to the same conclusion for the related Fe system. In addition, however, they show the absence of a kinetically viable direct protonation of the metal site under the same experimental conditions in which the isomerization process occurs. For the previous studies carried out by Heinekey and Puerta on the hydridoruthenium complexes, as well as for the previous study by Hamon et al. on Cp*FeH(dppe), the proton transfer process is carried out at very low temperature, where the direct protonation of the metal site may be disfavored by large negative activation entropies. Those studies, therefore, do not necessarily prove that a competitive metal protonation would not occur at ambient temperature.

Conclusion

The direct and independent observation, at the same temperature, of the kinetics for the faster protonation of the hydride ligand site in complex Cp*FeH(dppe) and for the isomerization of the resulting Cp*Fe(H2)(dppe) + intermediate to the thermodynamically preferred classical dihydride product has allowed to obtain strong evidence in favor of a direct isomerization process, not involving the reversible deprotonation of the hydride site and protonation of the metal site. The unambiguous presence of a direct proton transfer pathway to the metal site, for any complex which contains hydride ligands, remains to be established.

Further investigations will be necessary in order to probe the generality or limitations of the results reported here.

  Scheme 1

region provided the spectral changes shown in the upper part of Figure 1 .

 1 The starting hydride complex is characterized by a relatively strong (MH) band at 1844 cm -1 ( = 8.81 and 6.32 L mol -1 cm -1 at 200K and 290K, respectively) in CH2Cl2 solution, which is displaced relative to the solid state (1869 cm -1 ),8 plus a few weaker features in the 2000 -1750 cm -1 region which correspond to overtones of aryl group vibrations, see spectrum (a). The HBF4 addition at 200 K causes the essentially complete disappearance of this band, see spectrum (b).

Figure 1 .

 1 Figure 1. Spectral changes (IR, above; UV-visible, below) observed for the protonation of Cp*Fe(dppe)H by HBF4 in CH2Cl2. (a) Before addition of HBF4 (T = 200K); (b) after addition of HBF4 (T = 200K); (c) T = 290 K (after a few minutes); (d) T = 290, after several hours. [Fe] = 0.03 M (IR); 0.01 M (UV-visible); HBF4/Fe = 1.

AbsorbanceFigure 2 .

 2 Figure 2. IR spectra in the (RFO-H) stretching region showing the interaction between Cp*Fe(dppe)H and MFE or TFE in CH2Cl2. Solid line: [Fe] = 0.15 M; [MFE] = 0.015 M. Dashed line: [Fe] = 0.13 M; [TFE] = 0.01 M.

Figure 3 .

 3 Figure 3. Temperature dependence of lnK for the interaction between Cp*Fe(dppe)H and TFE in CH2Cl2. [Fe] = 0.13 M; [TFE] = 0.01 M.

  .

47

 47

Figure 4 .

 4 Figure 4. IR study of the interaction between Cp*Fe(dppe)H (0.038 M) and TFE in CH2Cl2 at 200 K. (a) Without TFE. (b) With TFE (3 equiv).

Figure 5 .

 5 Figure 5. UV-visible study of the interaction between Cp*Fe(dppe)H and HFIP (5 equiv) in CH2Cl2. [Fe] = 0.02 M. (a) Before the HFIP addition. (b) T = 270 K. (c) T = 200 K. The other intermediate spectra correspond to 265 -225 K (5 K step).

Figure 6 .

 6 Figure 6. Temperature dependence of lnK for the interaction between Cp*Fe(dppe)H and HFIP in CH2Cl2.

[

  Fe = Cp*Fe(dppe)]. Although the initial equilibrium may include, in principle, the hydrogen-bonded adducts involving the hydride site and at the metal site, [FeH•••HA] and [HFe•••HA], the latter species was not found in the present case.

  Scheme 3

Figure 7 .

 7 Figure 7. Pseudo-1 st order rate constants for the first step of the proton transfer from HA to

Table 1 .

 1 IR (Fe-H), 1 H (hydride resonance) and 31 P NMR data in CD2Cl2 for Cp*Fe(dppe)H and for the products deriving from its interaction with HA.

	Compound	Fe-H/cm -1	H(JHP/Hz)	P
	Cp*(dppe)FeH	1844 (s)	-17.32(67)	107.97
	Cp*(dppe)FeH•••HA a 1830 (sh)	b	b
	[Cp*(dppe)Fe(H2)] +	c	-12.50(broad)	93.39
	[Cp*(dppe)Fe(H)2] + 1940 (w)	-7.98 (73)	90.97

a Observed for HA = TFE. b Not significantly shifted from the values of free Cp*Fe(dppe)H (see text). c Not observable (see text).

The relaxation times as a function of temperature are shown in Table

2

. It can be noted that complex Cp*Fe(dppe)H does not show a T1 minimum. Rather, T1 keeps decreasing as the temperature increases.

Fitting the T1 data for the dihydrogen complex [Cp*Fe(dppe)(H2)] + , on the other hand, gives a T1min time of 9.7 ms at 230 K, in good agreement with the minimum of 7 ms obtained by Hamon et al. at 223 K and 300 MHz.

Table 2 .

 2 Longitudinal relaxation times at 400 MHz in CD2Cl2 for Cp*Fe(dppe)H and for the products deriving from its interaction with TFE (TFE/Fe = 3).

	T/K		T1/ms
		FeH	FeH + TFE	Fe(H2) +
	190		25	24
	200		12	15
	220	349.7	3.9	11
	225	328.2
	230	311.4		9.6
	235	280
	240	241.4	0.67	10.2
	245	223.4
	250	200
	255	182.8
	260	173

In a subsequent experiment carried out in CH2Cl2, IR monitoring in the Fe-H stretching

Table 3 .

 3 Parameters of the dihydrogen bonding interaction between Cp*Fe(dppe)H and MFE or TFE in CH2Cl2. Hº parameter calculated by Equation1, mean error ±0.4 kcal mol -1 . b Ej parameter calculated by Equation2, H°11 = -4.6 kcal mol -1 for CH2Cl2.16

	ROH	Pi	OH(free)	OH(bonded)		Hº a	Ej b
			cm -1	cm -1	cm -1	kcal mol -1
	MFE	0.74	3608	3362	246	-4.6	1.35
	TFE	0.89	3594	3240	354	-5.9	1.38

a

Table 4 .

 4 Band widths at half height for [Cp*Fe(dppe)(H2)] + and hydrogen-bonded Cp*Fe(dppe)H adducts in CD2Cl2 as a function of temperature. a

	T/K		w1/2/Hz
		FeH•••TFE FeH•••HFIP FeH•••PFTB
	180		65	triplet
	200	190	142	170
	220	270	230	230
	230	350	320	290
	240	510	480	440
	250	760
	260		1100	1280
	a C°FeH = 0.05-0.07 M. Identical values were observed for different alcohol/Fe ratios.

Table 5 .

 5 Observed rate constants for the reaction between Cp*Fe(dppe)H and HA. a

	HA	k3obs[HA] -1 /s -1 M -1	k4obs/s -1	k5obs/s -1
	TFA	b	(6.4±0.2)x10 -2
	PFTB	156±5	(7.6±0.2)x10 -2	(2.3±0.3)x10 -4
	HFIP	5.4±0.9	(6.8±0.2)x10 -2	(2.1±0.3)x10 -4
	TFE	(1.5±0.3)x10 -3	c	(2.2±0.6)x10 -4
	a See Scheme 3. b Too fast to be measured by stopped-flow. c Not measurable because faster
	than k3obs.

Table of Contents Entry

	FeH + HA	FeH•••HA
	FeH•••HA + HA	Fe(H2) + + A2H -
	Fe(H2) +	Fe(H)2 +
	(HA = MFE, TFE, HFIP, PFTB, TFA)

Acknowledgement

This work was supported by the European Commission's RTN Programme (HPRN-CT-2002-00176), by INTAS (00-00179), by RFBR (No. 02-03-32194, 03-03-6283) and by NATO (PST.CLG.978453).

Supporting Information

Derivation of equations for the observed rate constant of the tautomerization process under the two different mechanistic hypotheses (1 page).


BFW, Austria Gernot Hoch

James Connell

Alain Roques

Fera, UK Neil Audsley

NVWA Antoon Loomans

INIAV, Portugal Netherlands Edmundo De Sousa

Jeremy Allison

Fera

Nvwa

Edmundo De Sousa

Using a multi

come

Background

 Sex and aggregation pheromones and kairomones (host volatiles, bark beetle pheromones) known for many cerambycid species (reviewed in Millar & Hanks 2017)  Blends of several pheromones used in experiments in North America (e.g. Hanks et al. 2012, Hanks et al. 2018, Millar et al. 2018): Multi-lures feasible for trapping many cerambycids  Multi-lure (ethanol, α-pinene; frontalin, ipsenol, ipsdienol) Trapping in high risk areas:

Port of Vienna

  traps successfully tested to catch woodborers (particularly Scolytinae) at ports of enty in Italy (Rassati et al. 2014)  Multi-component blends for trapping Cerambycidae at ports of entry in France (PORTRAP projects) (Fan et al. 2018) Lures used in MULTIRAP based on previous experiments by project partner INRA Pheromones Fuscumol (= geranyl acetol): pheromone of certain species in Aseminae/Spondylidinae, attractive for many Lamiinae Fuscumol acetate: Attractant for some Lamiinae Geranyl acetone: Fuscumol synthesized by reduction of geranyl acetone Monochamol: Pheromone of Monchamus (Lamiinae) 3-hydroxy-2-hexanone: Pheromone of many Cerambycinae 2-methyl-1-butanol: Pheromone component of several Cerambycinae 2,3-hexanediol: Pheromone component of many Cerambycinae Prionic acid: Female produced sex pheromone of Prionus spp.KairomonesEthanol and α-Pinene: Attractive kairomone for many bark and wood boring beetles(Hanks et al. 2012, Millar & Hanks 2017, Sweeney et al. 2010, Barbour et al. 2011) mean trappings per lure and per trap type. Different letters in the same type of trap indicate signif. differences (Friedmann's Q test followed by Dunn-Nemenyi multiple comparison test, P < 0.05).

  of specimens from different subfamilies caught in different blends in cross vane (CT) and multi funnel (MF) traps from different subfamilies caught in different blends in Austrian pine forest (trap types combined) 1 and 2 for Lamiinae and Cerambycidinae, respectively:  Lure 1+a: only 1 specimen (Phymatodes testaceus) in out of 140 Cerambycidinae in 2017, and 1 specimen (Clytus lama) out of 499 in 2018  Preference of Lamiinae for Lure 1+a  Specific response of Prionus coriarius to prionic acid (but low no.) (Results from pine forest Neunkirchen) EtOH + alpha pinene leads to higher catch (Lure 3 vs. 3a):  Vienna (port + forest) 140 vs. 236 specimens  Neunkirchen 288 vs. 787, mostly due to Spondylidinae and Monochamus Combination of lures 1 + 2 (= 3) + a is promising for multitrapping Conclusions from MULTITRAP Potential benefits from replacing single-lure trapping programmes with combined multi-lure programmes for early detection: Trap density can be increased (compared to single-lure trapping) for better early detection Not only regulated pest species arrive at ports of entry Many specimens have to be identified Additional use of method in faunistic studies (two years trapping in Vienna: half of cerambycid fauna of this area)


Jenny Gun

Alexandre Modestov

Ovadia Lev

Rinaldo Poli
email: poli@u-bourgogne.fr

Reduction of (C5Me5)2Mo2O5 and (C5Me5)2Mo2O4 in Methanol -Water -Trifluoroacetate Solutions investigated by Combined On-Line Electrochemistry/Electrospray Mass Spectrometry

Molybdenum/ Oxo ligands / Cyclopentadienyl ligands / Electrospray ionisation mass spectrometry / Electrochemistry Complexes Cp*2Mo2O5 (Cp* =  5 -C5Me5) and Cp*2Mo2O4 were investigated by combined on-line electrochemical (EC) reduction and electrospray ionisation mass spectrometry (ESI-MS) in an trifluoroacetic acid-buffered watermethanol solution. It is shown that the reduction products at the larger negative potentials are identical for both compounds. The studies reveal the existence of a wide class of previously unknown bi-and trinuclear Mo V , Mo IV , Mo III and mixed-valence complexes which were identified on the basis of their masses and characteristic isotope patterns. The structures of the initial compounds and the m/z = 713-729 product of electroreduction were supported by in-situ MS n experiments with the elucidation of the fragmentation pathway for the collision induced dissociation.

Introduction

The development of the Electrospray Ionization Mass Spectrometers (ESI-MS) by Yamashita and Fenn [1] has provided a very powerful technique for analyzing various charged inorganic, [2] organic [3] and organometallic species in solution. [4] Electrospray is an atmospheric pressure technique that makes use of an electrostatic sprayer to assist the transfer of the ionic analytes that are initially present in solution into the gas phase. [5] In contrast to conventional ionization methods which create undesirable fragment ions, the ESI technique usually gives simpler spectra. [6] The on-line combination of electrochemistry with ESI-MS has been an achievement of the very last decade. The preparative electrochemistry, followed by ESI-MS analysis of the isolated products or products generated in situ have been extensively explored by Bond et al. [7,8] Three other groups, Cole et al., [9,10] Van Berkel et al., [11,12] and Brajter-Toth et al., [13] used an electrochemical cell coupled on-line with mass spectrometry for the investigation of redox reactions.

In our previous publication which focused only on the reductive process of compound Cp*2Mo2O5, [14] we have shown how, from the analytical point of view, various operating conditions may be optimized in order to minimize spurious reactions in the electrospray chamber and obtain spectra that reflect the solution composition. The previous study was carried out in an acetic-buffered solution and most of the resulting reduction products were found to incorporate the acetate ligand. A subsequent chemical reduction study has allowed the isolation and structural characterization of compound [Cp*MoO(O2CCH3)]2, which is closely related to one of the species observed by EC-ESI-MS. [15] With the goal of generating more reactive aqua species and to reach the lower oxidation states of Mo, we have expanded the previous study to solutions containing the stronger trifluoroacetic acid, whose conjugate base has a poorer coordinating ability. In order to verify the electroreduction pathway of the compound (C5Me5)2Mo2O5 we have expanded the electroreduction experiments to the previously described [16][17][18] Mo(V) compound (C5Me5)2Mo2O4.

Experiments on sequential product ion fragmentations (MS n ) by collision induced dissociation (CID) were performed in order to elucidate the gas phase degradation pathway for the starting and product compounds. In general, low energy CID is a powerful technique due to the simplicity and reliability of performing sequential ion fragmentation MS n experiments. Collisions between analyte ions and helium gas that serves as a damping buffer gas in the ion trap are used for analyte ion fragmentations and excitations. Collision-induced decomposition produces ionic and neutral species simultaneously in the ion trap. However, only the charged species can be directly identified in conventional multistage mass spectrometry. The chemical structure of the neutral species can only be hypothesized on the basis of the most favored fragmentation mechanisms. Valuable information on the molecular structures of organic compounds has also been acquired by ESI-MS n but very few investigations have been conducted for the organometallic compounds. [19,20]

Results and Discussion

Positive mode ESI-MS spectrum of the starting compounds.

The positive mode ESI-MS spectra of the starting compounds are shown in Figure 1, frames A and B. The inserts show the characteristic isotopic patterns of mono-, di-and trinuclear Mo fragments. For mononuclear Mo species, the isotopic envelope consists of 7 lines with two gaps located between the first and second and between the sixth and seventh lines, respectively. The isotopic envelope of a dinuclear Mo ion consists of 14 peaks and has a width of 16 m/z units, while that of a trinuclear Mo species has width of 23 m/z units The width of the isotopic envelope is an important characteristic in our MS n fragmentation experiments for the perfect isolation of the whole set of the molecules with the same structure.

We shall refer to the various species by the m/z value of the full isotopic pattern in the isotopic distribution and will compare it to the calculated distributions [START_REF] Gordon | The Chemist's Companion. Handbook of Practical Data, Techniques and References[END_REF] for the molecular assignment.

<Figure 1>

Spectrum A was measured for a water -methanol solution of Cp*2Mo2O5 containing 0.1 M trifluoroacetic acid. It reveals three major regions of peaks, corresponding to mono-, diand trinuclear species, respectively. The ESI-MS spectrum of this compound in watermethanol containing 0.1 M ammonium acetate (pH = 4) has been previously described. [14] The lower pH (pH = 1.8) used in the present study changes significantly the product distribution in every region. The mononuclear region exhibits the Cp*MoO2 + peak (m/z = 259-267) and the Cp*MoO3H2 + peak (m/z = 277-285) at approximately the same relative intensity as in the previous acetate buffer study. The strongest peak, however, corresponds to the methanol addition product, Cp*MoO2(MeOH) + (m/z = 291-299), which had only a very small intensity in the acetic buffer. The formation of adducts with solvent molecules in the ion trap is a well known phenomenon. [START_REF] Byrdwell | [END_REF]23] The main peak for dinuclear species corresponds to the protonated starting compound Cp*2Mo2O5H + (m/z = 535-551), as in the corresponding acetic buffer study. [14] The other most pronounced dinuclear peaks, corresponding to daughter ions formed in the ESI-MS chamber, had not been previously described, since their intensity was much lower under the higher pH conditions. They increase in relative abundance at increasing heated capillary temperatures. The lighter one (m/z = 517-533) corresponds to loss of one water molecule.

This process must of course involve at least one of the Cp* methyl H atoms, leading to a tetramethylfulvene ligand. Therefore, the peak is assigned to complex Cp*(C4Me4CH2)Mo2O4 + . The methylene group of this ligand may be bonded to the molybdenum atom or to one of the oxygen atoms. The envelope at higher masses, on the other hand, results from the superposition of two envelopes, one (major) at m/z = 549-565 which is attributed to a MeOH adduct of the just described tetramethylfulvene complex, and one (minor) at m/z = 553-569 probably resulting from water addition to the starting compound. The Cp*3Mo3O7 + peak (m/z=793-816) is the only relevant feature in the trinuclear species region.

The differences between Figure 1 and the corresponding spectrum resulting from the acetic buffer experiment (see Figure 1 of ref. [14]) are consistent with our parallel speciation studies. We have shown that complex Cp*MoO2 + is the predominant species at pH < ca. 3, while a mixture of Cp*MoO2 + and Cp*MoO3 -(in equilibrium with Cp*MoO2(OH) and Cp*2Mo2O5 depending on the H2O:MeOH ratio) is present at pH ca. 4. [24] Indeed, the peaks related to the dinuclear Cp*2Mo2O5 species are less abundant at the lower pH, while all those related to species Cp*MoO2 + (mononuclear and trinuclear species) are comparatively more abundant. Indeed, the trinuclear species results from the addition of Cp*MoO2 + to Cp*2Mo2O5 (vide infra).

The ESI-MS investigation of complex Cp*2Mo2O4, reported here for the first time, was carried out under the same conditions used for compound Cp*2Mo2O5, for comparison reasons. The spectrum of the starting solution is shown in Frame B of Figure 1. The main peak corresponds to the protonated starting compound, (C5Me5)2Mo2O4H + (m/z=519-535), and the second most important component (m/z = 1058-1085) is assigned to the tetranuclear Cp*4Mo4O6(OH)3 + complex. The latter is formally obtained by dimerization of the starting compound and addition of a water molecule and a proton. Evidence for the existence of a tetranuclear species has been provided for the corresponding Cp system. [25] The spectrum shows fewer peaks relative to that of the pentaoxo analogue. This can result from either a simpler solution composition (the speciation of this compound in methanol-water is yet unknown) or from a greater gas phase stability and therefore a less pronounced collision-induced decomposition. Both compounds adopt a neutral molecular structure in organic solvents. [16,26] In order to further explore this point, we proceeded with a detailed investigation of gas phase stability and MS n fragmentation pathways of the starting compounds.

Gas phase stability of the starting compounds

Figure 2 shows the relative abundance of the starting compound peaks (m/z = 535-551 for Cp*2Mo2O5H + and m/z = 519-535 for Cp*2Mo2O4H + as a function of the applied collision energy during the collision activated decomposition events in the ion trap mass spectrometer.

Both dissociation profiles lead to S-shaped curves. The half wave collision energy (E½) corresponds to the energy (in the percentage of the maximum tickling voltage) at which the relative abundance in fraction of the total ion current of the starting ion is 0.5. [27][28][29][30] For the same charge state (+1), the larger the E½ value the more stable the compound in the gas phase.

As it is evident from Figure 2, the E½ of Cp*2Mo2O4 is almost twice the E½ of Cp*2Mo2O5, underscoring the greater gas-phase stability of the former compound. This difference can be rather easily rationalized on the basis of the known structural and bonding differences between the two compounds. While the tetraoxo Mo(V) complex contains a direct metalmetal bond and two oxo bridges, the pentaoxo compound contains one oxo bridge and no metal-metal interaction.

<Figure 2>

Fragmentation pathways of the starting compounds

The dissociation profiles provided us with information about the stability of Cp*2Mo2O5 and Cp*2Mo2O4 complexes in the ion trap. An analysis of their degradation in the ion trap was carried out by their isolation therein followed by the observation of their sequential As concluded above from the dissociation profiles, Cp*2Mo2O5 is less stable than Cp*2Mo2O4 in the ion trap. The spectrum in Figure 3A shows that the first CID step for the Cp*2Mo2O5H + ion generates only one daughter ion (m/z = 517-533) by loss of a water molecule. MS 3 fragmentation of this daughter ion leads to many products by further loss of water and of a neutral dioxo(fulvene)molybdenum fragment. To the best of our knowledge, the C-H activation of a Cp* methyl group which is associated to the elimination of a water molecule and to the transformation to a tetramethylfulvene ligand has not previously been highlighted for an MS study. There is, however, at least one precedent in solution chemistry for compound [Cp*RuCl2]2O. [31] The main peaks are tentatively assigned in Scheme 1, though it is not possible to establish the exact methyl group that furnishes the hydrogen atoms. The m/z = 359-367 peak has the characteristic shape of a monomolybdenum species and is not observed at all in the regular ESI-MS. This compound would results from the elimination of a molybdic acid molecule with abstraction of two H atoms from the Cp* methyl groups. This assignment is in further agreement with related fragmentations or more reduced species, as will be shown later. It must be further pointed out that the exact structure of the proposed species containing two or more activated Me groups, e.g.

[(C5Me4CH2)2Mo] + or [{C5Me2(CH2)3}{C5Me3(CH2)2}Mo2O] + , may implicate formation of C-C bonds or C-O bonds.
The CID of complex (C5Me5)2Mo2O4 gives fewer peaks than its pentaoxo analogue.

Most dinuclear peaks are related for the two compounds (16 fewer mass units corresponding to one oxygen atom). The main mononuclear peak (m/z = 360-369) is one mass unit higher than the main mononuclear peak derived from the pentaoxo solution, thus indicating that it is a one-electron reduced form of it. It probably derives from the explusion of a MoO(OH)3 species from the parent dinuclear Mo(V) complex, paralleling exactly the formation of the mononuclear species in Scheme 1 . The currentpotential curves at the forward and backward scans go in parallel for both compounds and the heights are virtually scan rate independent over a rather wide range of scan rates (up to 100 mV s -1 ). This means that the electrochemical reduction of both Cp * 2Mo2O4 and Cp * 2Mo2O5 is apparently controlled by an interfacial step rather than being diffusion limited. In separate studies that are out of the scope of the present paper, we have witnessed the formation of a polymer film on the electrode surface under the electroreduction conditions. This makes the elucidation of the Cp * 2Mo2O4 and Cp * 2Mo2O5 electroreduction mechanisms and the identification of the electroreduction products impossible tasks solely on the basis of the voltammetric experiments.

Electrochemical experiments

<Figure 4>

Coupled electrochemistry -ESI-MS studies: consumption of the starting species

The coupled EC/ESI-MS study was carried out with a slow (0.5 mV/s) linear potential sweep between 0 and -1 V and back to 0, similar to the experiment described in our previous study. [14] Figure 5 shows the volt-spectrograms of the major mono-, di-, and trinuclear species. During the backward, anodic scan, the concentration profiles follow approximately the same pattern observed in the cathodic scan, indicating that the experiment is not polluted by film deposition processes.

<Figure 5>

The concentrations of both mononuclear species and of the dinuclear species start to decrease at a potential slightly less negative than E = -0. Interestingly, the major electroreduction products observed in the current study derive from subsequent 2-electron additions, whereas the previous acetic buffer study also showed a dinuclear mixed-valence (Mo2 IV,V ) species. Mononuclear Mo V and Mo IV acetato complexes where also evident in the previous study, whereas corresponding trifluoroacetato species are not observed in the present investigation.

Another notable difference is that whereas the observed Mo2 V,V products have the same stoichiometry under both conditions, the Mo2 IV,IV product is Cp*2Mo2(O)(OH)(O2CCF3)2 + in the present study and Cp*2Mo2O(O2CCH3)3 + in the previous acetate study. A peak for the corresponding Cp*2Mo2O(O2CCF3)3 + ion is not observed in the present study. This seems counterintuitive, because the stronger trifluoroacetic acid should be better capable to replace the hydroxo ligand. A possible, though somewhat speculative interpretation, relies on two fundamental differences between the two acids and the corresponding conjugate bases: while CF3COOH is the stronger acid, CH3COO -is the better coordinating agent. Thus, reduction of the same Cp*2Mo2O3(O2CCX3) + (Mo2 V,V ; X = H, F) complex would lead to the same Cp*2Mo2O2(O2CCX3)2 + (Mo2 IV,V ) ion according to Scheme 4. This, however, would only be observed for the acetato system because the stronger CF3COOH acid would immediately protonate it. The CH3COOH system could, on the other hand, lead to a triacetato species because of the stronger coordinating properties of the acetate ion. If this hypothesis holds true, each final Mo2 IV,IV product would be observed merely because the exchange process between hydroxide and (trifluoro)acetate, though thermodynamically favorable, is too slow under the present conditions. It is also worth mentioning that the neutral diacetato compound Cp*2Mo2O2(O2CCH3)2 (i.e. a deprotonated version of the observed product for the trifluoroacetato system) has been isolated from a reductive chemical synthesis and crystallographically characterized. [15] <Figure 7>

While the electroreduction did not afford any dinuclear ion with oxidation states below IV when carried out in an acetic buffer, it also provides a Mo2 III,III complex in the presence of trifluoroacetic acid. This difference may again be attributed to the stronger acidic properties of CF3COOH, favoring the protonation of the oxo ligands and their removal as water molecules.

<Scheme 4>

The following approximate potential values for the appearance of every dinuclear reduced species may be measured on the volt-spectrograms in Figure 7:

Cp*2Mo2O4H + (Mo2 V,V ), -0.55 V; Cp*2Mo2O3(O2CCF3) + (Mo2 V,V ), -0.60 V; Cp*2Mo2(O)(OH)(O2CCF3)2 +
(Mo2 IV,IV ), -0.68 V; Cp*2Mo2O(O2CCF3)2 + (Mo2 III,III ), -0.76 V. A comparison of the onset reduction potentials with those observed in the previous acetate study is useful. The Cp*2Mo2O4H + product is not acid dependent. In both studies, the onset of this peak is at approximately the same potential as the onset for the Cp*2Mo2O5H + consumption and the two numerical values are quite similar (these numbers may be measured only with limited precision). The [Cp*2Mo2O3(O2CCF3)] + Mo2 V,V species starts to form at a more negative potential relative to the acetato analogue (-0.55 V). Further reduction to the Mo2 IV,IV species seems easier for the trifluoroacetato system, in agreement with expectations (cf. -0.74 V and -0.78 V for the acetato Mo2 IV,V and Mo2 IV,IV products, respectively). Furthermore, reduction to the Mo2 III,III product starts at an even lower potential, while this process is not observed for the acetato system, as mentioned above.

Structural elucidation of the m/z = 713-729 product.

The main product of both Cp*2Mo2O5 and Cp*2Mo2O4 electroreductions, namely the Mo2 IV,IV ion interpreted as Cp*2Mo2O(OH)(O2CCF3)2 + , was the subject of more detailed MS n studies. There are two reasons for this choice. First, we wanted to prove the general approach of our product identification. Second, we wanted to check whether any observed peaks are in fact attributable to daughter ions of this major peak rather than to primary products of the electrochemical reduction.

Figure 8 shows the main results of the MS n experiment. The spectrum shown in part A is produced from the isolation of the species at m/z = 713-729 and subsequent CID. There are four main daughter ion products, the main one being that with m/z = 599-615. In fact, the lighter products increase in subsequent MS n experiments, while the heavier one (32 mass units higher) is obviously the solvent (methanol) adduct formed by gas-phase reactions in the ion trap. [28] This assignment is further supported by the next MS n step. The m/z = 599-615 product ion was isolated and consequently subjected to further collision activation (MS 3 ), giving the spectrum shown in Figure 8B. The further generation of the 634-650 peak can be easily understood in terms of the solvent adduct formation. Additional fragmentation by CID (MS 4 ) of the product ion with m/z = 502-518 results in one main product with m/z = 485-501 and a mononuclear peak with m/z = 361-369), see Figure 8C.

<Figure 8>

The proposed product ion compositions and fragmentation pathways are shown in Scheme 5. The first fragment (m/z = 599-615) results from the loss of 114 units, which correspond to the molecular weight of trifluoroacetic acid. Therefore, this fragment is still a dinuclear compound of Mo IV . Under the conditions of the experiment (CID = 30% for every MS n step) the next step of the fragmentation results in a prominent peak with the loss of 97 units, corresponding to a trifluoroacetyl group, and a minor peak with the loss of an additional 17 units (OH). The latter peak becomes the prominent one in the MS 4 . Loss of the acetyl group results in a formal oxidation to a mixed-valence Mo2 IV,V ion which is not observed under the electroreduction conditions, whereas the subsequent OH loss gives back a Mo2 IV,IV ion. The OH and the MoO2(OH) expulsion processes must involve one of the Cp* methyl protons yielding a tetramethylfulvene ligand, as already observed above for the CID of the starting complexes. It is quite interesting to observe that, for the three fragmentation processes leading from dinuclear to mononuclear species with loss of a neutral inorganic Mo complex, the latter always corresponds to an oxide/hydroxide in the highest possible oxidation state, i.e. Mo VI from Mo2 VI,VI (Scheme 1), Mo V from Mo2 V,V (Scheme 2) and Mo V from Mo2 IV,V (Scheme 5).

It can be concluded from Scheme 5 that the fragmentation pathway undertaken by the m/z = 713-729 compound is similar to that of the starting compounds. It is noteworthy that the m/z = 485 and 361 peaks generated by MS 4 are related to the fragments obtained from the Mo VI precursor (m/z = 481 and 359, see Figure 3A) and to those obtained from the Mo V precursor (m/z = 483 and 360, see Figure 3B). Therefore, these mass values further support the notion that the m/z 713-729 ion corresponds to a Mo2 IV,IV complex.

<Scheme 5>

Electrochemical formation of reduced trinuclear species

As can be seen by comparing Figure 1 and Figure 6 there are prominent spectral changes in the trinuclear m/z region following the electrochemical reduction. Voltspectrograms of the reduced trinuclear compounds are present in Figure 9. Because of overlap between the isotopic envelopes of different trinuclear species, the volt-spectrogram integrations were carried out over limited m/z ranges rather than over the entire envelope. The ranges used for the integrations are marked in the figure. As for the previous acetic buffer study, the envelope of the reduced trinuclear species is shifted exactly by 16 units to lower m/z relative to its precursor complex Cp*3Mo3O7 + , suggesting its formulation as a proton-free hexaoxo species. In contrast to the previous acetic buffer study, on the other hand, further reduction occurs by successive oxygen atom removal steps, resulting in the observation of Cp*3Mo3O6 + (Mo3 V,V,VI ), Cp*3Mo3O5 + (Mo3 IV,V,V ) and Cp*3Mo3O4 + (Mo3 IV,IV,IV ). Once again, this behavior can be attributed to the stronger acidity of CF3COOH. As for the dinuclear products discussed above, the formation of the trinuclear species in trifluoroacetic acid takes place through 2-electron transfer steps and the reduction potential becomes more negative as the oxidation state decreases. The onset potential values for the various reduced species, as measured from the volt-spectrograms in Figure 9, are -0.54 V; -0.61 V and -0.75 V, in order of decreasing number of oxygen atoms.

<Figure 9>

The structures assigned to various reduced species are only tentative and were drawn on the basis of the maximum symmetry principle. A [(Cp*Mo)3(-O)6] core has been shown to

exist in complex [(Cp*Mo)3(2-OH)n(2-O)6-n] 2+
(the value of n, probably 5, was not clearly established in the X-ray study) [32] and in related Cp*4Mo5O11 and Cp*6Mo8O16 clusters. [32,33] The oxidation state in these Mo3O6 cores, however, is lower (Mo3 11+ -Mo3 14+ ). The dicapped Mo3(3-O)2(2-O)3 unit is quite common in inorganic clusters but has not been found as yet, to the best of our knowledge, for Cp*-substituted triangular clusters of any metal. Finally, the monocapped Mo(3-O)(2-O)3 unit is also common for inorganic clusters. The first documented example containing half-sandwiched metal atoms comes from our own recent study of the chemical reduction of Cp*2Mo2O5 by zinc in methanol-water containing CF3COOH. [34] The isolated compound contains the

[(Cp*Mo)3(3-O)(2-O)3(2-O2CCF3)3] + ion, corresponding to a Mo3 V,V,V species.
No trinuclear species containing trifluoroacetate groups was detected in the present study, suggesting that this weakly coordinating anion has a high tendency to remain dissociated in the highly dielectric aqueous medium. The above mentioned isolation of the [(Cp*Mo)3O4(O2CCF3)3] + ion, on the other hand, suggests the existence of a transient [(Cp*Mo)3O4] 4+ species. Therefore, it is quite likely that a much wider variety of [(Cp*Mo)3On] m+ derivatives may be isolated under different pH, solvent, and counterion conditions. It is also likely that these complexes display a very rich redox chemistry. Further synthetic studies are currently ongoing to test these hypotheses and to test the electrofunctionalization of organic substrates for possible catalytic applications.

Conclusion

The sign or formation of species deriving from the protonolysis of this bond. This feature, together with the proven redox richness of the system, holds promise for potential electrocatalytic applications in an aqueous environment.

Experimental Section

General Remarks: The starting compounds, (C5Me5)2Mo2O5 and (C5Me5)2Mo2O4, were prepared according to the literature procedure. [18] Unless otherwise stated, all solutions of test compounds were prepared in H2O/MeOH mixture (1:1 by volume) containing 0.1 M CF3COOH. CF3COOH was purchased from Aldrich.

Methanol HPLC grade was purchased from Baker. The water was triple distilled. Torr. In all experiments, helium was introduced at an estimated pressure of 1 mTorr to improve the ion trapping efficiency. The background helium also served as collision gas during collision-activated decomposition events. The compounds were isolated in the ion trap with isolation width of 15 m/z units and activated by using increased collision energy to obtain collision-energy-dissociation profiles.

Electrospray Mass Spectrometry with an Ion

Coupled Electrochemistry -Electrospray Mass Spectrometry: The electrochemical part of experimental set up consisted of a PARC 273 potentiostat/galvanostat (Princeton, USA), connected to a specially designed flowthrough electrochemical cell that has been described and characterized elsewhere. [35] The schematic of the electrochemical flow cell is shown in Figure 10. The working glassy carbon disk electrode (1) (Metrohm, Herisau, Switzerland), is placed on the bottom of the cell. Compartments of counter (2) and reference (3) electrodes were separated from the main flow of electrolyte by filters (Upchurch Scientific, Oak Harbor, WA, USA). Both electrodes were Ag/AgCl -1M KCl. All reported potentials are given relative to the reference electrode. The analyte stream arrives on the working electrode surface through the outer tube (4) of the two coaxial system and the products of the electrode reaction are transferred to the mass spectrometer through the inner tube (5). The distance between the opening of the coaxial system and the working electrode was 50 m.

  Scheme 1 and Scheme 2, respectively.

Figure 4

 4 Figure4shows the voltammogram recorded in our flow-through electrochemical cell in

Figure 7

 7 Figure6shows the mass spectrum of the Cp*2Mo2O5 solution recorded at -1 V.

  present study has provided new information on the nature of the Cp*2Mo2O5 in an aqueous medium and on electrochemical versatility of Cp*Mo species in water. Selected molecular structure assignments from m/z values and isotopic distributions for Cp*Mo V and Cp*Mo IV reduction products where confirmed by MS n fragmentation experiments in comparison with the Cp*Mo VI precursor. The study has shown that the lower pH conditions established by CF3COOH open the access to a Cp*Mo III species, while reduction under acetic conditions went as far down as the oxidation state IV. The study has confirmed the robustness of the Cp*Mo bond under aqueous conditions. Even at very low pH, there is no

  Trap: A Finnigan (San Jose, USA) LCQ quadrupole ion trap mass spectrometer equipped with an electrospray ionization (ESI) interface was used for data acquisition. The ESI was operated in the positive ion mode with a spray voltage of 4.5 kV, a capillary voltage of 3.5 V, and a source temperature of 120°C. Mass spectra were obtained by scanning the mass analyzer from m/z 100 to 2000 with 5 total microscans. The maximum injection time into the ion trap was 50 ms. The analyzer was operated at a background pressure of 2*10 -5

  All tubes were from PEEK (Upchurch Scientific, Oak Harbor, WA, USA). The inner tube was 125 m inner diameter, 1.6mm outer diameter, 15 cm long. The solution flow was driven by a syringe pump. The product stream was diluted with excess methanol. After mixing with methanol, the sample stream was delivered to the ESI interface by a 30 cm long, 125 m inner diameter tubing. The flow rate of the product stream was ca 35 l*min -1 . The methanol flow in the auxiliary tubing was 30 l min -1 . The reagent flow rate was set to 5 l min -1 .Current cell design permitted us to measure mass spectrograms and linear sweep voltammograms simultaneously.

Figure 1 .

 1 Figure 1. Positive mode electrospray mass spectra in 0.1 M CF3COOH in H2O/MeOH (1:1) (pH 1.2) of: (A) Cp*2Mo2O5 (0.1 mM) and (B) Cp*2Mo2O4 (0.1 mM).

Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .

 23456 Figure 2. Dissociation profiles of the starting compounds ((A) Cp*2Mo2O5 and (B) Cp*2Mo2O4) in the same charge state as a function of the increased collision energy in the ion trap. The solutions are as described in Figure 1.

Figure 7 .Figure 8 . 4 +Figure 9 . 4 . 1 [

 784941 Figure 7. Potential dependence of the relative abundance of the dinuclear species generated during the coupled electrochemistry -ESI-MS of Cp*2Mo2O5. Left: Volt-spectrograms with the range of m/z used for the integration. Center: expanded isotopic pattern. Right: tentative chemical structure. The experimental conditions are identical with those of Figure 4.

Acknowledgments

RP thanks the CNRS and the Ministry of Research for funding. ADM and OL acknowledge the financial support of the Israel Science Foundation. We are grateful to the French Embassy in Israel for an Arc-en-Ciel/Keshet travel grant and to Prof. M. Vorotyntsev for helpful discussion. We thank a reviewer for most useful comments.

WE (1) to MS RE (3) CE (2) (5) (4)

Graphical Content Entry

Graphical Abstract

An electrospray ionisation mass spectrometric analysis coupled on line to a flow through electrochemical shows that the aqueous reduction chemistry at pH 1.8 for compounds Cp*2Mo2O5 and Cp*2Mo2O4 is identical, whereas differences are noted relative to the same study at pH 4.0.