Dr Chiara Dinoi

Dr Marco Ciclosi

Dr Eric Manoury

Laurent Maron
email: laurent.maron@insa-toulouse.fr

Dr Lionel Perrin

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Prof Rinaldo Poli

Prof Laurent Maron

Olefin epoxidation by H2O2/MeCN catalysed by cyclopentadienyloxidotungsten(VI) and molybdenum(VI) complexes: experiments and computations

Keywords: Molybdenum, Tungsten, Epoxidation catalysis, Hydrogen peroxide, DFT calculations

Compounds [Cp*2M2O5] (M = Mo, 1; W, 2) are efficient precatalysts for the cyclooctene (COE) epoxidation by aqueous H2O2 in MeCN/toluene. The reaction is

quantitative, selective and takes place ca. 50 times faster for the W system (kobs = 4.32 (9)•10 -4 s -1 at 55°C and 3•10 - 3 M concentration for the dinuclear complex, vs. 1.06 (7)•10 -5 s -1 for the Mo system). The rate law is first order in catalyst and COE substrate (k = 0.138(7) M -1 s -1 for the W system at 55°C), whereas increasing the concentration of H2O2 slows down the reaction because of an inhibiting effect of the greater amount of water. The activation parameters for the more active W systems (H ‡ = 10.2(6) kcal mol -1 ; S ‡ = -32(2) cal mol -1 K -1 ) were obtained from an Eyring study in the 25-55°C temperature range. The H2O2•urea adduct was less efficient as an oxidant than the aqueous H2O2 solution. Replacement of toluene with Et2O did not significantly affect the catalyst efficiency, whereas replacement with THF slowed down the process. The epoxidation of ethylene as model olefin catalysed by the [Cp*MO2Cl] systems (M = W, Mo) in the presence of H2O2 as oxidant and CH3CN as solvent has been investigated by DFT calculation with the use of the conductor-like polarisable continuum model (CPCM). For both metal systems, the rate limiting step is the transfer of the hydroperoxido O α atom to the olefin, in accordance with the first-order dependence on the substrate and the zero order dependence on H2O2 found experimentally in the catalytic data. The activation barrier corresponding to the rate limiting step is 4 kcal lower for the W complex than for the corresponding Mo analogue (32.3 vs. 28.3 kcal mol -1 ). This result reproduces well the higher catalytic activity of the W species. The different catalytic behaviour between the two systems is rationalised by NBO and NPA analyses. Compared to Mo, the W VI center withdraws more electron density from the σ bonding [O-O] orbital and favours, as a consequence, the nucleophilic attack of the external olefin on the σ*[O-O] orbital.

Introduction

Differently from the [Cp*2Mo2O5] compound, for which several aspects of its aqueous organometallic chemistry have been recently investigated by our group, [1][START_REF] Collange | Results and perspectives of high oxdation state organomolybdenum chemistry in water[END_REF][START_REF] Dinoi | [END_REF][4][5] the analogue W complex has been barely investigated in aqueous media. In particular, while high oxidation state cyclopentadienyl Mo-oxido compounds have found large applications in the olefin epoxidation reaction, [6][7][8][9][10][11][12] the corresponding cyclopentadienyl W-oxido derivatives have never been tested in this application to the best of our knowledge. However, the well known catalytic activity of several peroxidotungstate systems, often greater than that of molybdenum analogues, [13] as well as the recent progress related to oxotransferase tungsten-containing enzymes [14][15][16][17][18][19][20] suggest that the investigation of olefin epoxidations catalyzed by organometallic derivatives of the WO2 fragment could be particularly rewarding. A higher catalytic activity of [Cp*2W2O5] compared to the related Mo system has been recently underlined for the oxidation of thiophene derivatives. [21] By using H2O2 as oxidant in MeCN solution, the W compound was found more active than the Mo analogue by a factor of ca. 100.

These results, as well as the improved and simplified synthetic procedure described for the [Cp*2W2O5] complex, [START_REF] Dinoi | [END_REF] encouraged the extension of our investigations to the olefin epoxidation reaction. Concerning the Cp*Mo VI -catalysed olefin epoxidation mechanism, some of us have recently reported a computational study in organic and aqueous media using the Cp*Mo(O)2Cl and [Cp*MoO2] + complexes as models for the catalysts and ethylene as model for the substrate. [22] The computed lowest energy pathway involves the formation of an activated hydroxide hydroperoxido intermediate capable of receiving the exogenous nucleophilic attack by the olefin substrate. The study also provided a possible explanation for the lower activity of peroxido derivatives relative to the oxido analogues, as experimentally observed for the Cp*Mo(O)2Cl and Cp*MoO(O2)Cl complexes, [23,24] as well as for the lower activity of H2O2 relative to tert-butylhydroperoxide (THBP). [6] On the basis of our new experimental results and the previous computational work, we extended the computational mechanistic investigation to the olefin epoxidation process catalyzed by the W system.

Results and Discussion

(a) Experimental investigations

The epoxidation of cyclooctene (COE) was studied in a mixture CH3CN/toluene (4 mL) using H2O2 as oxidant at various temperatures by use of 1 mol % of the bimetallic compounds [Cp*2M2O5] (M = Mo, W) as precatalysts (i.e., cyclooctene/metal = 50). These conditions are similar to those used in a recent contribution dealing with the catalytic action of the same compounds in the oxidation of thiophene derivatives by H2O2. [21] The only difference relative to the previous report is the need of toluene as a co-solvent (25% v/v) in order to insure homogeneous conditions (cyclooctene is not miscible with the H2O2-H2O-MeCN phase). Reaction monitoring by gas chromatography revealed the total consumption of the substrate and the final formation of the corresponding epoxide as the sole terminal product, indicating that the reactions are selective and quantitative. The result for one typical run is shown in Figure 1 for the W precatalyst. Figures for all other kinetic runs are given as Supporting Information. The reactions took place at convenient rates for GC monitoring in the 25-55°C range when using the W catalyst. As [Cp*2Mo2O5] is much less active, only one run was carried out at the highest temperature used for the W analogue. The data fitted well a firstorder rate expression as a function of the substrate concentration, providing the rate constant values collected in Table 1. From an overview of the kinetic data, the W compound is more active than the Mo compound by a factor of ca. 50 (cf. runs 1 and 2). This activity ratio is similar to that found for the related oxidation of thiophene derivatives under analogous catalytic conditions (kW/kMo ~ 100). [21] A remarkable observation is that the Mo system, albeit less active than the W system, still affords selective epoxidation under these conditions. This observation is in stark contrast with previous reports, where cyclooctene oxidation by Cp*Mo VI catalysts was described as efficient only when using tBuOOH as an oxidant in a non aqueous solvent, while the addition of water or the use of H2O2 as an oxidant was reported to give rise to catalyst deactivation. [6] Table The more active W system was investigated in the presence of different pre-catalyst amounts (runs 2-4), confirming the first-order dependence of the rate on the catalyst concentration (see Figure 2). From the slope of the straight line the second-order rate constant at 55°C could be derived, k = 0.138(7) M -1 s -1 . This catalytic system was investigated at several temperatures (runs 2 and 5-7), allowing the determination of the activation parameters through an Eyring analysis, after conversion of the kobs values to the true second-order rate constant values (Figure 3). The analysis gave ΔH ‡ = 10.2(6) kcal mol -1 and ΔS ‡ = -32(2) cal mol -1 K -1 . The large negative activation entropy suggests an associative mechanism with a high level of ordering in the transition state relative to the reactants. The COE oxidation experiment with the W catalyst at 55 °C was run also with different H2O2 concentrations (runs 8-10). These experiments required higher dilution (half concentration for both substrate and catalyst) in order to avoid phase separation upon addition of the greater amount of the H2O2 solution. Given the above-determined first-order dependence of the rate on the catalyst concentration, kobs should decrease by a factor of two for the same [H2O2]/[COE] ratio, which closely corresponded to the experimental observation (cf. runs 2 and 8). If the rate law had either a first or a zero-order dependence on the oxidant, the observed rate constant should increase or remain constant, respectively, when raising [H2O2]. Instead, kobs decreased by a factor of ca. 6 upon quadrupling [H2O2] (cf. runs 8 and 9). A similar phenomenon was previously observed for the catalyzed oxidation of thiophene derivatives. [21] The hypothesis that the larger excess of H2O2 partially decomposes the catalyst in run 9 is not supported by the data, because the kinetic model of the first-order reaction still provides an excellent fit of the data and thus indicates that the catalyst does not significantly change in concentration over the time range of the kinetics experiment. It is possible to rationalize this behaviour by invoking a stabilization of the reactant state through hydrogen bonding with the excess water, or by the involvement of other equilibria, favoured by the excess water, between the precatalyst and other complexes, some of which are less active or inactive. The importance of the amount of water in the system (as opposed to the H2O2 concentration) on the rate was tested in run 10. In the latter, a [H2O2]/[COE] ratio of 2 was maintained, while three additional volume equivalents of pure water were added, in order to have a similar water concentration in runs 9 and 10 (the water amount is actually slightly greater in run 10 because the volume equivalents of the H2O2 solution that are replaced by pure water contain only 70% water). The result (slightly lower kobs for run 10 relative to run 9) confirms that the presence of a big water excess is the cause of the activity decrease, the rate not having a linear dependence on [H2O2].

In an attempt to remove this negative water effect on catalysis, we have tested the stable H2O2•urea adduct as oxidant (run 11). However, the resulting activity is lower by a factor of 36 than that of the aqueous H2O2 solution under the same conditions (run 2). The computational section will further explore this point providing, on the basis of the energy profile calculated in the presence of one explicit urea molecule, a rational explanation of this result. Two final experiments were run in different solvents, replacing toluene with Et2O (run 12) or THF (run 13). A different solvent combination (2:1 v/v) and a slightly greater solvent volume (4.5 mL) were needed in order to maintain the reaction mixture in a single phase. It is interesting to observe that the rate constant in run 12 is essentially the same as that of run 2, whereas that of run 13 is lower by a factor of ca. 2. THF is a stronger proton acceptor in H-bonding than Et2O, [25] thus energetically stabilizing the reactant state. This result seems to confirm the above-formulated hypothesis of a H-bonding stabilization effect by the excess water. We can safely conclude that the rate has a first-order dependence on the substrate and a zeroorder dependence on H2O2. Hence, the substrate is involved in the catalytic cycle between the resting state and the rate-determining transition state, whereas H2O2 is not.

Before turning to the computational part of this study, we wish to remark here the difference between the results shown here and the previous knowledge on the olefin epoxidation catalyzed by high oxidation state organometallics, all these precedents being related only to Mo. [6-12, 23, 24, 26] Non-aqueous solvents were used for all previous studies in conjunction with tBuOOH in decane as oxidant. It has been occasionally mentioned that the presence of water impurities poisons the catalyst, 7 or that the use of H2O2 results in much reduced catalytic activities or selectivities. [6] In this study, we have shown that although excess water indeed reduces the catalytic activity, its presence does not prevent efficient catalysis, especially for the tungsten system. In fact, the aqueous H2O2 solution is more efficient than H2O2•urea in a dry medium.

(b) Computational investigation

Taking into account the above described experimental data, we extended the recently reported computational study on the [Cp*MoO2Cl] catalyst [22] to the corresponding W system. Use of the Cl atom for the calculations on the Mo system was justified because the chloride complex has indeed been used in experimental investigations of epoxidation catalysis, [23] but can also be considered to model the oxido-bridged [Cp*MoO3] -group in the [Cp*2Mo2O5] compound. In the corresponding W version, we continued to use Cl as a model anionic ligand for the sake of comparison, even though the known [27] [Cp*WO2Cl] complex has not been tested as an epoxidation catalyst. The cyclooctene substrate was modelled with the simpler ethylene molecule.

The main goal of this investigation was to rationalize the higher catalytic activity of the W derivative relative to its Mo analogue. Since the metal change is not expected to dramatically affect the catalytic mechanism, we have initially computed for the [Cp*WO2Cl] complex the same path previously outlined for the analogous Mo system. The calculated decrease of the overall activation barrier on going from Mo to W, in agreement with the experiment, will constitute supporting evidence that the mechanism is indeed the same for both systems and a comparison of the calculated and experimentally determined (vide supra) activation parameters will comfort us on the validity of the pathway. In addition, it will be possible to rationalize the metal effect on the catalytic activity upon inspection of the bonding feature at the resting state and rate-determining transition state levels.

The study is mainly divided in five sections. In the first one, we computationally analyze the olefin epoxidation mechanism in the presence of H2O2 as oxidant and CH3CN as solvent, considering the [Cp*WO2Cl] compound as a model for the dinuclear [Cp*2W2O5] complex. The presence of the solvent (CH3CN) has been implicitly considered by performing CPCM continuum model single point and frequency calculations on gas phase optimized geometries. In this approach, the solute molecule, possibly supplemented with some explicit co-solvent water molecules belonging to the first solvation shell, is placed in a cavity surrounded by a polarizable continuum whose reaction field modifies the energy and the properties of the solute. Among the several approaches available for the solvent effect description, continuum models are quite popular [28] since they efficiently account for both the electrostatic and non-electrostatic interaction energy between the solute and the continuum. [29] The second part examines the explicit participation of water molecules in the chemical reaction and in particular the influence of water assistance on the activation barrier heights as well as on the relative energy of the catalytic intermediates. The subsequent section concerns the influence of urea on the energy profile and in particular the presence of a higher energetic barrier for the rate determining olefin epoxidation step. Finally, the fourth and fifth parts computationally rationalize, through an NBO analysis, the different catalytic activities of W and Mo, providing an explanation for the higher activity of the W complex relative to its Mo analogue.

The computational study employed the DFT approach with the B3PW91 functional using Stuttgart-Dresden RECPs in association with their polarized basis sets. This level of calculation differs slightly from the approach used in the previous study. [22] Furthermore, while the previous study addressed the catalytic activity in chloroform and used the single-point CPCM in that solvent, the present work investigates the catalytic behaviour of the complexes in MeCN. For these reasons, the previously reported pathway for the Mo catalyst has been recalculated at the same level as the W system for comparison purposes. In order to check the adequacy of the level of theory used for the coordination properties of W, we first optimized the geometry of the [Cp*2W2O5] complex, whose X-ray crystal structure has been recently reported. [START_REF] Dinoi | [END_REF] A view of the optimized geometry is available in Figure 4 and Table 2 summarizes the main measured and computed geometrical parameters. The remarkable agreement between the experimental and optimized structure traduces the adequacy of the level of theory used to the molecular system studied. The computed energy profile shown in Figure 5 is similar to the one previously reported for the corresponding Mo system. [22] The first step involves the H2O2 activation with protonation of an oxido ligand, via an initial hydrogen-bonded adduct 2 and the transition state ts23. The latter leads to the hydroxido hydroperoxido intermediate 3. The reaction profile continues with the approach of the olefin to make adduct 4 which, after the hydroperoxido O α atom transfer to the olefin via ts45, results in the formation of ethylene oxide and complex [Cp*WCl(O)(OH)2] (6). The latter regenerates the initial [Cp*WO2Cl] compound after water elimination. This last step requires an intramolecular proton transfer from one hydroxide ligand to the other one through transition state ts67. According to this energy profile, the resting state of the catalytic cycle is the Hbonded adduct of the Cp*WO2Cl complex with H2O2, 2, whereas the rate determining transition state is ts45, giving rise to an energy barrier of 28.3 kcal mol -1 . [a] Experimental structural data are from ref. [START_REF] Dinoi | [END_REF] .

(b2) Assisted proton transfer by one water molecule

In order to further probe the mechanistic details of this reaction, especially regarding the intramolecular proton transfer process involved in transition state ts23, one explicit water molecule was added to the system. Water molecules, which are present in solution together with H2O2, can facilitate proton-transfer pathways by shuttling the proton since they can establish H bonds with both the proton donor and the acceptor, as pointed out for other systems. Several theoretical studies have already reported the active participation of water clusters in tautomerisation [30] and protonexchange processes, [5,31] acting as bifunctional catalysts. The energy profile computed with one additional water molecule is shown in Figure 7. In Figure 8, the geometries of the optimized structures are depicted. As shown in Figure 7, the presence of an additional water molecule drives the whole energy profile to a significant energetic stabilization due to the formation of a number of hydrogen bonds. However, the most significant effect is observed at the relative barrier height of ts23a, which is now only 15.8 kcal mol -1 (vs. 23.5 kcal mol -1 without water). This stabilization is related not only to the ability of the additional water molecule to act, at the same time, as a proton acceptor for the donating O-H ligand and as a proton donor for the receiving O-H ligand, but also to the smaller distortion of the O-W-O moiety (wider O-W-O angle of 79.3° in ts23a, see Figure 8, in comparison with the 65.6° value optimized for the ts23 structure, Figure 6). The O-W-O angle in ts23a is much closer to that of the proton transfer product 3a, compared to the rearrangement of ts23 to 3. Interestingly, the proton that originates from the donating HOO-H group has already migrated onto the water molecule in ts23a (HOO•••(HOH2) + bond = 1.127 Å), whereas the proton that eventually ends up in the hydroxido ligand is still bonded to the added water molecule ((H2O-H) + •••O = 1.136 Å). As a result, the transition state is better described as a hydronium ion (H3O + ) interacting with the anionic [Cp*WO2Cl(OOH)] molecule through the establishment of two hydrogen bonds with the oxido and hydroperoxo ligands. The assistance of a water molecule leads also to a stabilization of ts45a. The barrier from the resting state (2a) to the rate-determining transition state (ts45a) decreases only slightly to 27.8 kcal mol -1 , relative to 28.2 kcal mol -1 for the calculated pathway without added water. For the corresponding CPCM single point enthalpy H, the value of 12.7 kcal mol -1 for the ratedetermining transition state (ts45a) has been obtained.

The calculated energy difference associated with the H2O2 activation pre-equilibrium (between systems 2 and 3) is of special interest. The experimental evidence (absence of a first order dependence of the rate law on the H2O2 concentration, cf. runs 9 and 10 in Table 1) suggests that this pre-equilibrium is shifted toward the hydroperoxido intermediate 3 (i.e. this is the real dormant state of the catalytic cycle) whereas an equilibrium shifted toward 2 would entail a first order dependence in H2O2. The decrease of the energy cost needed to go from 2 to 3, 10.3 kcal mol -1 , after adding a water molecule (8.3 kcal mol -1 on going from 2a to 3a) shows a trend in the correct direction. Under the real conditions of catalysis, a greater amount of water probably interacts with species 2 and 3, further tipping the balance in favour of the hydroperoxido intermediate.

(b3) Effect of urea

As shown by the experimental studies, the substitution of water by urea led to a decrease of the reaction rate by a factor of 36. Taking into account the stronger binding ability of urea as proton acceptor for H2O2 relative to water, we firstly compared the ability of urea and water in stabilizing the reactant mixture. In a first approximation, we optimised the geometry of the H2O2-H2O and H2O2-NH2CONH2 adducts (even though both can form extended hydrogen bonding networks in the presence of more solvent molecules) and we calculated the ΔG° of the exchange reaction reported in Equation 1.

H2O2•H2O + NH2CONH2 → H2O2•NH2CONH2 + H2O (1)
The exoergonicity of this reaction (-2.66 and -3.22 kcal/mol in the gas phase and in CH3CN solution, respectively) indicates that the H2O2•NH2CONH2 system is more stable than the corresponding H2O2•H2O one. Among the different H2O2•NH2CONH2 and H2O2•H2O optimised structures, the most stable ones are those shown in Figure 9. [32,33] Both adducts exhibit a cyclic structure that involves two hydrogen bonds. The higher stability of the H2O2•NH2CONH2 adduct is due to its stronger hydrogen bonds, as illustrated by the hydrogen bond lengths and angles reported in Figure 9. The two hydrogen bonds of the H2O2•NH2CONH2 system present length and angle values (1.96, 158° and 1.73, 171° respectively) compatible with high-strength hydrogen bonds (1.8 Å and 180°). In the H2O2•H2O adduct, on the other hand, due to the rigidity of the five-membered ring, the bond lengths increased as the angles decreased (2.11, 125° and 1.87, 145° respectively) resulting in hydrogen bond interactions of lower strength compared to those of the H2O2•NH2CONH2 system. The lower catalytic activity observed in the presence of the H2O2•urea adduct, therefore, may be related to the stronger binding ability of urea for H2O2 relative to water, stabilizing the reactant mixture by a value of -3.2 kcal/mol, relative to the rate-determining transition state of the catalytic cycle. In order to verify this point, further probing the mechanistic details of the reaction, the energy profile was calculated in the presence of one explicit urea molecule (Figure 10). Geometries of the optimized structures are depicted in Figure 11. As shown in Figure 11, differently from the water behavior, the urea molecule does not facilitate the proton transfer in the transition state ts23b which requires the same geometry strain previously observed for the water free [Cp*WO2Cl] catalyzed cycle reported in Figure 6 (O-W-O = 64.9° in ts23b vs. O-W-O = 76.9° in 3b). The activation energy barrier corresponding to this step (25.2 kcal mol -1 ) is very close to that computed for the water free W system in Figure 5 (23.5 kcal mol -1 ). In complex 3b, the hydroperoxo β-O atom is significantly oriented toward the metal center (W-O β = 2.398 Å, W-O α = 2.017 Å), forming a strained three-membered WOO(H) cycle with a W-O α -O β angle of 85.7°. Concerning the rate-determining step, the activation barrier from the resting state (2) to the oxygen transfer step (ts45) is greater in the presence of urea (31.5 kcal mol -1 ) than in the presence of water (27.8 kcal mol -1 ). This result is in accordance with the experimentally observed lower reactivity of the tungsten-H2O2-urea system compared to the corresponding water analogue.

While the H2O2•urea compound stabilizes the reactant mixture by ca. 3.2 kcal/mol compared to the H2O2•water system, the adduct 2-1b, relative to which the ratedetermining barrier is measured, is 3.7 kcal/mol higher than the corresponding 2a water complex. The stabilizing effect of urea is thus lost during the coordination of the H2O2-urea adduct to the metal, indicating that the higher energetic barrier obtained in the presence of urea is not a consequence of the stabilisation of the reactants but it is rather due to the destabilisation of the transition state ts45b relative to the water containing ts45a. In order to directly compare the energetic barriers of the Mo and W metals and to better understand the effect of the metal on the catalytic activity, the energy profile computed for W has been recomputed for Mo at the same level of theory (functional, basis set and solvent model, Figure 12). The geometries of the corresponding structures are available as supporting information (Figure S1). In accordance with the experimentally observed lower reactivity of the molybdenum hydroperoxo complex, the rate-determining barrier from the resting state (2c) to the oxygen transfer step (ts45) is greater for Mo (32.3 kcal mol -1 ) than for W (28.3 kcal mol -1 ). A plausible explanation for this behavior and for the specific role of the metal center in the activation/deactivation of the hydroperoxo group will be addressed in the following section. The geometries as well as the bonding parameters of the optimized Mo complexes are very similar to those of the W analogues described in Figure 6. The only significant geometrical changes on going from the W to the Mo system concern the M-O α -O β moiety in 3/3c and ts45/ts45c. On going from 3b to ts45c, the Mo-O α distance lengthening (to 2.081 Å), the Mo-O β distance shortening (to 2.166 Å), and the Mo-O α -O β angle closing (to 68.2°) are more pronounced than in the case of W (the corresponding values being 2.072 Å, 2.176 Å and 68.8°), in agreement with a slightly earlier transition state for the W system.

(b5) Natural Bond Orbital (NBO) analysis of rate-determining transition states.

The greater activity of tungsten complexes relative to the Mo analogues in the epoxidation reaction has been noted earlier but only limited attention to this phenomenon has been devoted by theoretical studies. [34,35] In order to explain the factors that govern the activity of transition metal hydroperoxido intermediates in olefin epoxidation, and in particular the influence of the metal in activating or deactivating the hydroperoxido group, we carried out an NBO analysis on the [Cp*M(O)(OH)Cl(OOH)(C2H4)] (M = W, ts45; Mo, ts45c) transition states. All the possible interactions between "filled" (donor) Lewis-type NBOs and "empty" (acceptor) non-Lewis NBOs have been considered and quantified according to their energetic contribution computed at the 2 nd -order perturbation analysis.

Concerning the electrophilic character of the oxygen transfer, the NBO analysis reveals a strong overlap between the ethylene π(C-C) and the hydroperoxido unoccupied O-O antibonding orbital, σ*(O-O), for both ts45 and ts45c. The back donation from one of the O α lone pairs to the π*(C-C) has a much lower contribution to the interaction, demonstrating the electrophilic nature of the oxygen transfer process in the present epoxidation reaction. This is in accordance with previously reported DFT calculations on Mo and W peroxo and diperoxo complexes. [34][35][36][37][38][39][40] Another important contribution to the higher catalytic activity of the W derivative compared to that of Mo belongs to the β-O atom coordination of the O α O β (H) moiety to the metal center. For both metals, the NBO second order perturbation analysis reveals a strong overlap between a lone pair localized on the β-O atom and the formally empty d and p orbitals of the metal. This interaction is stronger for W than for Mo (161.3 vs. 64.2 kcal mol -1 ), in agreement with the well known higher oxophilic character of W. Hence, the incipient bond formation between the metal and the β-O atom withdraws electron density from the bonding [O-O] orbital, lowering consequently the σ* [O-O] orbital energy and rationalizing the earlier transition state for W than for Mo, as suggested by the bond parameters (vide supra). On the basis of previous works [37,38] and our NBO analysis, the transition state can be described as two fused three-membered rings in which the metalladioxirane MO α O β (H) moiety plays a key role in the transfer of the hydroperoxido O α atom to the olefin (Figure 13). The nature of the metal center strongly influences the electrophilic character of the [O-O] moiety, controlling indirectly the hydroperoxido O α atom transfer to the olefin. In order to correlate the metal oxophilicity with the charge distribution in the [M-O α -O β (H)] fragment, we carried out a partial charge analysis (NPA) of the two transition states and the two olefin adducts, [Cp*M(O)(OH)Cl(OOH)•••(C2H4)] (M = W, 4; Mo, 4c) that precede them (Figure 13 and Table 3). In agreement with the participation of the metal to the olefin epoxidation reaction, the electrophilic character of the [M-O α -O β ] fragment has to be related not just to the O α charge (almost identical for both compounds), but rather to the sum of the partial charges at the metalladioxirane moiety, q(M) + q(O α ) + q(O β ). [37] On going from Mo to W, while the charge at the metal center increases, the charges at the hydroperoxo oxygen atoms are almost identical. The comparison between the Mo and W systems, therefore, clearly indicates that the [W-O α -O β ] fragment is more positive by +0.2 electrons than the Mo analogue. This result well matches the higher electrophilicity of the W-based metalladioxirane system as well as its higher catalytic activity observed both experimentally and computationally.

Conclusions

The present study constitutes the first report of the activity of a cyclopentadienyltungsten derivative as an epoxidation catalyst. Consistent with a recent report dealing with the oxidation of thiophene derivatives by H2O2 in MeCN, [Cp*2W2O5] has a much greater activity than its Mo analogue under identical conditions (by a factor of 50 in COE epoxidation vs. 100 in the oxidation of benzothiophene). The results of a comparative computational study of the epoxidation pathway for the W and Mo systems agree with the experimental evidence of the greater activity for the W system.

The assistance of an additional water molecule drives the whole energy profile to a significant energetic stabilization, lowering in particular the relative barrier height of the H2O2 activation step. The lower catalytic activity obtained by replacing aqueous H2O2 with the H2O2•urea reagent was rationalized on the basis of a transition state destabilization effect, related to the ability of urea to establish stronger hydrogen bonds with H2O2. Finally, the NBO and NPA analyses have rationalized the greater activity of the W-based catalyst on the basis of the stronger electron density withdrawal from the σ bonding [O-O] orbital, consequently lowering the energy of the σ* [O-O] orbital which is susceptible to the nucleophilic attack by the external olefin.

Experimental Section

Materials and Instrumentation. Cyclooctene (Ega-Chimie), cyclooctene oxide (Fluka), dodecane (Aldrich), and H2O2 (30% in water, Fluka) were used as received. Compounds [Cp*2M2O5] (M = Mo, 1; W, 2) were prepared as described in the literature. [START_REF] Dinoi | [END_REF] MeCN (SDS, synthesis grade) and Toluene (VWR Prolabo) were used as received. The gas chromatographic analyses were carried out with a Fisons 8000 Series instrument equipped with a SPB-5 capillary column.

General procedure for the catalytic runs. The reactions were carried out in a Schlenk tube with magnetic stirring and held at the chosen temperature by immersion in a thermostated oil bath. The typical experiment was run by charging the tube with the catalyst (1.2•10 -2 mmol) followed by 4 mL of a MeCN/toluene mixture (3/1 v/v), 1.2 mmol of COE (132 mg, 156 L) and the internal standard (dodecane, ca. 119 mg, 160 L, 0.7 mmol). The solution was then warmed to the chosen temperature, followed by dropwise addition of the aqueous H2O2 solution (273 L, 2.4 mmol) in 2-3 minutes. The reaction progress was monitored by gas chromatographic analysis of periodically withdrawn samples (ca. 0.2 mL), after quenching the excess H2O2 with MnO2, filtering and diluting with ca. 0.2 mL of diethyl ether. This allowed the independent measurement, for each sample, of the final epoxide product and the residual COE.

Computational details. Calculations were carried out using Gaussian 03 package at the DFT level by means of the hybrid density functional B3PW91. [41,42] For the W, Mo [43,44] and Cl [45] atoms, the Stuttgart-Dresden pseudopotentials were used in combination with their associated basis sets augmented by a set of polarization functions (f-orbital polarization exponents of 1.043 and 0.823 for Mo and W respectively, [46] whereas a dorbital polarization exponent of 0.643 for Cl [47] ). For the C, O and H atoms the all electron 6-311G(d,p) [48] basis sets were used. The nature of the optimized stationary point, minima or transition state, has been verified by means of analytical frequency calculation at 298.15 K and 1 atm. The geometry optimizations have been achieved without any geometrical constraints. IRC calculations were carried out in order to confirm the connectivity between reactant(s), transition state and product(s). Solvent effects were included implicitly by means of CPCM single point calculations in CH3CN (εr = 36.64). The energy data presented correspond to the free enthalpy of the computed compounds in which thermal, vibrational, electrostatic and non-electrostatic contributions have been included. The contribution of the cavitation energy to the solute-solvent interaction energy has been recalculated by the formula of Tunon, [49,50] which more rigorously considers the geometry of the cavity and the occupancy of the solvent molecules inside the cavity (for CH3CN at 25°C γ = 0.0412 kcal Å -2 , Vs = 40.31 Å 3 , and ρ = 0.015 molecules Å -3 ). The calculation has been performed using the Pauling atomic radii with explicit hydrogen atoms and the solvent accessible surface (SAS) cavity model, which better reproduces the border line between the solute and the solvent. Within the CPCM approximation, the translational and rotational modes are those of the gas phase. The electron density and partial charge distribution in selected transition states were examined in terms of localized electron-pair bonding units by using the NBO program. [51] Through this method, the input atomic orbital basis set is transformed via natural atomic orbitals (NAOs) and natural hybrid orbitals (NHOs) into natural bond orbitals (NBOs), which correspond to the localized one center ("lone pair") and two-center ("bond") elements of the Lewis structure. All possible interactions between "filled" (donor) Lewis-type NBOs and "empty" (acceptor) non-Lewis NBOs orbitals, together with their energetic quantification (stabilization energy), has been obtained by a second order perturbation theory analysis of the Fock matrix, as indicated in the NBO program. Only stabilization energy higher than 10 kcal mol -1 has been considered. The NBO program has also been used to perform the natural population analysis (NPA), which affords the natural atomic charges (nuclear charge minus summed natural populations of NAOs on the atom) and total core, valence and Rydberg populations on each atom.

S 1

Entry for the Table of Contents

Figure 1 .

 1 Figure 1. Time dependence of the molar fraction of substrate (diamonds) and epoxide (squares) for the COE oxidation catalyzed by [Cp*2W2O5] using H2O2 in MeCN/Toluene 3/1 at 55°C. The lines are the corresponding fits to first order kinetics (see text).

  [a] Solvent = MeCN/Toluene (3:1 v/v) unless otherwise stated. [b] Additional water (3 volume equivalents) was added to the aqueous solution containing 2 mole equivalents of H2O2 to match the total volume of run 7 (8 mole equivalents of H2O2). [c] H2O2•urea was used as oxidant. [d] Solvent = MeCN/Et2O (2:1 v/v). [e] Solvent = MeCN/THF (2:1 v/v).

Figure 2 .Figure 3 .

 23 Figure 2. Dependence of observed rate constant on the pre-catalyst concentration.

(

  b1) Olefin epoxidation with H2O2 in acetonitrile catalyzed by the [Cp*WO2Cl] system.

Figure 4 .

 4 Figure 4. DFT optimized structure of the dinuclear [Cp*2W2O5] complex.

Figure 5 .

 5 Figure 5. CPCM-corrected free energy profile (in kcal mol -1 ) for the H2O2 activation and C2H4 epoxidation by [Cp*WO2Cl] in CH3CN solution. The reference energy corresponds to the separate reagents ([Cp*WO2Cl] + H2O2 + C2H4).

Figure 6 .

 6 Figure 6. Optimized geometry of systems ts23, 3, 4 and ts45.

Figure 7 .

 7 Figure 7. CPCM-corrected free energy profile (in kcal mol -1 ) for the H2O2 activation and C2H4 epoxidation assisted by one water molecule in CH3CN solution. The reference energy corresponds to the separate reagents ([Cp*WO2Cl] + H2O2 + C2H4 + H2O).

Figure 8 .

 8 Figure 8. Optimized geometry of systems ts23a, 3a, 4a and ts45a.

Figure 9 .

 9 Figure 9. Optimised structures of the most stable H2O2•NH2CONH2 and H2O2•H2O adducts.

Figure 10 .

 10 Figure 10. CPCM-corrected free energy profile (in kcal mol -1 ) for the H2O2 activation and C2H4 epoxidation assisted by one urea molecule in CH3CN solution. The reference energy corresponds to the separate reagents ([Cp*WO2Cl] + H2O2•NH2CONH2 + C2H4).

Figure 11 .

 11 Figure 11. Optimized geometry of systems 2-1b, 2-2b, ts23b, 3b, 4b and ts45b.

( b4 )

 b4 Olefin epoxidation with H2O2 in Acetonitrile catalyzed by the [Cp*MoO2Cl] system, 1c.

Figure 12 .

 12 Figure 12. CPCM-corrected free energy profile (in kcal mol -1 ) for the H2O2 activation and C2H4 epoxidation by [Cp*MoO2Cl] in CH3CN solution. The reference energy corresponds to the separate reagents ([Cp*MoO2Cl] + H2O2 + C2H4).

  In addition, the comparison of the C2H4→ [O-O] charge donation for the Mo and W transition states ts45 and ts45c, shows that the nature of the metal does not directly influence the stabilization energy associated to this interaction. To gain further insights in the driving force of the reaction, we have then analyzed the M-[O-O] orbital interaction. The NBO analysis reveals a strong donation from the occupied σ bonding [O-O] orbital to the formally empty d and p metal orbitals. The involvement of the metal d and p orbitals reduces the bonding character of the σ[O-O] orbital and thus leads to a weakening of the O-O bond. At this point it is important to understand how these interactions between the metal center and the hydroperoxido group vary for different metal centers. By comparing the [O-O] → [M] charge donation in both Mo and W transition states, we have observed that the corresponding stabilization energy of the donoracceptor interaction computed at the NBO second order of perturbation is considerably higher for W than for Mo, the associated stabilization energy measuring 82.0 and 31.0 kcal mol -1 respectively. The W atom, therefore, is likely to attract a higher electron density from the σ bonding [O-O] orbital, affording a bigger weakening of the O-O bond and consequently lowering the energy of the σ* [O-O] orbital which is susceptible to the nucleophilic attack by the external olefin.

Figure 13 .

 13 Figure 13. The nature of the [Cp*M(O)(OH)Cl(OOH)•••(C2H4)] adducts (M = W, 4; Mo, 4c) and [Cp*M(O)(OH)Cl(OOH)(C2H4)] transition states (M = W, ts45; Mo, ts45c).

Table 2 .

 2 Comparison of the DFT-optimized geometry (main distances [Å], angles [°])for the [Cp*2W2O5] complex with the X-ray structure of the same compound. a

	Bond distances X-Ray (Å) DFT (Å)	Angles	X-Ray (°) DFT (°)
	W(1)-O(1)	1.881(6)	1.900	O(12)-W(1)-O(11) 105.2(3)	106.04
	W(1)-O(11)	1.729(6)	1.724	O(12)-W(1)-O(1)	105.5(3)	104.55
	W(1)-O(12)	1.727(6)	1.732	O(11)-W(1)-O(1)	104.6(3)	105.67
	W(1)-CT1	2.0993(3) 2.131	O(22)-W(2)-O(21) 105.7(4)	106.03
	W(2)-O(1)	1.892(6)	1.902	O(22)-W(2)-O(1)	104.2(3)	104.55
	W(2)-O(21)	1.711(7)	1.724	O(21)-W(2)-O(1)	105.7(3)	105.60
	W(2)-O(22)	1.732(7)	1.731	W(1)-O(1)-W(2)	170.1(4)	169.69
	W(2)-CT2	2.0957(3) 2.131

Table 3 .

 3 NPA partial charges of the two transition states and adducts represented in Figure13.

	N	Metal	q(M)	q(O α )	q(O β )	Sum
	4	W	0.97	-0.35	-0.41	0.21
	4c	Mo	0.75	-0.32	-0.41	0.01
	ts45	W	0.93	-0.40	-0.56	-0.03
	ts45c	Mo	0.70	-0.39	-0.56	-0.24

Acknowledgements

We are grateful to the Institut Universitaire de France and the Laboratoire Européen Associé LTPMM (financed by the Centre National de la Recherche Scientifique) for funding and to the Centre Interuniversitaire de Calcul de Toulouse (CICT, Project CALMIP) for granting free CPU time. This work was also performed using HPC resources from GENCI-[CCRT/CINES/IDRIS] (Grant 2009-[c2009086188]). We also gratefully acknowledge a ENASARCO fellowship to CD.


Chiara Dinoi

M Fátima

Elisabete C B C Guedes Da Silva

A Alegria

Luísa M D Piotr Smoleński

R S Martins

Rinaldo Poli

Armando J L Pombeiro
email: pombeiro@ist.utl.pt

] M Fátima

Elisabete C B A Alegria

Luísa M D R S Martins

Molybdenum Complexes Bearing the Tris(1-pyrazolyl)methanesulfonate Ligand: Synthesis, Characterization and Electrochemical Behaviour

Keywords: Mo complexes bearing Tpms Mo O Molybdenum, tris(1-pyrazolyl)methanesulfonate ligand, scorpionate, crystal structure, carbonyl complexes, electrochemistry, high oxidation state

come

Introduction

Facially coordinating tridentate nitrogen donor ligands have experienced a rapid development in the last years, and namely tris(pyrazolyl)borates constitute a highly successful type of ligands, forming complexes with applications in various fields such as catalysis, metal extraction and bioinorganic modelling. [1][START_REF] Pettinari | Scorpionates II -Chelating Borate Ligands[END_REF][3][START_REF] Garcia | [END_REF][5][6][7][8][9][10][11] Their success results largely from the ability to tune steric [1,12] and electronic [12,13] properties. Since Trofimenko synthesized the poly(1pyrazolyl)borate ligands, [14,15] they have experienced a growing application, becoming the most commonly used tripodal ligands. However, they have the disadvantage of being insoluble in water and unstable towards hydrolysis. [1,10] In order to obtain complexes that are soluble and stable under physiological conditions and that could therefore be used as better enzyme models, Kläui and co-workers reported [25] the synthesis of the novel tris(1pyrazolyl)methanesulfonate (Tpms) ligand that relates to the conventional hydrotris(1pyrazolyl)borate (Tp) species (Figure 1). Both can act as a monoanionic, C3v-symmetrical Ndonor ligands, but instead of the boron hydride unit of Tp, Tpms bears a methanesulfonate group which affords good stability towards hydrolysis and increased solubility in polar solvents, e.g. water and methanol. In spite of these useful features, the coordination chemistry of Tpms remains little explored.

Hence, also in view of our interest on the synthesis of Tpms complexes with diverse transition metals, namely V, [26] Re, [27] Fe [26] or Cu [26,28] , and their application in catalysis, [26,29,30,31] and in pursuit of our long standing studies on the chemistry of molybdenum, [32,33] we decided to investigate the unexplored Tpms-Mo coordination chemistry. The main goals of the current study are as follows: i) extend the promising coordination chemistry of Tpms to Mo; ii) investigate the structural role of the sulfonate group in the resulting complexes; iii) and compare the geometrical and electronic properties of the Tpms-Mo complexes with those of the Tp analogues.

Several Mo complexes with the neutral unfunctionalized hydrotris(1-pyrazolyl)methane Tpm [Tpm = HC(pz)3] and its substituted derivatives are known. [5,6] The first one, [Mo(Tpm)(CO)3], has been obtained from the reaction of [Mo(CO)6] with Tpm [34] and shown to be a versatile starting material. Several mononuclear and binuclear molybdenum compounds with the methyl substituted [HC (3,5-Me2pz)3] ligand were also reported. [35] The oxidation of [{HC (3,5-Me2pz)3}Mo(CO)3] by various oxidants, such as SOCl2, Br2, I2, and HNO3, was described, affording mononuclear Mo III and Mo VI complexes of the type [{HC (3,5-Me2pz)3}MoX3] (X = Cl, Br, I, and O). [35] Dimolybdenum complexes possessing the thermodynamically stable (Mo2O4) 2+ core were also obtained. [35] More recently, the syntheses of the molybdenum(0) [Mo(CO)3(L3)], [Mo(CO)4(L2)],

[Mo(CO)3(NCMe)(L2)] and molybdenum(VI) [MoCl2O2(L2)] complexes were performed with the tridentate (L3) and bidentate (L2) poly(pyrazolyl)methane ligands (Gn-dend)OCH2C(pz)3 (L3) and (Gn-dend)CH(3,5-Me2pz)2 (L2) [Gn-dend = poly(benzylether) dendron]. [36] Reactions of [MoO2Cl2(thf)2] with potential mono-and bi-dentate pyrazole type ligands lead to octahedral complexes of formula [MoO2Cl2(L1)2] and [MoO2Cl2(L2)] (L1 = 3,5-Me2pz; L2 = Me2C(pz)2), [36] whereas reactions of [MoO2X2(thf)2] (X = Cl or Br) with potential tridentate ligands, e.g. tris(pyrazolyl)methane or tris(pyrazolyl)borate, lead to the replacement of both co-ordinated solvent molecules and one of the halogen ligands to give [MoO2X(L3)]X [X = Cl or Br, L3 = HC(pz)3, HC(3,5-Me2pz)3]. [36] These complexes were shown to catalyse cyclooctene epoxidation with t BuOOH. [37] In addition, oxidation of [Mo(Tpm)(CO)3] by I2 or Br2 gave rise to the seven-coordinated [Mo(Tpm)X(CO)3]X derivatives (X = I or Br), whereas protonation with HBF4 afforded the hydrido complex [Mo(Tpm)H(CO)3][BF4]. [38] In contrast to the above described Tpm Mo derivatives, the reactivity with Mo of the related anionic sulfonate functionalized species, i.e. the tris(1-pyrazolyl)methanesulfonate (Tpms),

had not yet been investigated, to our knowledge. This has encouraged us, within the above goals of the present study, to prepare the first Mo-Tpms compounds, to examine their properties and, when appropriate, to compare them with those of the neutral Tpm analogues.

Results and Discussion

Synthesis and characterization of Li[Mo(Tpms)(CO)3] (1) and its conversion to [Mo(Tpms)(CO)2(-CO)Li(thf)2]n (2)

Reaction of the lithium salt LiTpms [Tpms = SO3C(pz)3] with [Mo(CO)6] in refluxing CH3CN afforded Li[Mo(Tpms)(CO)3] (1), isolated in high yield as a yellowish-green powder upon concentration of the reaction solution (Scheme 1). Compound (1) is air-sensitive, in particular in solution, and is readily soluble in water (S25ºC  16 mg•mL -1 ) and in other polar solvents such as MeOH, CH3CN, thf, CHCl3 and CH2Cl2. The 1 H and 13 C NMR spectra of (1) in CD3OD are indicative of a C3v symmetry, with the Tpms ligand coordinated to the Mo atom through the three equivalent pyrazolyl groups. Its solid-state infrared spectrum shows strong (CO) bands at 1916 and 1756 (broad) cm -1 . These values are considerably lower than those for [Mo(CO)6], 1990 cm -1 , [39] consistent with a greater extent of Mo ), the hydrotris(1-pyrazolyl)borate and cyclopentadienyl analogues, respectively, reveals that the CO IR frequencies decrease in the order Tpms  Cp  Tp (e.g. 1900, 1796, 1747 cm -1 for [Mo(Cp)(CO)3] -, [40] and 1890, 1750 cm -1 for [Mo(Tp)(CO)3] - [41] ), suggesting the opposite order of the electron-donor character of those ligands, i.e. Tp  Cp  Tpms.

The electrospray ionisation (ESI) MS spectrum of compound 1 in CH3CN (Figure S1 in Supplementary information), in the negative mode, shows a very clean isotopic pattern centred at m/z = 475, which agrees with the theoretical isotopic distribution of [Mo(Tpms) (CO)3] -. The same pattern is also observed in CH2Cl2. Selected angles (º): O11-Li1-O22, 112.3 (11); N11-Li1-N24, 107.1 (10); O11-Li1-N11, 94.6(9); O22-Li1-N24, 94.9(9); O22-Li1-N11, 126.4(12); O11-Li1-N24, 124.4(12); O13-Li2-O21, 116.2(11); N21-Li2-N14, 104.5(9); O13-Li2-N14, 96.7(8); O21-Li2-N21, 92.2(8); O13-Li2-N21, 126.0(9); O21-Li2-N14, 123.5 (9). Symmetry codes to generate equivalent atoms: i) x,1+y,z; ii) x,-1+y,z.

Upon recrystallization of (1) from thf/Et2O at 0°C, green crystals of the new compound

Reactions of Li[Mo(Tpms)(CO)3] (1) A) With I2

The controlled oxidation of complex (1) with 1 equiv. of iodine gave a reddish-brown product formulated as [Mo(Tpms)I(CO)3] (3) on the basis of IR, 1 H NMR and 13 C NMR spectroscopies. Complex 3 is diamagnetic, air sensitive and readily soluble in chlorinated hydrocarbons, CH3CN, thf and also in water (S25ºC  5 mg•mL -1 ), not considerably soluble in alcohols, diethyl ether and hydrocarbon solvents. In accordance with a Cs symmetry, the infrared spectrum of (3) in the solid state shows three carbonyl stretching bands (2 A' + A'') at 2039, 1966 and 1944 cm -1 . They are shifted to higher frequencies compared with those of the starting Mo 0 tricarbonyl complex, reflecting the decrease in electron density at the central metal atom as a result of the oxidation. This pattern is consistent with either a 4:3 "piano stool" structure [38,41] or a 3:3:1 structure with axial CO [39] (Scheme 2 a) and b), respectively). Both these possibilities have already been reported for related complexes, such as [Mo(Tp)Br(CO)3] [41] and [Mo(Tpm)Br(CO)3] [38] (4:3 "piano stool" structure) or [Mo(Tp iPr )I(CO)3] [42] A comparison with the hydrotris(1-pyrazolyl)borate analogue [Mo(Tp)I(CO) 3 ] reveals that the Tpms complex exhibits (CO) frequencies higher than those of the latter compound (2016, 1950, 1910 cm -1 ), [41] also in accordance with the lower electron-donor character of Tpms relative to Tp.

The 1 H NMR analysis of complex (3) shows one set of equivalent pyrazolyl rings, therefore indicating a fluxional process on the NMR time scale. The fluxionality of 7-coordinated molybdenum(II) and tungsten(II) complexes is well known. [38,41,42,43] Slow diffusion of Et2O into a saturated CH3CN solution of complex (3) afforded reddishbrown crystals suitable for X-ray diffraction study. The data revealed the tetraoxo Mo The asymmetric unit of 4 comprises half of the complex molecule and one CH3CN molecule.

The binuclear complex contains a crystallographically imposed twofold axis which passes through the centre of the Mo1-Mo1 i bond, perpendicularly to the central Mo2O2 plane. The molecular geometry is that of a typical edge-sharing bioctahedron, with two terminal oxo ligands placed on the same side (syn) of the Mo2O2 place, and two bridging O-atoms, displaying the well-known (Mo2O4) 2+ bridging structure found in many compounds. [44][45][46][47][48] The and 1.438(2) Å). The Mo-Mo distance is 2.5545(4) Å, similar to that in related Mo V dinuclear compounds, [46,48,49] and is consistent with a metal-metal single bond. The disparity in the terminal and bridging molybdenum-oxygen distances (Figure 4) is also found in other Mo2O4

anionic
structures. [49] The O2-Mo1-N14 i ) [50] and 152° in [Mo(Cp)O2]2 [51] ] and was shown to allow the formation of the Mo-Mo bond without excessively distorting the Mo-O-Mo angles in the bridge, which usually take values very close to the 82.50 (7)º angle found in our case (83.3° in [Mo(Cp*)O 2 ] 2 [50] and 84.2° in [Mo(Cp)O2]2 [51] ).

The structure of ( 4

B) With HBF4

Complex 1 is readily protonated with HBF4 in dry acetonitrile to give the hydrido species [Mo (Tpms)H(CO)3] (5) which was isolated in good yield as a green powder. The compound is significantly air-sensitive as a solid and highly sensitive in solution, turning immediately to black in the presence of O2. It is sparingly soluble in water (S25ºC  1 mg•mL -1 ). The three carbonyl stretching absorptions in the solid state IR spectrum (2025, 1945 and 1912 cm -1 ) are indicative of a Cs 7-coordinated structure, as observed for the previously described complex

(3) and other related hydride complexes involving a facially coordinating ligand. [41,[52][53][54] The lower (CO) values of (5) as compared to those of (3) reflect the stronger electronegativity of iodide relative to hydride and therefore the higher metal electron density in the latter, as observed for similar systems. [38,41,52,55] In contrast, 5 displays higher (CO) values than the Tp analogue [Mo(Tp)H(CO)3] (2000, 1906, 1887 cm -1 , in THF), [41] consistent with the weaker electron-donor ability of Tpms in comparison with Tp (see above). The 1 H NMR spectrum (CDCl3) exhibits one set of equivalent pyrazolyl rings, suggesting a fluxional behaviour. The chemical shift for the hydridic proton (-3.23 ppm) is similar to those reported for [Mo(Tpm)H(CO)3][BF4] ( -3.12) [38] and analogous Tp compounds. [41,[52][53][54] By slow evaporation of a chloroform solution of 5, reddish-brown crystals were obtained. The X-ray diffraction analysis disclosed the structure of the oxidized [Mo(Tpms)OCl]2(-O) complex 6 (Scheme 1) which crystallized with 6 chloroform molecules. An ORTEP drawing of complex ( 6) is shown in Figure 5. Due to the rather poor quality of this structure, only a restricted discussion is presented. The asymmetric unit contains half of the molecule, with the bridging O2 atom sitting on an inversion centre, which imposes a perfect linearity to the Mo-O-Mo group. The molecule includes two terminal oxygen atoms, displaying the well-known (Mo2O3) 4+ moiety. Each Mo V centre adopts a pseudo-octahedral coordination geometry with the anionic tridentate Tpms ligand bonding facially the metal atom. A similar structure containing the related hydrotris(1-pyrazolyl)borate ligand has been previously described. [56] Figure 5. An ORTEP view of [Mo(Tpms)OCl]2(µ-O).6CHCl3 (6.6CHCl3). The ellipsoids are drawn as 30% probability; the chloroform molecules and hydrogen atoms are omitted for clarity. Symmetry code to generate equivalent atoms: (i) 1-x,2-y,z.

C) With AgBF4

Reaction of compound 1 with 1 equiv. of AgBF4 in thf gave a reddish-brown powder which, upon attempted crystallization by slow diffusion of Et2O into a thf solution afforded yellow crystals of a product (7) suitable for X-ray diffraction analysis. Compound 7 is the oxo-Mo VI complex [Mo(Tpms)O2]2(-O) which crystallized with one molecule of thf. The two anionic tris(1-pyrazolyl)methanesulfonate groups in the molecule (Figure 6) act as  3 :N,N,N ligands, bonding facially the Mo VI centres which display the well-known (Mo 2 O 5 ) 2+ bridging structure found in many related cyclopentadienyl containing compounds. [57][58][59][60] Each molybdenum atom bears two terminal oxo-ligands and displays a pseudo-octahedral geometry, with the two heavily distorted octahedron sharing the O2 atom as common vertices. Such a distortion may be a consequence of the large intramolecular repulsion between each pair of oxo-ligands (O1-Mo1-O3 and O4-Mo2-O6 angles of 104.47( 16)° and 104.87 (15)º, respectively). The deviation of the Mo1-O2-Mo1 angle [176.14( 16)°] from linearity can be ascribed to packing forces in the crystal, as it has already been recognized in other cases. [57,58] The Mo=O and Mo (3) or 0.51 (5) V vs. Fc/Fc + , respectively.

In complex 1, the second oxidation process is assigned to the further oxidation yielding the 16-electron [Mo(Tpms)(CO)3] + complex of Mo II , its irreversibility being probably associated to fast coordination of a solvent molecule, yielding an electronically saturated product. The irreversibility of the first oxidation wave of compounds ( 3) and ( 5) signals the instability of the resulting cationic Mo III complexes, which then rapidly decompose with probable CO loss [61] and, for the hydride compound (5), by deprotonation. [62][63][64][65] The first oxidation potentials of all these tricarbonyl complexes are much lower than that (0.98 V vs. Fc/Fc + ) of the parent hexacarbonyl compound, on account of the replacement of three carbonyls in the latter by other more electron-donating ligands. [66][67][68][69] The lower first oxidation potential of 5 in comparison with 3 reflects the stronger electron-donor character of the hydride relatively to the iodide ligand. [68] The first oxidation potential of the Mo 0 complex (1) (-0.35 V vs. Fc/Fc + ) is comparable to those (in the -0.53 to -0.23 V vs. Fc/Fc + range) reported for analogous molybdenum complexes of the type [LMo(CO)3] n , where L is a tridentate N-donor ligand [Tp (n = -1), [70] Tpm Me (n = 0), [71] 1,4,7-trimethyl-1,4,7-triazacyclononane (n = 0), [72] or 1,4,

triazacyclononane (n = 0)], [70] reflecting [66][67][68][73][74][75] the relative donor/acceptor abilities of these different L ligands. The more negative value (-0.53 V vs. Fc/Fc + ) is reported [70] for complex [Mo(Tp)(CO) 3 ] -and the less negative one (-0.234 V vs. Fc/Fc + ) for [Mo(Tpm Me )(CO) 3 ], [71] while that of 1 is intermediate, in accord with the order of the net electron-donor ability of the corresponding tripodal ligands. This is shown by the values of the electrochemical Lever EL

E/V vs. Ferrocene -2 -1.5 -1 -0.5 0 0.5 1 -2.5 I II E/V vs. Ferrocene -2 -1.5 -1 -0.5 0 0.5 1 -2.5 E/V vs. Ferrocene -2 -1.5 -1 -0.5 0 0.5 1 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 -2.5 I II E / V vs. Fc/Fc +
parameter (the lower this value, the stronger is the ligand electron-donor character) that parallel the above E1/2 ox values of the complexes: EL = -0.23 V (Tp), [76] -0.09 V (Tpms) [27] and 0.14 V (Tpm). [77] In addition, the hydride complex [Mo(Tpms)H(CO)3] 5 is oxidized at a higher potential (-0.44 V) relative to the following related hydride compounds, in the order: [Mo(Cp)H(CO)3] (-0.50 V) [78]  [Mo(Tp)H(CO)3] (-0.53 V) [78]  [Mo(Tp*)H(CO)3] (-0.59 V) [78] (Tp* = hydrotris(3,5dimethylpyrazolyl)borate)  [Mo(Cp*)H(CO)3] (-0.71 V) [78] (values are vs. the ferrocene/ferrocinium redox couple). This points out the following order of the electron-donor ability of the involved polyhapto ligands, also in agreement with the IR results discussed above and with the electron donation by the methyl substituents: Tpms  Cp  Tp  Tp*  Cp*.

Conclusion

This work shows that [Mo(CO)6] is a convenient starting material for the syntheses of both low and high oxidation state molybdenum complexes bearing the sulfonated Tpms ligand. In fact, three carbonyls are readily displaced by the Tpms tripodal ligand to afford a tricarbonyl Tpms-Mo 0 complex (1) which, upon metal halogenation or protonation, forms 7 Interestingly, the sulfonate group of the Tpms ligand is not innocent and plays a determining role on the properties of some of the complexes, not only by imparting hydrosolubility, but also by coordinating to the molybdenum atom (in 4) or to the Li + ion of a contiguous molecule forming a 1D polymeric chain (in 2 and also in the [Li(Tpms)]n salt).

The above features of the sulfonate group of Tpms deserve to be further explored for the syntheses of other water soluble (i) organometallic low oxidation state Mo complexes, (ii) middle and high oxidation state Mo compounds, and (iii) polymeric and supramolecular assemblies.

In addition, the availability of comparable Tp, Tp*, Cp and Cp* complexes allows the ordering of the corresponding and related polyhapto ligands according to their electron-donor character on the basis of the oxidation potentials of those complexes, as measured by cyclic voltammetry which provides a rather convenient tool for this purpose.

Experimental Section

General Techniques

Unless stated otherwise, all preparations and manipulations were carried out with Schlenk techniques under an oxygen-free argon atmosphere. All glassware was oven-dried at 120 °C.

Tetrahydrofuran and diethyl ether were dried over Na/benzophenone, toluene was dried over Na, and pentane and dichloromethane were dried over CaH2. Each of them was distilled under dinitrogen prior to use.

Materials and Instrumentation

IR spectra were recorded on a BIO-RAD FTS 3000MX spectrophotometer, in KBr pellets.

Wavenumbers are in cm -1 ; abbreviations: vs = very strong, s = strong, m = medium, w = weak br = broad. 1 H NMR spectra were recorded on a Bruker Avance II 300, operating at 300 MHz. )2] 0/+ redox couple (E1/2 ox = 0.525 V vs. SCE in CH2Cl2 [79] ) as internal standard.

Hydrotris(1-pyrazolyl)methane and lithium tris(1-pyrazolyl)methanesulfonate were prepared in accordance with published procedures. [25,80] Mass spectra were recorded with a Varian 500-MS LC Ion Trap Mass spectrometer equipped with an electrospray-ionization (ESI) interface.

Mass spectra were obtained by scanning the mass analyzer from m/z 50 to 2000 with a total of 3 microscans. Elemental analyses were carried out at the Analytical Laboratory of the Instituto Superior Técnico, Universidade Técnica de Lisboa.

Preparation of Complexes

Lithium tricarbonyl{tris(1-pyrazolyl)methanesulfonato}molybdenum(0) Li[Mo(Tpms)(CO)3] (1) and its conversion to [Mo(Tpms)(CO)2(-CO)Li(thf)2]n (2)

A suspension of [Mo(CO)6] (0.1420 g, 0.538 mmol) and lithium tris(1pyrazolyl)methanesulfonate LiTpms (0.1774 g, 0.591 mmol) was refluxed in CH3CN (60 mL) for 6 h under a dinitrogen atmosphere. The initially white suspension turned into a clear yellow solution and then to green. After cooling to 20 °C, the reaction mixture was filtered in order to remove the unreacted ligand (not very soluble in CH3CN), and the resulting solution was evaporated under reduced pressure affording a yellowish-green powder of 1 in powder of 3. Yield: 0.098 g (0.164 mmol, 60 %). Compound 3 is readily soluble in chlorinated hydrocarbons, CH3CN, thf and also in water (S25ºC  5 mgmL -1 ), not considerably soluble in alcohols, diethyl ether and hydrocarbon solvents. IR (KBr pellet): (CO) = 2039, 1966, 1944 cm -1 . 1 H NMR (300 MHz, CDCl3, 20°C):  9.17 (d, 3H, 3 JHH = 2.9 Hz, H (5) or H(3), pyrazolyl), 8.51 (d, 3H, 3 JHH = 2.2 Hz, H(3) or H(5), pyrazolyl), 6.56 (dd, 3H, 3 JHH = 2.7 Hz, H(4) pyrazolyl). 13

Tricarbonylhydrido{tris(1-pyrazolyl)methanesulfonato}molybdenum(II) [Mo(Tpms)H(CO)3] (5).

Complex (1) (130 mg, 0.2715 mmol) was dissolved in CH3CN (30 mL) at 20 °C. HBF4 (69.42 L, 54% in diethyl ether, 0.5068 mmol) was added dropwise, resulting in a colour change of the reaction mixture from greenish-yellow to green. After 1 h, the reaction mixture was taken to dryness under vacuum and the resulting solid was extracted with 5 mL of distilled CH3COCH3. The suspension was filtered and the resulting solution was evaporated under Crystal Structure Determinations. Single crystals of [LiTpms]n, 2, 4, 6 and 7 were obtained as indicated above. Intensity data were collected at 150 K, using a Bruker AXS-KAPPA APEX II diffractometer with graphite monochromated Mo-Kα ( 0.71073) radiation. Data were collected using omega scans of 0.5º per frame and full sphere of data were obtained. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT [START_REF] Bruker | APEX2 & SAINT[END_REF] on all the observed reflections. Absorption corrections were applied using SADABS. [START_REF] Bruker | APEX2 & SAINT[END_REF] Structures were solved by direct methods by using the SHELXS-97 package [START_REF] Sheldrick | [END_REF] and refined with SHELXL-97. [83] Calculations were performed using the WinGX System-Version 1.80.03. [84] All hydrogen atoms were inserted in calculated positions. Least-squares refinements with anisotropic thermal motion parameters for all the non-hydrogen (except C17, N22 and N27 of [LiTpms]n which were refined isotropically) atoms and isotropic for the remaining atoms were employed. atoms and isotropic ones for the remaining atoms were employed. Crystal data and refinement parameters are shown in Table 1. CCDC numbers 749680 to 749684 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

  Figure 1

  -CO -backbonding in the [Mo(Tpms)(CO)3] -unit than in the hexacarbonylmolybdenum compound. The Tpms ligand, in fact, replaces three stronger -acceptor carbonyl groups in [Mo(CO)6], increasing the metal electron density. A comparison with [Mo(Tp)(CO)3] -and [Mo(Cp)(CO)3] -(Cp =  5 -C5H5

Figure 2 -

 2 Scheme 1

[

  Mo(Tpms)(CO)2(-CO)Li(thf)2]n (2) were obtained and analysed by X-ray diffraction (Figure3, Scheme 1). Compound 2 is a 1D-type coordination polymer in which the [Mo(Tpms)(CO)3] -moiety acts as a building block, leading to an asymmetric unit containing one Mo centre and one Li(thf) 2 moiety bound to the O12 atom of the sulfonyl group and to the O3 atom of a bridging CO (Figure3). The Tpms group acts as a bridging tetradentate ligand in the N,N,N,O ( 4 :N,N,N,O) coordination mode, bound to the Mo atom through the pyrazolyl nitrogens N11, N14 and N17, and to the Li atom through the O12 oxygen. The N-Mo-N angles are restrained by this chelating ligand and range from 77.29(16) to 77.79(16)º; the wider C-Mo-X (X = C or N) angle ranges from 86.3(2) to 175.0(2)º, therefore leading to a highly distorted octahedron around the Mo atom. The Mo-N-N-C torsion angles of the pyrazolyl rings display values from 174.90 to 177.75º thus revealing a low degree of tilting of the rings. The O4 environment around the Li + cation shapes an almost regular tetrahedron (O-Li-O angles in the 108.1(5) -114.4(5)º range). The solid state structure of compound 2 also exhibits the presence of intermolecular C-H--- interactions (Figure S2, in Supplementary Information) between the thf C(202)-H(20D) and the N17-N18 pirazolyl ring (H•••centroid distance of 2.777 Å with a C-H•••centroid angle of 145.23º), as well as (although considerably weaker) between the C(202)-H(20C) and the N11-N12 pirazolyl ring (H•••centroid distance of 3.061 Å with a C-H•••centroid angle of 139.34º). These interactions, together with several hydrogen bonds involving all the oxygen atoms, help to stabilise the structure in the solid state. Structure 2 can be qualified as a heterometallic supramolecular chain. The nearest distance between two Mo atoms or two Li atoms in a chain is 10.794 Å, the same as the unit cell parameter b. The shortest interchain Mo•••Mo, Li•••Li and Mo•••Li distances are of 7.196, 9.734 and 7.462 Ǻ, respectively. These distances considerably exceed the sum of the van der Waals radii of two Mo or two Li atoms and hence the chains are effectively separated, as shown in Figures S3 and S4 (Supplementary information).

Figure 3 .

 3 Figure 3. ORTEP view of the asymmetric unit of [Mo(Tpms)(CO)2(-CO)Li(thf)2]n (2) with atom labelling scheme. Thermal ellipsoids are drawn at 30% probability. All hydrogen atoms were omitted for clarity. Selected distances (Å): Mo1-N11, 2.226(4) ; Mo1-N14, 2.245(5) ; Mo1-N17, 2.232(4) ; Mo1-C1, 1.922(7) ; Mo1-C2, 1.911(6) ; C1-O1, 1.174(7) ; C2-O2, 1.159(6) ; C3-O3, 1.194(7) ; O3-Li1, 1.898(10) ; O12-Li1 i , 1.933(10) ; Li1-O100, 1.929(10); Li1-O200 1.894(11); S1-O11, 1.432(4) ; S1-O12, 1.451(4) ; S1-O13, 1.433(4). Selected angles (°): N11-Mo1-N14, 77.29(16); N11-Mo1-N17, 77.47(16) ; N17-Mo1-N14, 77.79(16) ; C3-Mo1-N17, 175.0(2) ; C3-Mo1-C2 ; 86.5(2) ; O1-C1-Mo1, 178.8(5) ; O2-C2-Mo1, 177.1(5) ; O3-C3-Mo1, 177.0(5) ; C3-O3-Li, 164.9(5). Symmetry codes to generate equivalent atoms: (i) x,-1+y,z.

Scheme 2 .

 2 Scheme 2. Possible isomers for the 7-coordinated complex 3: a) 4:3 "piano stool" structure (Cs symmetry); b) 3:3:1 structure with CO in the axial position (Cs symmetry); c) 3:3:1 structure with I in the axial position (C3v symmetry).

  V dimer [Mo(Tpms)O(-O)]2 4 (Scheme 1 and Figure 4) which crystallized with two molecules of CH3CN. Compound (4) derived from the slow oxidation of (3) with air, as a consequence of O 2 diffusion during the crystallization procedure.

Figure 4 .

 4 Figure 4. An ORTEP view of compound [Mo(Tpms)O(-O)]2•2CH3CN (4.2CH3CN). The CH3CN molecules and the hydrogen atoms are omitted for clarity. Selected distances (Å): Mo1-N11, 2.189(2); Mo1-N14, 2.211(2); Mo1-O1, 1.673(2); Mo1 i -O2, 1.9345(18); Mo1-O2, 1.940(2); Mo1-O12, 2.303(2); Mo1-Mo1 i , 2.5545(4); S1-O11, 1.435(2); S1-O12, 1.472(2); S1-O13, 1.438(2). Selected angles (º): N11-Mo1-N14, 82.47(9); O12-Mo1-N14, 75.80(8); O1-Mo1-O12, 162.95(9); Mo1-O2-Mo1 i , 82.50(7); O2-Mo1-O2, 94.25(8); O2-Mo1-Mo1 i , 48.66(5). Symmetry code to generate equivalent atoms i: 1-x,y,3/2-z.

  Tpms group acts as a tridentate N,N,O ligand. The basal positions in each octahedron are occupied by the N11 and N14 atoms and the bridging O2 oxygen atoms and the two distorted octahedra share the common O2-O2 i edge; the apical positions are taken up by the oxido O1 and O12 atoms from the sulfonate group. Due to the sulfonate coordination, the S1-O12 bond length of 1.472(2) Å is slightly longer than the other two S1-O distances (1.435(2)

  [163.61(9)º] and O2-Mo1-N11 i [156.12(9)º] angles in the equatorial plane, together with the O12-Mo1-O1 [162.95(9)º] angle in the axial plane, are rather compressed and force the Mo1 and the bridging O2 oxygen atoms to shift away, in opposite directions, from the least-square basal plane. As a consequence, the Mo-O2-Mo i and the Mo-O2 i -Mo i planes make an angle of 25.83º. A similar folding has already been reported for other related structures [149° in [Mo(Cp*)O2]2 (Cp* =  5 -C5Me5

  ) is further stabilized by weak hydrogen bonds [shortest d(DA) 3.158(5) Å; shortest (DHA) 133.00º] involving the non coordinated O11 atom of the sulfonate group which simultaneously interacts with one of the pyrazolyl groups of a vicinal dimer and with the methyl group of a CH3CN molecule whose nitrogen atom, in turn, interacts with a pyrazolyl ring of a third molecule. No specific π•••π stacking interactions are identifiable between the pyrazolyl rings, the shortest interaction between the N14 containing pyrazolyl rings of neighbouring molecules being of 4.360 Å.

  -O-Mo bond distances (in the 1.689(3)-1.694(3) Å and 1.879(3)-1.884(3) Å ranges, respectively) agree with their double and single bond character, respectively. The structure is stabilized by intramolecular - stacking contacts. The N17-N18 and the N21-N22 pirazole rings are mutually involved in a face-to-face interaction with a distance of 3.627 Å between the ring centroids, the H17 hydrogen atom of the former ring being directed towards the N24-N25 pyrazolyl  cloud, and the H21 atom of the latter being directed towards the N14-N15 pyrazolyl  cloud (H17centroid 2.771 Å, C17-H17centroid 150.13º; H21centroid 2.747 Å, C21-H21 centroid 149.95º).

Figure 6 -

 6 Figure 6 -An ORTEP view of [Mo(Tpms)O2]2(µ-O)thf (7thf). The ellipsoids are drawn as 30% probability; the thf molecule and hydrogen atoms are omitted for clarity. Selected distances (Å): Mo1-N11, 2.214(3); Mo1-N14, 2.302(3); Mo1-N17, 2.308(3); Mo1-O1, 1.690(3); Mo1-O2, 1.879(3); Mo1-

Figure 7 .

 7 Figure 7. Cyclic voltammogram of Li[Mo(Tpms)(CO)3] 1 (1.6 mM) (initial anodic scan), at a

  -coordinated Tpms-Mo II complexes (3 and 5). The latter are oxidized with air to form high oxidation state dinuclear Tpms-oxo-Mo complexes bearing the {Mo V 2O2(-O)2} 2+ and {Mo V 2O2Cl2(-O)} 2+ moieties, the latter upon chlorination with CHCl3. The Tpms-oxo-Mo VI complex, on the other hand, contains the {Mo VI 2O4(-O)} 2+ unit and was obtained by oxidation of the above Tpms-Mo 0 compound 1. Hence, the low oxidation state Tpms-Mo 0 complex provides a versatile entry to a diversity of high oxidation state Tpms-oxo-Mo species.

  Abbreviations: s = singlet, d = doublet, m = complex multiplet; dd = doublet of doublets, br =

  C NMR (75.5 MHz, CDCl3, 20°C):  148.94 [s, C(5) or C(3) pyrazolyl], 138.08 [s, C(3) or C(5), pyrazolyl], 107.96 [s, C(4), pyrazolyl]. Anal. for MoSO6N6C13H9I•1.5thf•CHCl3•2H2O (864.0): calcd. C 27.8, N 9.7, H 3.0; found: C 27.0, N 9.4, H 3.2. Reddish-brown crystals of 4•2CH3CN suitable for X-ray analysis were obtained upon slow diffusion of Et2O into a CH3CN solution of complex (3). IR (KBr): 1104, 1073, 1056, 1032 (br, s, S=O, Mo=O), 803 cm -1 (s, Mo-O-Mo). 1 H NMR (300 MHz, (CD3)2CO, 20°C):  9.19 (m, 6H, H(5) or H(3), pyrazolyl], 7,53 (m, 6H, H(3) or H(5), pyrazolyl), 6.63 (m, 6H, H(4) pyrazolyl). 13 C NMR (75.5 MHz, (CD3)2CO, 20°C):  167.59 [s, 3C, C(5) or C(3) pyrazolyl], 145.23 [s, 3C, C(3) or C(5), pyrazolyl], 95,10 [s, 3C, C(4), pyrazolyl].

Table 1 .

 1 Selected crystallographic and refinement parameters for compounds [LiTpms]n, 2, 4.2CH3CN, 6.6CHCl3 and 7.thf. R1 = ||Fo| -|Fc||/|Fo|; wR2 = [[w(Fo 2 -Fc 2 ) 2 ]/ [w(Fo 2 ) 2 ]] 1/2 .

	Compound	[LiTpms]n	2	4.2CH3CN	6.6CHCl3	7.thf
	Empirical formula	C20 H18 Li2 N12 O6 S2	C21 H25 Li Mo N6 O8 S	C24 H24 Mo2 N14 O10 S2	C26 H24 Cl20 Mo2 N12 O9 S2	C24 H26 Mo2 N12 O12 S2
	Formula weight	600.46	624.42	924.57	1613.57	930.57
	Crystal system	orthorhombic	Triclinic	Monoclinic	Monoclinic	Monoclinic
	Space group	P c a 21	P -1	C2/c	P21/c	P21/n
	a (Å)	18.6055(15)	10.469(2)	18.3013(5)	12.157(2)	13.222(3)
	b (Å)	8.3363(9)	10.794(2)	13.4344(4)	12.716(3)	19.685(4)
	c (Å)	16.6854(14)	11.685(4)	15.2516(4)	18.007(4)	14.048(3)
	 ()	90	90.156(17)	90	90	90
	 ()	90	99.50(2)	116.8860(10)	99.255(13)	117.084(5)
	 ()	90	94.365(18)	90	90	90
	Volume (Å 3 )	2587.9(4)	1298.4(6)	3344.53(16)	2747.6(10)	3255.4(12)
	Z	4	2	4	2	4
	Density (calculated) (Mg/m 3 ) 1.541	1.597	1.836	1.950	1.899
	Absorption coefficient (mm -1 ) 0.269	0.642	0.951	1.563	0.981
	F(000)	1232	636	1848	1584	1864
	Rfls collected	11746	17721	8097	16985	14365
	Unique rfls.	4493	4576	3045	5004	4725
	Rint	0.1589	0.1106	0.0329	0.2844	0.0418
	GOF	0.935	0.964	1.061	0.954	1.037
	Final R1, wR2 (I  2) [a]	0.0775, 0.1636	0.0538, 0.1016	0.0280, 0.0681	0.0949, 0.1676	0.0357, 0.0864
	R1, wR2 (all data)	0.1577, 0.2151	0.1016, 0.1189	0.0364, 0.0717	0.2572, 0.2349	0.0465, 0.0920
	[a]

Acknowledgements

We are grateful to the European Commission for funding of this work through the AQUACHEM research Training Network (Project n° MRTN- CT-2003-503864). This study has been also supported by the Foundation for Science and Technology (FCT) and its POCI 2010 and PPCDT programmes (FEDER funded), grant BPD/20869/04. The authors also thank to Dr. M. Cândida Vaz and Dr. Conceição Oliveira for the elemental analysis and ESI-MS services, respectively.