T Sebald

Uchimoto

S Zhang

B Ducharne

S Takeda

G Sebald

T Uchimoto

Identification of the ferromagnetic hysteresis simulation parameters using classic non-destructive testing equipment

Keywords: Ferromagnetic hysteresis simulation, magnetic incremental permeability, Eddy current testing, Dodd and Deeds model, Jiles-Atherton model

Identification of the ferromagnetic hysteresis simulation parameters using classic non-destructive testing equipment

-Introduction:

Electromagnetic Non-Destructive Testing (ENDT) regroups a large set of methods based on the electromagnetic signature of tested specimens [START_REF] Huang | New Technologies in Electromagnetic Nondestructive Testing[END_REF]. The objective is to use these signatures as an indirect way to evaluate a level of integrity.

Eddy Current Testing (ECT) is by far the most popular ENDT. Despite its relative simplicity, it conveys a large amount of information. Different configurations of ECT have already been described in the scientific literature: _ The single coil method [START_REF] Borovik | Single-coil eddy current sensors and their application for monitoring the dangerous states of gas-turbine engines[END_REF].

_ The Transmitter/Receiver (TR) probe method [START_REF] Kojima | Inverse methodology for eddy current testing using transmitterreceiver coil probes[END_REF].

_ The differential probe method [START_REF] Chady | Crack detection and recognition using an eddy current differential probe[END_REF].

If the specimen is ferromagnetic, the superimposition of a slowly varying high amplitude magnetic excitation to the ECT characterization enables the measurement of the Magnetic Incremental Permeability (MIP) signature (the butterfly loop: incremental permeability plotted as a function of the static magnetic excitation field leading typically to a butterfly shape). MIP is interesting as it is a remarkable indicator of residual stresses and strains [START_REF] Gupta | Magnetic incremental permeability non-destructive evaluation of 12 Cr-Mo-W-V Steel creep test samples with varied ageing levels and thermal treatments[END_REF]- [START_REF] Gupta | Comparison of electromagnetic inspection methods for creep-degraded high chromium ferritic steels[END_REF]. When ECT is mostly used to detect surface macroscopic flaws (cracks, grinding burns …), MIP is more adapted to evaluate the mechanical sub-surface properties of tested specimens and their evolutions (creep, structural health monitoring …).

ENDT is daily used in the steel industry. Due to their high electrical conductivities, industrial steels are well indicated for ECT method. Most of steel alloys are ferromagnetic. From the NDT point of view, this property can be seen as an advantage, it opens access to a large panel of NDT methods exclusive to the ferromagnetic materials (the magnetic Particle inspection (MPI) [START_REF] Wang | A review of three magnetic NDT technologies[END_REF], the Magnetic Barkhausen Noise (MBN) [START_REF] Ducharne | Micromagnetic nondestructive testing Barkhausen noise vs other techniques[END_REF][10], the Harmonic Analysis (HA) [START_REF] Dobmann | Magnetic tangential field strength inspection a further ndt-tool for 3MA[END_REF] …). The magnetic behavior of ferromagnetic steels is depending on the micro-structure and on the mechanical internal properties. The magnetic answer of a tested specimen under the influence of an external magnetic stimuli constitutes an indirect way to evaluate these properties. In [START_REF] Takahashi | Relationship between mechanical and magnetic properties in cold rolled low carbon steel[END_REF] [START_REF] Tomáš | Comparison of four NDT methods for indication of reactor steel degradation by high fluences of neutron irradiation[END_REF] authors have demonstrated the viability of the magnetic methods for the observation of the ductilebrittle transition temperature (DBTT) and the Vickers hardness of structural materials used for nuclear reactor pressure vessels. But ferromagnetism is also strongly nonlinear. When it comes to the interpretation of ENDT signals such nonlinear behavior constitutes an obstacle in the development and the spread of ENDT methods.

The capability of ENDT is well admitted in the industrial field but progresses in this domain are still highly expected. Among many options already tested to improve the efficiency of ENDT, the development of simulation tools seems especially promising. By combining simulation results and experimental data, the ENDT signals can be anticipated and used to prevent possible failures [START_REF] Calmon | Trends and stakes of NDT simulation[END_REF].

Different methods have been developed for the simulation of ENDT industrial situations. A space discretization of the Maxwell equations is one solution [START_REF] Augustyniak | Finite element method applied in electromagnetic NDTE: a review[END_REF], analytical approaches is another one [START_REF] Skarlatos | Study of the non-linear eddy-current response in a ferromagnetic plate: theoretical analysis for the 2D case[END_REF]. In most of the commercial simulation tools dedicated to ENDT, hysteresis is ignored and the relation between the magnetic stimuli and the magnetic state is always taken into account through linear relations. This restriction is clearly an issue, especially for the simulation of the magnetic methods (MBN, MIP …).

The absence of hysteresis in commercial simulation tools is due to multiple reasons, among these:

_ the complexity of hysteresis and the complexity of hysteresis models. The combined simultaneous resolution of the Maxwell equation and of a hysteresis model is most of the time indirect and needs the use of dedicated techniques such as the fixed point or the Newton-Raphson methods [START_REF] Saitz | Newton-Raphson method and fixed-point technique in finite element computation of magnetic field problems in media with hysteresis[END_REF] [START_REF] Belahcen | A fast fixed-point method for solving magnetic field problems in media of hysteresis[END_REF].

_ the difficulty of gathering reliable magnetic experimental characteristics, as required by the hysteresis model settings.

Progresses have already been done concerning the simultaneous resolution. Strong formulations coupling an hysteresis material law to the Maxwell equations resolution have been proposed and seem to provide positive results [START_REF] Raulet | The magnetic field diffusion equation including dynamic hysteresis: a linear formulation of the problem[END_REF] [START_REF] Ducharne | Dynamics of magnetic field penetration into soft ferromagnets[END_REF]. In the same time, very few scientific research works have been done to solve the problem of the experimental results acquisition.

In this manuscript, by coupling numerical tools, optimization parameter methods and experimental results accessible from classic NDT equipment, we propose a technique able to return the hysteresis simulation parameters. As a consequence, the geometry has no more influence on the results, and the method is useful even on oversize specimens impossible to characterize with the hysteresis characterization standard devices.

The first step of the method consisted in the acquisition of the MIP curve, evolution of the ECT single coil impedance modulus variations as a function of a DC magnetic field excitation |𝑍|(𝐻).

Standard ECT and MPI equipment can be used to achieve these measurements. D&D simulations were performed on a large window of simulation parameters (frequency f, permeability modulus |𝜇| and a reference surface Z ( f , |𝜇| ) was plotted. This surface constituted the support of the interpolation inverse procedure run to return the permeability vs the magnetic excitation curve IµI(H). In the last step of the process, the J-A model in a slightly modified configuration was operated to simulate the MIP behavior and plot the corresponding |𝜇|(𝐻) curves. Again, large windows of parameters were tested, the confrontation between the experimental and the simulated |𝜇|(𝐻) curves allowed to establish the best J-A combination of parameters. These parameters were set for the simulation of the MIP curve and conserved for the simulation of the hysteresis behavior B(H).

The structure of this manuscript can be summarized as follows:

The first section describes the experimental setup and the acquisition of the |𝑍|(𝐻) experimental results. A description of the D&D simulation method such as the inverse process to obtain the Z ( f , |𝜇| ) is proposed in the second section. The J-A model in its modified version is detailed in the third section and finally the whole process is run and tested on electric steel specimens.

Comparisons between simulated hysteresis cycles and experimental ones are proposed as a validation of the hysteresis parameter combination such as the validation of the whole method.

-Experimental setup, acquisition of the |𝒁|(𝑯) experimental results

2-a) Magnetic incremental permeability characterization setup

The first step of this research consists in the acquisition of the MIP butterfly signature: evolution of the sensor coil impedance modulus |𝑍| as a function of the tangent magnetic field excitation H. Standard NDT industrial equipment can be used for this acquisition (Fig. 1). The DC magnetic excitation can be generated through a MPI experimental setup and the evolution of the flat coil impedance monitored with an eddy current controller (Fig. 1). In our laboratory, our equipment is slightly different. A dedicated experimental setup has been developed especially to recreate the industrial conditions. An overall 3D view of this setup is depicted in Fig. 2 below. (please note that to get closer to the industrial conditions, this amplifier and the associated frequency generator can be replaced by a regulated power supply controlled manually. If f is low enough both methods give the same MIP curve. We did this test to set f and avoid any undesired dynamic contribution). The exciting coil is associated to a U-shaped large section magnetic yoke for the generation of the HDC magnetic field excitation contribution. The tangential magnetic field along the specimen in the field direction was measured using a Hall element. A pancake coil (ECT coil) was placed on the surface of the specimen between the legs of the yoke (see Fig. 1). The dimensions of the ECT coil were as follows: 3.3 mm inner diameter, 3.95 mm outer diameter, 3.0 mm height, 275 turns, and 0.05 mm wire thickness. The lift-off was 0.39 mm. The coil impedance was measured using a LCR meter (ZM2375, NF Corporation). The current of the ECT coil was controlled to an effective value of 1 mA using the LCR meter. The frequency of this AC magnetic field excitation was set to 50 KHz. The AC magnetic field contribution was maintained lower than a quarter of the coercivity to ensure reversible AC magnetization changes, i.e., to avoid irreversible domain wall jumps, as recommended by the literature [START_REF] Yashan | Measurements and semi-analytical modeling of incremental permeability using eddy current coil in the presence of magnetic hysteresis[END_REF]. The resulting data provided by the LCR meter consisted in the sensor coil impedance modulus, phase, real and imaginary part. A series of measurements for the different levels of DC magnetization was recorded using @Labview data acquisition software.

A calibration procedure was carried out to take into account the parasitic impedances due to the electrical wires and contacts. A comparison between experimental and simulation tests in the air (using the D&D analytical expression described in the next section for the simulation) was done and both a rotation 𝜑 and expansion 𝑀𝑜𝑑 factors were found so that the calculation results were consistent with the experimental ones.

𝑍 = |𝑍|. 𝑀𝑜𝑑 . 𝑒 ( ) (1)
This method gave us the MIP signature but it also enabled access to the Eddy Current Magnetic Signature (EC-MS) plot, impedance imaginary part as a function of the real part which has been described in recent articles as a particularly sensitive indicator of residual strains [START_REF] Matsumoto | Investigation of Electromagnetic Nondestructive Evaluation of Residual Strain in Low Carbon Steels Using the Eddy Current Magnetic Signature (EC-MS) Method[END_REF] [START_REF] Zhang | Simulation tool for the Eddy Current Magnetic Signature (EC-MS) non-destructive method[END_REF].

In parallel to this first experimental setup, a hysteresis cycle characterization setup was exploited to plot the tested specimen hysteresis cycles required for the validation of the method. A detailed description of this setup can be found in [START_REF] Zhang | Simulation tool for the Eddy Current Magnetic Signature (EC-MS) non-destructive method[END_REF]. The tested specimens were all coming from the same batch. They were all electric steel, FeSi laminations with a 3 wt % silicon content and referenced 35JN360. Tab. 1 below gives the physical properties as provided by the manufacturer.

Tab. 1 -Physical properties of the 35JN360 specimens as provided by the manufacturer.

The tested specimen geometrical information are available in Fig. 4. The electrical conductivity of these specimen was 2.13 10 6 S.m -1 , with Non-Oriented (NO) grains and isotropic magnetic properties.

3 -Inverse process, Dodd and Deeds analytical method, IµI(H) plots establishment.

In the late sixties, early seventies, Dodd and Deeds (D&D) derived closed form integral expressions for the impedance of the cylindrical coil above a layered conductive half-space [START_REF] Dodd | Analytical solutions to eddy current probe coil problems[END_REF]- [START_REF] Theodoulidis | Eddy Current Canonical Problems (With Applications to Nondestructive Evaluation)[END_REF]. This model continues to be used extensively to this day. In the industrial field, it allows to design eddy current tests and to optimize parameter measurements including thickness, conductivity and magnetic permeability [START_REF] Dodd | Measurement of small magnetic permeability changes by eddy current techniques[END_REF]- [START_REF] Dodd | Thickness measurements using eddy current techniques[END_REF]. In this study, the D&D simulation method was used to plot a reference surface |𝑍|(|𝜇 |, 𝑓) related to our experimental situation. This surface constituted the base of the interpolation method used to convert the experiment |𝑍|(𝐻) plots into |𝜇 |(𝐻) plots.

We opted for the D&D analytical solution of a cylindrical coil above a two-layer conductor. The top layer was our tested specimen (0.35 mm thick), the bottom layer (supposed extended to infinity) was set with a vacuum permeability and a low electrical conductivity in order to restrain its influence in the final calculus. The D&D solution is decomposed in two terms. The first term Z0 is related to the cylindrical coil and is expressed by:

𝑍 = ( ) ( ) ∫ ( , ) 𝑘(𝑧 -𝑧 ) + 𝑒 ( ) -1 𝑑𝑘 (2)
Where r1, r2, z1, z2, are the coil inner radius, the outer radius, the bottom position and the top position on the z axis (see Fig. 5 below). N is the number of turns. Z0 does not depend on the vertical position of the coil but rather on its width z2-z1. The second term ΔZ has to be added to Z0 to obtain the final solution. ΔZ is associated to the impedance changes due to the two-layers conductor and can be expressed as:

𝛥𝑍 = ( ) ( ) ∫ ( , ) (𝑒 -𝑒 ) ( )( ) ( )( ) ( )( ) ( )( ) 𝑑𝑘 (3)

𝜆 = 𝑘 + 𝑗𝜔𝜇 𝜇 𝜎

Where d is the first conductive layer thickness. The D&D analytical solution was configured with the sensor coil information provided in the first section and simulations were done for a frequency and relative permeability windows of [10 KHz -1 MHz] and [100 -5000] respectively.

The resulting surface |𝑍|(|𝜇 |, 𝑓) is depicted in Fig. 6.

-Modified Jiles-Atherton model and identification of the hysteresis parameters.

The inverse process based on the D&D analytical solution for a cylindrical coil above a layered conductive half-space gave us an estimation of |𝜇 | for all the values of H tested. It is important to keep in mind that μr is a tensor linking vector quantities. As the tested specimen is magnetized by the DC magnetic field contribution, |𝜇 | is supposed to be independent of the spatial position.

Even if we never verified it experimentally, our specimens are supposed to have anisotropic magnetic characteristics (non-oriented grains). In our experimental setup, the AC and the DC magnetic contributions were perpendicular (see Fig. 1). The quasi-static excitation amplitude is supposed to be 100 to 1000 times higher than the AC ones. Consequently, in the scanned area

4-a) Modified Jiles-Atherton model for a fast simulation of the magnetic incremental permeability

The J-A theory is based on domain-wall motion which is the principal cause of hysteresis in multidomain specimens. But the J-A model also considers the effect of domain rotation since the essence of the model is the establishment of a relationship between energy dissipation ("losses") and magnetization variations [START_REF] Jiles | Theory of ferromagnetic hysteresis[END_REF]- [START_REF] Jiles | Ferromagnetic hysteresis[END_REF]. On a one hand, the J-A model exhibits remarkable properties: a limited number of parameters, a capability of considering hysteresis through a unique equation. On the other hand, the J-A model (at least, in its early stage version) is suffering from various restrictions: the limitation to scalar situation (i.e., the magnetic excitation H and the magnetization M are assumed to be collinear), the limitation to quasi-static frequencies and the accommodation issue (i.e., the incapability to simulate correctly the minor loop situations). In the J-A theory, M is decomposed into a reversible Mrev and an irreversible Mirr contributions:

𝑀 = 𝑀 + 𝑀 (4)
The anhysteretic magnetization is described with a sigmoid-type equation (Langevin …) including at least two parameters:

_ Ms the saturation magnetization _ a, an anhysteretic trajectory parameter related to the domain walls density.

Eq. 5 and eq. 6 below are the classic options for this anhysteretic magnetization:

𝑀 = 𝑀 𝑐𝑜𝑡ℎ - (5)
𝑀 = 𝑀 𝑡𝑎𝑛ℎ (6)
According to the J-A theory, a can obtained theoretically using Eq. 7:

𝑎 = (7)
kB is the Boltzmann's constant, θ the temperature, and m the magnetic magnitude at a domain scale. In the J-A model, an effective field He is defined as the sum of H and an additional contribution related to the surrounding magnetized mater and modulated by α a mean field parameter (the inter domain coupling parameter, see J-A theory [START_REF] Jiles | Theory of ferromagnetic hysteresis[END_REF]- [START_REF] Jiles | Ferromagnetic hysteresis[END_REF]).

𝐻 𝐻 + 𝛼𝑀 (8)
He is the model input, by taking into account the influence of the environment magnetic state, congruency issues are avoided. Eq. 9 below connects the anhysteretic, the reversible and the irreversible magnetizations:

𝑀 = 𝑐(𝑀 -𝑀 ) (9)
As c is constant, a proportional ratio is assumed between the magnetization contributions. c can be adjusted experimentally from the ratio between the initial permeability observed on the first and on the anhysteretic magnetization curves (in the Rayleigh area). c has a strong influence in the simulation of the MIP butterfly signature where similarly to the Rayleigh zone, the reversible magnetization contribution is major.

A differential equation (Eq. 10) connects the anhysteretic and the irreversible magnetization: =

k is related to an average energy required by the domain walls to break pinning site and δ a directional parameter necessary to avoid physical non-sense.

𝛿 = +1 𝑖𝑓 ≥ 0 𝛿 = -1 𝑖𝑓 < 0 (11)
After recombination of the above equation (Eq.4 to 11) an expression of the susceptibility (assumed equal to the permeability) can be obtained:

= ( ) ( ) (12)
Eq. 12 permeability can be classified as differential if dH/dt has been conserving the same sign for a time lap long enough to irreversible domain wall motions. Oppositely, after a recent dH/dt sign changing, it can be assimilated to an incremental permeability. The H integration of eq. 12 leads to the field B and allow to plot the B(H) hysteresis behavior:

𝐵(𝑡) = 𝜇 (𝐻(𝑡) + ∫ (𝑡)𝑑𝐻 (13)
In [START_REF] Zhang | Simulation tool for the Eddy Current Magnetic Signature (EC-MS) non-destructive method[END_REF], we proposed a simple modification of the J-A model to describe quasi simultaneously a B(H) hysteresis cycle and the associated |𝜇 |(𝐻) MIP butterfly loop. In this method, the J-A model is excited with a high amplitude alternative magnetic field H and for every step time both the differential and the incremental magnetic permeability are calculated. In such configuration, H variations are long enough to allow irreversible domain wall motions and eq. 12 describes a differential permeability. The incremental permeability calculus requires the creation of a virtual MIP effective field HeMIP defined as:

⎩ ⎪ ⎨ ⎪ ⎧ 𝑖𝑓 𝑠𝑖𝑔𝑛 ( ) = 𝑠𝑖𝑔𝑛 ( ) 𝐻 (𝑡) = 𝐻(𝑡 -𝑑𝑡) -𝑑𝐻(𝑡) + 𝛼𝑀(𝑡 -𝑑𝑡) 𝑒𝑙𝑠𝑒 𝐻 (𝑡) = 𝐻 (𝑡) (14)
HeMIP simulates a sudden sign change in the excitation field time derivation and replacing He by HeMIP in the simulation process leads to a permeability expression (Eq. 15) which can be assimilated to the incremental permeability:

= ( ) ( ) (15)
The combination parameters which lead to the minimum value of the error function (Eq. 16)

was set as our specimen combination of parameters. This combination is depicted in Tab. 2. Eq. 5 sigmoid has been used for the J-A anhysteretic contribution.

Tab. 2 -Best combination of parameters for the J-A simulation of the 35JN360 specimens.

Finally, both the D&D and the J-A (Tab. 2 parameters) |𝜇 |(𝐻) curves are exposed in Fig. 9 -a.

-Conclusions and perspectives

ENDT is in constant progress, by coupling numerical tools to experimental results, the capability of detection and characterization could be improved drastically. A lot of research is supported in this field. An important current issue comes from the acquisition of the experimental data required for the configuration of the simulation methods. It is especially true for steel inspection.

Steel is ferromagnetic and exhibits strong magnetic non-linear behaviors (hysteresis, frequency dependence …). The magnetic characterization of steel is supposed to be done in very restrictive conditions described by international standards. In this manuscript, we proposed an alternative method to estimate the hysteresis simulation parameters using classic NDT equipment (ECT device and MPI inductor). The proposed method runs through 3 steps and combines experimental results (MIP butterfly loop) to numerical simulations (the D&D analytical solution and the J-A model). The first tests exposed in this manuscript for the FeSi laminations with a 3 wt% silicon content exhibit a maximum of ≈ 20 % relative error for the main hysteresis indicators.

Even if this percentage error can be judged as relatively high, it has to be balanced by the fact that the resulting hysteresis parameters open doors to comparisons between data from NDT experimental devices and situations never compared before. Assumptions have been advanced in the description of our method, therefore further developments will be proposed in a future work to confirm these hypothesis and improve the capability of the method. A study of reproducibility has to be performed such as the evolution of the simulation parameters as a function of creep and aging.

Fig. 1 -

 1 Fig. 1 -Illustration of the MIP and ECMS signature acquisitions with standard NDT equipment.

Fig. 2 -

 2 Fig. 2 -Overall 3D view of the industrial setup used for the acquisition of the incremental permeability.

Fig. 3 -

 3 Fig. 3 -a, b give the MIP and the EC-MS curves obtained at HAC = 50 Khz. Fig. 3 -c shows the

Fig. 3 -

 3 Fig. 3 -a. MIP Experimental curve (HAC = 50 KHz). Fig. 3 -b. ECMS Experimental curve (HAC = 50 KHz). Fig. 3 -c. B(H) Experimental curve (HDC = 0.1 Hz) .

Fig. 4 -

 4 Fig. 4 -Geometrical properties of the tested specimens.

Fig. 5 -

 5 Fig. 5 -Illustration for the D&D simulation of cylindrical coil above a layered conductive half-space.

Fig. 6 -

 6 Fig. 6 -Reference surface |𝑍|(|𝜇 |, 𝑓) calculated with the D&D simulation method.

Fig. 6 Fig. 7 -

 67 Fig. 6 surface and the corresponding matrix constitutes the data base we use to return by

Fig. 7 -

 7 Fig.7 -a. Relative permeability as a function of the impedance from the D&D simulation (f = 50 KHz). Fig.7 -b.|𝜇 |(𝐻) resulting curve for the FeSi 3% NO specimen.

  the direction of the vector cumulative excitation field 𝐻 ⃗ (𝐻 ⃗ + 𝐻 ⃗ ) remains almost constant during a large proportion of the measurement cycle. Based on this observation, we considered the scalar version of the Jiles-Atherton model (where 𝐻 ⃗ and 𝐵 ⃗ are collinear) as accurate enough to provide consistent simulation results. a b a

Fig. 8

 8 Fig. 8 below gives an example of a simulated major B(H) hysteresis cycle and its related

Fig. 8 -

 8 Fig. 8 -a. Simulated B(H) major hysteresis cycle. Fig. 8 -b. Corresponding J-A parameters. Fig. 8 -c. Simulated MDP curve. Fig. 8 -d. Simulated MIP curve.

Fig. 9 -

 9 Fig. 9 -b shows the resulting comparison between the experimental and the simulated B(H)

Fig. 9 -Tab. 3 -

 93 Fig. 9 -a. Comparison between the IZI(H) MIP curves obtained with the D&D interpolation method and with the J-A model (best combination of parameters). Fig. 9 -b. Comparison between the simulated and the measured B(H) curves

Finally, it

  is important to the weight of the lift off estimation in the success of the method. Large differences can be observed on the |𝜇 |(|𝑍|) (Fig.7 -a) and on the |𝜇 |(𝐻) (Fig.7 -b) curves after a small variation of the lift-off. Good simulation results can still be obtained by adjusting c (the reversibility parameter). c has a strong influence on the MIP simulation but relatively weak on the B(H) curve, it can be modified to match the MIP curve without affecting the B(H) simulation. Consequently, from a magnetic characterization/NDT point of view, a small error in the measurement of the lift-off can lead to a resulting c out of physical meaning.

  Many perspectives can be associated to this work including: _ To test new geometries (tubes, rods, gears …) _ To test new materials (steel, pure iron …)


Chiara Dinoi

Pelin Sözen

Gülnur Taban

Deniz Demir

Funda Demirhan

Petr Prikhodchenko

Jenny Gun

Ovadia Lev

Jean-Claude Daran

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

Aqueous Reduction of [Cp*2W2O5]: Characterization of the Triangular Clusters [Cp*3W3O4(OH)2]2+ and [Cp*3W3O6]+ -Comparison with Molybdenum

Keywords: Tungsten, Aqueous organometallic chemistry, Oxo ligands, Metal clusters, DFT calculations Tungsten, Aqueous organometallic chemistry, Oxo ligands, Metal clusters, DFT calculations

come   L'archive ouverte pluridisciplinaire

Introduction

Aqueous organometallic chemistry is attracting growing interest because of its ''green'' impact [1][2][3] and because of its potential in catalytic [4,5] and biomedical applications [6] . Although this area is widely explored for low-valent systems, usually upon appending hydrophilic functionalities to the ligand periphery in order to increase solubility in an aqueous environment [7][8][9] , studies of high oxidation organometallics are still rather scarse. [10][11][12] For redoxactive metals, these studies are particularly interesting as they may open the way to electrocatalytic applications. [12] In a recent contribution [13] , we have reported that the zinc reduction of Cp*2Mo2O5 in a strongly acidic (CF3COOH or CF3SO3H) mixed H2O-MeOH medium affords the trinuclear mixed-valence (Mo3 13+ ) cluster [Cp*3Mo3O2(OH)4] 2+ . The ion adopts a nearly equilateral triangular shape, each edge being bridged by two ligands. The distortion of the triangle is caused by one edge being bridged by two OH groups, while the other two edges are bridged by one O and one OH group. As a consequence, the former edge is slightly longer than the latter ones. Although the number of bridging OH vs. O groups was not revealed directly by the X-ray structural analysis, it was suggested indirectly by the same technique through the existence of short hydrogen-bonding contacts (or lack thereof) between the Mo3O6 core and the two CF3CO2 and CF3SO3 anions, and confirmed by a comparison with a DFT geometry-optimized model, by EPR spectrometry, and by a magnetic susceptibility measurement.

In this contribution, we examine the parallel behaviour of the pentamethylcyclopentadienyltungsten system and report the isolation and characterization of two new, redox-related tritungsten oxo clusters, [Cp*3W3O6] + and [Cp*3W3O4(OH)2] 2+ . MeOH is necessary to increase the solubility of the starting material. The reaction was carried out in the presence of two different acids, trifluoroacetic acid and trifluoromethylsulfonic (triflic) acid. Like for the Mo analogue, a series of color changes was observed over a period of three days, yielding a final green solution through an intermediate orange-red color. Crystallization of the final green product was successful in the case of the triflic acid reaction, revealing the nature of the compound as [Cp*3W3O4(OH)2](CF3SO3)2•2H2O, 1.

Results and Discussion

Thus, the reaction stoichiometry is as shown in equation 1. The oxidation state of tungsten in the final product is +5, yielding a 3-electron W3 15+ cluster. Compound 1 is quite soluble in CH2Cl2 and its solutions are air sensitive, turning rapidly to orange when exposed to the atmosphere. The nature of the orange product is shown in the next section.

3[Cp*2W2O5] + 3Zn + 10H +

 2[Cp*3W3O4(OH)2] 2+ + 3H2O + 3Zn 2+ (1) The structure of the dication is shown in Figure 1, whereas Figure 2 illustrates the arrangement of the anions and the interstitial H2O molecules relative to the trimetallic cluster.

Relevant bonding parameters are collected in Table 1. The H atoms on the bridging OH groups could not be directly located from the diffraction data, but their presence is clearly demonstrated by the H-bonding network. Hydrogen bond contacts for compound 1 are available in the Supporting Information. As shown in Figure 2, the bridging atoms O13 and O21 establish short contacts with atoms O100 and O200 of the interstitial water molecules. The latter, in turn, establish short contacts with two different atoms of triflate anions: O100 with O6 but also with O4 of a second asymmetric unit; O200 with O3 but also with O1 of a third asymmetric unit. Close inspection of the difference Fourier map obtained after refinement of all heavy atoms revealed in fact the position of the H atoms on the interstitial H2O molecules. This observation excludes the alternative formulation as a neutral [Cp*3W3O6] complex with two interstitial CF3SO3H molecules, because in the latter case only one of the two H2O protons would point toward an oxygen atom of the trifluoromethylsulfonate group, while the other one would point toward the bridging oxo group of the W3 cluster. While these considerations establish the presence of H atoms on O13 and O21, they provide no information on the absence or presence of H atoms on the other bridging O atoms, O11, O12, O22 and O23. This point will be addressed in further detail later, by inspection of the spectroscopic properties and by comparison of the experimental (X-ray diffraction) and computed (DFT) bonding parameters. Table 1. Selected bond distances (Å) and angles (°) for the trinuclear clusters in all compounds. [a] (a) Distances As stated above, exposure of solutions of compound 1 to air rapidly yields orange-red solutions. Crystallization from such oxidized solution yielded red crystals of compound 2, shown by an X-ray analysis to correspond to [Cp*3W3O6](CF3SO3). This formulation is consistent with the spectroscopic properties and with a comparison between the experimental (X-ray diffraction) and computed (DFT) structural parameters, as shown in a later section. The cation is a mixed-valence W3 V,V,VI cluster, namely a 2-electron cluster. Thus, the transformation generating this cluster can be described as in equation 2.

[Cp*3W3O4(OH)2] 2+ + [ox]  [Cp*3W3O6] + + 2H + + [red] (2)
Like compound 1, compound 2 is soluble in CH2Cl2, but is also fairly soluble in MeOH-H2O solvent mixtures. These solutions are relatively stable in air, but not indefinitely so, to ultimately yield colorless solutions, apparently faster at high pH. An 1 H NMR monitoring in MeOD-D2O reveals the formation of a single resonance ( 2.05), attributed to [Cp*WO3] -, [14] at neutral or basic pH, whereas several other products, yet to be identified, are obtained under strongly acidic conditions (pH 2: resonances at  = 2.22, 2.19, 2.15).

The structure of 2 contains a simple anion and no interstitial solvent molecule, thus firmly establishing the +1 charge of the cluster. However, the cation exhibits a symmetry imposed disorder, because of a twofold axis which passes through atom W2 and through the midpoint of the W1-W1 i bond. Therefore, there is a 50:50 distribution of two symmetry related orientations of the Cp* ligand bonded to atom W2. In addition, the triflate anion is also located on a special position (another C2 axis, orthogonally cutting through the C-S bond), yielding again a 50:50 distribution of two symmetry related orientations. In spite of this problem, the structural model refined to quite acceptable final residuals yielding relatively precise bond distances and angles.

The crucial question concerns the absence of H atoms on the bridging O atoms. No residual electron density that could be assigned to such atoms was visible from the final difference Fourier map. However, even more convincing is the absence of close contacts between the two ions. The anion is located on the same plane as the W3 triangle, separated from it by one of the Cp* ligands, see Figure 3. Two of the Cp* methyl H atoms are in close contact with the anion: H11B

•••O2, 2.448; H11B•••F11, 2.506; H12C•••O11, 2.428; H12C•••F12, 2.540 Å.
It may be reasonably argued that, if any OH group was present on the triangular W3 cluster, this would establish H-bonding contacts with the triflate anion, as observed for the reduced analogue 1 (see above). Rather, the space above and below the W3 triangle, in close contact with the bridging O atoms, is occupied by CH3 groups of [Cp*3W3O6] + ions located in other asymmetric units.

b.2 Two trifluoroacetate structures (compounds 3 and 4)

Before discussing the structural parameters within the [Cp*3W3O6] + ion in more detail, we introduce two additional compounds, related to 2. These were obtained from the same synthetic procedure shown above, but using CF3COOH in place of triflic acid. Although the green product could not be obtained in single crystalline form, red crystals of the oxidation product were obtained after exposure to air. From two batches, crystallized under slightly different conditions, two different crystals containing the same [Cp*3W3O6] + ion were obtained:

[Cp*3W3O6][Zn3(3-O)(-CF3COO)4(CF3COO)(CF3COOH)- (H2O)3]•CF3COOH, 3, and [Cp*3W3O6]2[Zn(H2O)6](CF3COO)2- (CF3COOHOOCCF3)2•4H2O, 4.
The bonding parameters for the tritungsten cluster of each compound are collected in Table 1, together with those of compounds 1 and 2.

Compound 3 contains a complex, apparently unprecedented trinuclear zinc anion, as well as an interstitial CF3COOH molecule, shown in Figure 4. The closest relatives appear to be Zn3(3-O)(-CF3COO)x clusters with x= 5 [15] or 6 [16] , with similar frameworks and geometries. The anion in compound 3 contains one 4coordinate (Zn2) and two 6-coordinate (Zn1, Zn3) zinc ions, joined together by a triply bridging ligand, O(4). One carboxylate ligand bridges atoms Zn1 and Zn2, a second one bridges atoms Zn2 and Zn3, two additional ones bridge atoms Zn2 and Zn3, and a fifth one is terminally bonded to the 4-coordinate Zn2 atom. The latter one is disordered among two different positions, one of which features a H-bond to the aqua ligand identified by atom O2 (O71a•••O2 = 2.70(2) Å). The other position features two weaker H-bonding interactions, the first one with the same aqua ligand (O71•••O2 = 3.11(3) Å) and the second one (not shown in Figure 4) with atom O92 of a coordinated acid ligand (O71•••O92 = 3.02(2) Å). The latter ligand is terminally bonded to atom Zn3 via the carbonyl oxygen atom O91, while the OH function of this ligand (O92) is engaged as a proton donor in H-bonding with the triply bridging oxygen atom O4. The most relevant H-bonding contacts in compound 3 are supplied in the Supporting Information. The coordination sphere of 6-coordinate Zn1 is completed by two water molecules (O1 and O2), whereas that of 6-coordinate Zn3 is completed by a third water molecule (O3). The interstitial acid is a proton donor to the OH group of the coordinated acid (O92) and an acceptor from one of the aqua ligands (O2). The anion also establishes H-bonding interactions, as a proton donor, with the W3 cation: three stronger ones via the aqua ligands O1 and O3 (with atoms O21, O23 and O13 i ), plus two weaker ones via the aqua ligands O3 (with atom O12 i ) and O2 (with atom O22), see the aqua ligands of the [Zn(H2O)6] 2+ cation, see Figure 6. The opposite face of the W3 triangle (atoms O11, O12 and O13) does not establish H-bonds. In addition, the aqua ligands of the [Zn(H2O)6] 2+ cation are H-bond donors toward the interstitial water molecules (O111 i to O2W and O113 i to O1W) and to the free CF3COO -ion (O112 to O51). The other O atom of the latter (O52) is a H-bond acceptor from O1W. The homoconjugate anion, CF3COOHOOCCF3 -, also establishes H-bonds as acceptor with both interstitial water molecules.

The relevant H-bonding parameters are provided in the Supporting Information. The Hbonding network yields an arrangement of the structure in 2D layers (see supplemental figure S2). According to the formulation of the two clusters, deriving from the X-ray analyses shown above, as [Cp*3W3O4(OH)2] 2+ for the green derivative and [Cp*3W3O6] + for the red-orange derivatives, the first cluster has three metal electrons and should therefore be paramagnetic, whereas the second one has only two, which should be located in a symmetric metal-metal bonding combination, [13] and should therefore be diamagnetic. These expectations are in agreement with the experiment.

Compound 1 does not exhibit any strong 1 H NMR signal. On the other hand, it has an EPR resonance, although this is clearly visible only at low temperatures (< 200 K). The frozen glass spectrum in CH2Cl2 solution is shown in Figure 8. The overall shape is close to that expected for tetragonal symmetry, though a slight splitting of the perpendicular component is clearly visible. This is in nice agreement with the asymmetry of the bridge system, which introduces a slight perturbation to the threefold symmetry of the W3 triangle as also indicated by the W-W distances. The perpendicular component does not have sufficient resolution to reveal the 183 W satellites. The parallel component, on the other hand, shows them quite clearly (see enlargement in Figure 8b). This feature was satisfactorily simulated as the sum of three Lorentzians, plus a quadratic function to account for the baseline drift. The satellites correspond to ca. 15% of the total intensity, in agreement with coupling of the unpaired electron with only one of the three cluster W atoms (theoretical values of 14.4, 24.7 and 31.6% are expected for coupling to 1, 2 or 3 equivalent W atoms). This observation will be fully rationalized on the basis of the computational results (vide infra).

Contrary to compound 1, compounds 2-4 do not show any EPR signal. They exhibit instead an NMR resonance at  2.28 in CDCl3, consistent with the equivalence of the three Cp* ligands. Since this single 1 H NMR resonance is not conclusively proving the chemical constitution of the isolated bulk material, further characterization was sought by mass spectrometry (MS).

An electrospray ionisation (ESI) MS study of compound 4 in

CH2Cl2 gives a very clean isotopic cluster, which agrees with the theoretical isotopic distribution of [Cp*3W3O6] + , see Figure 9. The same pattern is also observed in MeOH and in MeOH-H2O mixtures. Note the absence of fragmentation peaks, reflecting the robustness of the molecular scaffold. In order to learn about the fragmentation pattern, the collision-induced decay (CID) of this ion was investigated by MS n . As shown in Figure 10 6) for compounds 2-4, were subjected to geometry optimization in order to find further supporting evidence for their stoichiometry (notably the number of OH vs. O groups) and to investigate their electronic structure. The calculations were run with no symmetry restrictions. Both calculations converged smoothly, giving final geometries in remarkably good agreement with the corresponding experimental ones for the Cp* derivatives (see Table 3). The only slight discrepancy consists of slightly longer bond lengths from the calculations relative to the experiment (W-O distances by 0.02 -0.05 Å; W-W distances by 0.06 -0.10 Å), as is typically observed for DFT calculations. [a] The experimental parameters (X-ray structures) that are chemically and geometrically equivalent are averaged; for the individual parameters, see Table 1. [b] Label W refers to the two geometrically equivalent W atoms; W' refers to the unique atom sitting on the ideal C2 symmetry axis; CG and CG' are the centers of gravity of the rings bonded to atoms W and W', respectively. [c] For the labels assigned to geometrically different W-O and W-OH bonds (a-f), refer to Scheme 1. [d] From the structure of compound 1.

[e] Averaged values from the structures of compounds 2-4.

The geometrical trends for the less symmetric dihydroxo cluster 5 (see Scheme 1) are perfectly reproduced. Particularly notable is the difference between the W-W and W-W' distances. Excellent agreement is also found between the optimized geometry for the [Cp3W3O6] + cluster and the experimentally observed structure for the Cp* analogue. In particular, the geometry is now close to the ideal D3h symmetry, with three very similar W-W distances. Note that the average optimized W-W distance in 6 is essentially identical to the shorter W-W distance for the dioxo-bridged bond in 5. The same phenomenon is observed in the experimental structures. The three W-CG distances are also essentially equivalent.

The electronic structure analysis is particularly interesting. The qualitative interaction diagram for the metal based orbitals in this type of molecular scaffold has been previously discussed [17,[START_REF] Albright | Orbital Interactions in Chemistry[END_REF] and is reproduced in Scheme 2 for convenience. The relevant metalbased molecular orbitals calculated for the two systems are shown in Figure 11, whereas more extensive data are available in the Supporting Information. For system 6a 2-electron W3 V,V,VI clusterthe two metal electrons are located in the a(z 2 ) orbital, as expected. This orbital interaction insures the metal-metal attractive force. The additional electron in system 5a 3-electron W3 V,V,V clusteris located in one of the two orbitals derived from the e(xy) set. Since the ideal symmetry is reduced from D3h to C2 by the addition of the two protons, the e(xy) set splits (e1 of a-type + e2 of b-type) and the unpaired electron is located in the symmetric a component. Note that, whereas the antisymmetric (b) component must be nil on the unique tungsten atom (W') (see Supporting Information), there is no symmetry restriction for the composition of the a-type orbital. However, the composition of this orbital is such that the contribution from the two symmetry-equivalent W atoms is negligible, whereas atom W' contributes greatly. Thus, the spin density is very much localized on the W' atom (0.911), whereas it is negligible on the two W atoms (0.028 and 0.036), in perfect agreement with the measured relative intensity of the satellites in the EPR spectrum (vide supra). In this respect, it should be noted that the related molybdenum complex [Cp*3Mo3O2(OH)4] 2+a 5-electron Mo3 IV,IV,V clusterwas shown to have the same ordering for the metal-based molecular orbitals. [13] Therefore, the unpaired electron for that complex is located in the antisymmetric b orbital and couples to only two Mo nuclei, again consistent with the experimental evidence from EPR spectroscopy.

(e) Comparison with molybdenum and redox considerations

The reduction process reported here parallels that reported recently for the reduction of Cp*2Mo2O5. [13] However, the Mo system leads to a deeper reduction with generation of a 5-electron cluster, the trinuclear Mo3 IV,IV,V [Cp*3Mo3O2(OH)4] 2+ complex. This product has the same molecular architecture as the two trinuclear W clusters described here, differing only by the number of protons and electrons. This difference agrees with the general trend of greater stability for higher oxidation states upon descending the same group of transition metals.

It may be predicted that, by treatment with suitable oxidizing agents (or at a suitable electrochemical potential in an electrochemical experiment) the 5-electron Mo cluster could produce a 3-electron or a 2-electron cluster with the same structure found in the present study for the W system. Conversely, suitable reductive treatment of the W system could afford a 5-electron system such as the previously reported Mo cluster. Other electronic configurations, e.g. 4-electron M IV,V,V , could also exist. Given that proton gains/losses are associated with these transformations (at least that between the two types of W clusters reported in the present contribution), it may also be expected that the redox potential is pH-dependent in a protic environment. A detailed analysis of the electrochemical behaviour of these clusters is beyond the scope of the present investigation and may be carried out at a later time.

Previous studies of the Cp*2Mo2O5 system by a coupled electrochemical-electrospray mass spectrometry methodology, using a flow-through electrochemical cell, [START_REF] Gun | [END_REF]20] revealed a very rich redox behaviour, including the formation of trinuclear clusters that are related to the isolated cluster. We have now attempted to carry out a similar investigation for Cp*2W2O5, but no sufficient quantities of soluble products were revealed by the mass spectrometric analysis. At the potential at which the Mo system started to yield reductive processes, the W system showed no activity, whereas the rapid formation of a film on the surface of the electrode, which blocked any further electrochemical transformation, was observed at lower potentials. Therefore, it seems that the Cp*2W2O5 is more difficult to reduce than the Mo analogue and the primary reduction products (perhaps including the isolated trinuclear clusters described here) are unstable under these highly reducing conditions and proceed further to yield insoluble products.

Conclusions

Like its Mo analogue, [13] compound [Cp*2W2O5] can be reduced in an aqueous medium to a triangular metal-metal bonded cluster. However, only 2-and 3-electron clusters are obtained for W, whereas the Mo system undergoes a deeper reduction, yielding a 5electron cluster.

Like for the Mo cluster, the oxo/hydroxo asymmetry in the bridge system splits the degeneracy of the "e"type frontier orbitals, as indicated by the EPR spectrum of the 3electron cluster and confirmed by the DFT calculations. The variability of the cluster electrons for this system will be further explored for applications in organometallic chemistry and catalysis.

Experimental Section

General procedures. All preparations and manipulations were carried out with Schlenk techniques under an oxygen-free argon atmosphere. All glassware was oven-dried at 120°C. Solvents were dried by standard procedures and distilled under dinitrogen prior to use. 1 H NMR spectra were recorded on a Bruker AM 250, operating at 250 MHz. Chemical shifts are expressed in ppm downfield from Me4Si. Coupling constants are given in Hertz. Mass spectra were recorded with a Finnigan (San Francisco, USA) LCQ quadrupole ion-trap mass spectrometer equipped with an electrosprayionization (ESI) interface. The ESI was operated in the positive ion mode with a spray voltage of 4.5 kV. The capillary voltage was 20 V and the source temperature was 100°C. Mass spectra were obtained by scanning the mass analyzer from m/z 100 to 2000 with 5 total microscans. Maximum inject time was 400 ms. The analyzer was operated at a background pressure of 2•10 -5 Torr. In all experiments, helium was introduced at an estimated pressure of 1 mTorr to improve the ion trapping efficiency and as collision gas for the collision-induced dissociation events. The compounds were isolated in the ion trap with an isolation width of 12 m/z units and activated by using increased collision energy to obtain collision-energydissociation profiles. EPR measurements were carried out at the X-band microwave frequency on a Bruker ESP300 spectrometer. The spectrometer frequency was calibrated with diphenylpicrylhydrazyl (DPPH, g = 2.0037). The starting compound, [Cp*2W2O5], was prepared as described in the literature. [14] Synthesis and crystallization of [Cp*3W3O4(OH)2](CF3SO3)2•2H2O, 1. Metallic zinc (20 mesh, 0.415 g, 6.34 mmol) was added to a solution of [Cp*2W2O5] (50 mg, 0.070 mmol) in 6 mL of MeOH-H2O (1:1 v/v). The mixture was acidified with 10 drops of CF3SO3H and stirred under argon at room temperature for three days, during which time it changed from a yellow solution to a green suspension. The mixture was filtered and the solid was extracted with THF (0.5 mL). Addition of 1.5 mL of Et2O to the filtrate yielded the product as a green precipitate (0.047 g, 72%). A single crystal for the X-ray analysis was obtained by diffusion of an Et2O layer into a THF solution at room temperature. EPR (X-band, CH2Cl2, 120K): g┴ = 1.89; g|| = 1.72 (aW|| = 114 G).

Synthesis and crystallization of [Cp*3W3O6](CF3SO3), 2.

A sample of compound 1 (61 mg, 0.044 mmol) was dissolved in CH2Cl2 and stirred for 1 h at room temperature in open air. The color of the green solution turned orange. Evaporation to dryness left an orange power, which was recrystallized from CH2Cl2/pentane to give red crystals (25 mg, 47% yield). 1 H NMR (CDCl3):  2.28.

Reduction of [Cp*2W2O5

] by zinc in the presence of CF3COOH. Formation of compounds 3 and 4. Metallic zinc (20 mesh, 0.415 g, 6.34 mmol) was added to a solution of [Cp*2W2O5] (50 mg, 0.070 mmol) in 6 mL of MeOH-H2O (1:1 v/v). The mixture was acidified with 10 drops of CF3COOH and stirred under argon at room temperature for five days, during which time it changed from a yellow solution to a green suspension. The mixture was filtered and the solid was dried under vacuum (31 mg). Attempts to produce single crystals from this material failed, only giving powdery precipitates. Red single crystals of compound 3 were eventually obtained from CH2Cl2/hexane, after exposure of the solution to air. 1 H NMR (CD2Cl2):  2.26. In a parallel identical procedure, from one of the numerous attempts to crystallize the green solid, the yellow mother solution of the green powdery precipitate was recovered, dried, and the yellow-orange residue was set again for crystallization from THF/hexane, yielding single crystals of compound 4. 1 H NMR (CDCl3):  2.28. MS (ESI, CH2Cl2): envelope at m/z = 1053, M + (see Results and Discussion section). X-ray diffraction studies. A single crystal of each compound was mounted under inert perfluoropolyether at the tip of a glass fibre and cooled in the cryostream of an Oxford-Diffraction XCALIBUR CCD diffractometer. Data were collected using the monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (SIR97) [21] and refined by least-squares procedures on F 2 using SHELXL-97. [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] All H atoms attached to carbon were introduced at idealised positions and treated with the riding model. All H atoms attached to oxygen were either located on difference Fourier syntheses or their coordinates were calculated on the basis of hydrogen bonding geometry and energy considerations. [23] They were then treated as riding on their parent O atoms.

In compound Cp*3W3O6(CF3SO3), the cation, Cp*3W3O6, is arranged around a twofold axis resulting in one of the Cp* to be disordered over two positions. The CF3SO3 anion is also disordered and arranged around another twofold axis. For both the disordered parts, the atomic positions and anisotropic thermal parameters were restrained to reasonable values using the tools available within SHELXL-97. [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] The drawing of the molecules was realised with the help of ORTEP3. [24] Crystal data and refinement parameters are shown in Table 4. CCDC-643997 -644000 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. Computational details. The DFT calculations were carried out on model systems where the Cp* ligands were replaced on the simpler Cp rings. The starting geometries were based on the crystallographically determined structures and no symmetry restrictions were imposed. The geometries were fully optimized and the resulting minima of the potential energy surface (PES) were verified by the positive value of all second derivatives of the energy. The calculations were performed using the B3LYP threeparameter hybrid density functional method of Becke [25] and the standard LANL2DZ basis set, which included the Hay and Wadt effective core potentials (ECP) for the tungsten atoms, [26] as implemented in the Gaussian03 suite of programs. [START_REF] Frisch | Gaussian 03, Revision B.04[END_REF] The calculation on the open-shell system 5 was carried out using the spin-unrestricted formulation. The value of <S 2 > resulting from the calculation is 0.7575, indicating negligible spin contamination.

Supporting Information (see footnote on the first page of this article): Packing diagrams for compounds 3 and 4; energy diagrams and representative molecular orbitals for systems 5 and 6 (5 pages).

  (a) The [Cp*3W3O4(OH)2] 2+ cluster. The reduction of [Cp*2W2O5] was carried out under the same conditions previously used for the reduction of [Cp*2Mo2O5], namely by metallic zinc in an acidic H2O-MeOH mixed solvent.

Figure 1 .

 1 Figure 1. An ORTEP view of the dication in compound [Cp*3W3O4-(OH)2](CF3SO3)2•2H2O, 1. A most informative structural feature is the variation of the W-W distances: the two bonds bridged by one O and one OH group, W1-W2 (2.7416(3) Å) and W2-W3 (2.7485(3) Å), are almost 0.1 Å longer than the bond bridged by two O groups, W1-W3 (2.6545(3) Å). In addition, the W-OH distances (in the range 2.038-2.080 Å; average 2.06(2) Å) are statistically longer than the W-O distances (in the range 1.872-2.009 Å; average 1.94(6) Å). Furthermore, the W-(-OH)-W angles (83.39(13) and 83.73(13)°) are smaller than the W-(-O)-W angles in the W(-O)2W moiety (average 85.54(14)°). The W-(-O)-W angles in the W(-O)(-OH)W moiety are even larger (average 90.40(15)°). This is certainly a secondary effect, related to the longer W-W distance.

Figure 2 .

 2 Figure 2. An ORTEP view of compound 1, highlighting the H-bonding interactions between dication, anions, and interstitial water molecules. The Cp* ligands bonded to the W3 cluster are omitted for clarity [symmetry codes: (i) 3/2-x, y-1/2, 1-z; (ii) 1/2-x, y-1/2, -z].

  [a] CGn is the center of gravity of the Cp* ligand bonded to atom Wn. [b] This structure has a symmetry imposed C2 axis passing through atoms W2 and the midpoint of the W1-W1 i bond. Numbering scheme correspondence: W3 = W1 i ; O12 = O22 i ; i: -x,y,-z+3/2. (b) The [Cp*3W3O6] + cluster. b.1 The triflate structure (compound 2)

Figure 3 .

 3 Figure 3. A view of the relative orientation of cation and anion in the structure of [Cp*3W3O6](CF3SO3) , 2. Only one of the two symmetryrelated orientations of the trifluoromethylsulfonate anion and of the Cp*(2) ligand are shown for the sake of clarity.

  Figure 5. H-bonding interactions are established on both opposite sides of the triangular W3 face, resulting in zig-zag 1D H-bonded chains (see supplemental figure S1). The most significant bonding parameters of the [Zn3(3-O)(-CF3COO)4(CF3COO)-(CF3COOH)(H2O)3] -anion are collected in Table2.

Figure 4 .

 4 Figure 4. A view of the [Zn3(3-O)(-CF3COO)4(CF3COO)-(CF3COOH)(H2O)3] -anion and the interstitial CF3COOH molecule in compound 3. Only one component (C71a, C72a and O71a) of the disordered CF3COO group is shown for the sake of clarity.

Figure 5 .

 5 Figure 5. A view of the arrangement between the [Zn3(3-O)(-CF3COO)4(CF3COO)(CF3COOH)(H2O)3] -anion, the interstitial CF3COOH molecule, and the [Cp*3W3O6] + cation in compound 3. The cation Cp* ligands and the aqua H atoms have been removed and only the coordination spheres of the Zn atoms are shown for clarity [Symmetry code: (i) ½+x, 3/2-y, z-1/2].

Figure 6 .

 6 Figure 6. A view of the arrangement between the [Cp*3W3O6] + , [Zn(H2O)6] 2+ , CF3COO -, and CF3COO•••H•••OOCCF3 -ions, and the two interstitial H2O molecules, in the structure of compound 4. The cation Cp* ligands and all H atoms have been removed for clarity [symmetry code: (i)x, -y, -z].

Figure 7 .

 7 Figure 7. An ORTEP view of the [Cp*3W3O6] + ion from the structure of compound 3.

Figure 8 .

 8 Figure 8. EPR spectrum of a frozen CH2Cl2 solution of compound 1 at 120 K: (a) experimental spectrum; (b) enlargement of the 3800-4050 G range; (c) simulation of (b), see text.

Figure 9 .

 9 Figure 9. ESI-MS of a CH2Cl2 solution of compound 4. An expansion of the molecular ion isotopic cluster is shown in part (b) and the theoretical simulation in part (c).

Figure 10 .

 10 Figure 10. MS n studies of the [Cp*3W3O6] + ion. (a) MS/MS(1053) at increasing collision energies, plus expansion (b) and simulation (c) of the

  (a), the ion displays a major decay pathway by expulsion of a [Cp*WO2] fragment with transformation into [Cp*2W2O4] + (isotope cluster centered at m/z = 702). However, the isotope pattern (Figure 10(b) with simulation in Figure 10(c)), reveals the presence of a second species one mass unit lighter. Therefore, elimination of a [Cp*WO2H] unit, with the presumed involvement of a CH3 group and generation of a tetramethylfulvene ligand, constitutes a competitive fragmentation pathway. Further trapping of these species and new CID in an MS 3 experiment, see Figure 10(d), is less selective but the major decay pathway is once again expulsion of a [Cp*WO2] fragment from [Cp*2W2O4] + , yielding [Cp*WO2] + (isotope cluster centered at m/z = 351). The latter ion is not produced in a major pathway by direct expulsion of [Cp*2W2O4] from the trinuclear species. (d) DFT calculations Model compounds of the triangular W3 clusters, obtained by replacing the Cp* ligands with the simpler Cp ring, i.e. [Cp3W3O4(OH)2] 2+ (5) for compound 1 and [Cp3W3O6] + (

1 .

 1 Scheme 1. Labeling of geometrically different W-O bonds in the structure of [Cp3W3O4(OH)2] 2+ . The ideal C2 symmetry axis passes through atom W' and through the midpoint of the W-W vector. The Cp ligands are not drawn for the sake of clarity.

2 .Figure 11 .

 211 Scheme 2. Qualitative metal-metal orbital interactions for a Cp3M3L6 molecule of ideal D3h symmetry.

Table 4 .

 4 Selected crystallographic and refinement parameters for all compounds.

	Compound	1	2	3	4
	Empirical formula	C32 H49 F6 O14 S2 W3	C31 H45 F3 O9 S W3	C44 H53 F21 O24 W3 Zn3	C36 H53 F9 O17 W3 Zn0.5
	Formula weight	1387.38	1202.28	2112.52	1513.02
	Temperature	180(2) K	180(2) K	180(2) K	180(2) K
	Wavelength	0.71073 Å	0.71073 Å	0.71073 Å	0.71073 Å
	Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic
	Space group	P 21/a	C 2/c	C c	P -1
	a, Å	19.6490(7)	16.7981(11)	12.9884(12)	12.8993(12)
	b, Å	11.1968(4)	15.1424(7)	21.8922(15)	13.2879(13) Å
	c, Å	19.7744(8)	14.9343(8)	22.862(2)	16.6848(18) Å
	°	90	90	90	96.016(8)
	°	106.433(3)	113.172(5)	90.493(7)	104.993(9)
	°	90	90	90	116.737(9)
	Volume, Å 3	4172.8(3)	3492.3(3)	6500.5(9)	2384.7(4)
	Z	4	4	4	2
	Density (calculated), Mg/m 3	2.208	2.287	2.159	2.107
	Absorption coefficient, mm -1	8.438	9.984	6.509	7.564
	F(000)	2644	2272	4040	1446
	Crystal size, mm 3	0.27 x 0.14 x 0.09	0.26 x 0.205 x 0.043	0.32 x 0.2 x 0.126	0.34 x 0.15 x 0.034
	 range, °	2.79 to 26.37	3.65 to 30.03	3.20 to 28.28	2.61 to 25.03
	h, k, l ranges	-24÷24, -13÷13, -23÷24	-23÷16, -21÷21, -19÷21	-16÷17, -29÷29, -30÷29	-15÷15, -15÷14, -19÷18
	Reflections collected	29412	17199	28767	14697
	Independent reflections	8524 [R(int) = 0.0348]	5088 [R(int) = 0.0369]	11663 [R(int) = 0.0547]	8107 [R(int) = 0.0691]
	Completeness to  = 25.00°, %	99.8	99.7	99.9	96.3
	Absorption correction	Multi-scan	Multi-scan	Multi-scan	Multi-scan
	Max. and min. transmission	0.3589 and 0.1896	0.3236 and 0.1509	1.0 and 0.2368
	Refinement method	Full-matr. least-sq. (F 2 )	Full-matr. least-sq. (F 2 )	Full-matr. least-sq. (F 2 )	Full-matr. least-sq. (F 2 )
	Data / restraints / parameters	8524 / 8 / 547	5088 / 32 / 226	11663 / 32 / 857	8107 / 0 / 436
	Goodness-of-fit on F 2	1.124	1.182	0.923	0.867
	R1, wR2 [I>2(I)]	0.0324, 0.0738	0.0417, 0.0883	0.0382, 0.0619	0.0502, 0.0921
	R1, wR2 (all data)	0.0413, 0.0772	0.0484, 0.0909	0.0502, 0.0649	0.1356, 0.1097
	Absolute structure parameter			0.007(6)
	Largest diff. peak and hole, e•Å -3 3.747 and -1.748	2.455 and -3.030	1.489 and -1.302	4.859 and -1.751

Acknowledgments

We are grateful to the European Commission for funding of this work through the AQUACHEM Research Training Network (Project n° MRTN-CT-2003-503864). We also thank CINES and CICT (project CALMIP) for a grant of free computer time. Supplemental travel support was provided by a Bosphorus bilateral Program of Integrated Actions, co-sponsored by the French Ministry of Foreign Affairs in France and by TUBITAK in Turkey (TBAG-U/142(105T256)). ____________

Entry for the Table of Contents


Chiara Dinoi

Gülnur Taban

Pelin Sözen

Funda Demirhan

Jean-Claude Daran

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

Improved syntheses of [Cp*2Mo2O5] and [Cp*2W2O5]: Structural characterization of Na[Cp*MoO3]•5H2O and [Cp*2W2O5]

Keywords: Molybdenum, Tungsten, aqueous organometallic chemistry, oxo ligands, pentamethylcyclopentadienyl ligands

Use of aqueous conditions and the ROOH (R = tBu or H) compounds as oxidizing agents affords a simple two-step, one-pot synthetic procedure leading to [Cp*2M2O5] (M = Mo, W) from [M(CO)6] selectively and in high yields. The hexacarbonyl compounds are converted to Na[Cp*M(CO)3] in refluxing THF, then the mixture is diluted with water (W) or aqueous NaOH (Mo) and directly treated with 6 equivalents of ROOH. Acidification of an aqueous solution of the Na[Cp*MO3] product affords spectroscopically pure [Cp*2M2O5] in high isolated yields (global yields from [M(CO)6], for R = tBu: Mo, 91%; W, 87%). The Xray structures of Na[Cp*MoO3]•5H2O and [Cp*2W2O5] are also reported.

Introduction

High oxidation state organometallic compounds have found useful applications in a variety of catalytic processes [1,2], mostly related but not limited to oxidations processes [3].

In particular, the catalytic activity of organometallic oxo molybdenum compounds in olefin epoxidation [4] has recently attracted renewed interest [5][6][7][8][9][10][11][12]. Together with other derivatives, compound [Cp*2Mo2O5] was found an excellent catalytic precursor for the epoxidation of cyclooctene by tBuOOH in CHCl3, even in the presence of water [12]. To the best of our knowledge, the analogous [Cp*2W2O5] derivative has not been tested as an oxidation catalyst, although high oxidation state tungsten derivatives are often found to possess comparable or greater catalytic activity than the molybdenum analogues [13]. The synthesis of these compounds, however, suffers from time investment and/or moderate global yields.

Compound [Cp*2Mo2O5] was first obtained in 40% yields by aerial oxidation of Cp*Mo(CO)2(NO), a precursor that must itself be prepared from commercially available starting materials [14]. Similar considerations apply to subsequently reported syntheses, starting from [Cp*Mo(CO)2]2 (69% yields) [15,16], [{Cp*Mo(CO)2(OMe)}n] (7.5% yields) [17], and [Cp*MoBr(CH3)(NO)]2 (unspecified yields) [18], again by aerial oxidation. The analogous aerial oxidation of [Cp*W(CO)2]2 afforded [Cp*2W2O5] in 14% yield [15]. Using a different strategy, basic (NaOH) hydrolysis of Cp*MoCl4 in air yields the product in two steps via the Cp*MoO2Cl intermediate, the reported yield of each step being 78% and 82% [19]. The alternative hydrolysis using tBuNH2 yields the same final product in two steps via the [tBuNH3][Cp*MoO3] intermediates, with yields of 68% and 58%, respectively (the tungsten analogue is similarly prepared via the [tBuNH3][Cp*WO3] intermediates in 80% and 67% yields for the two steps) [19]. An alternative hydrolytic approach by protonolysis of [Cp*M(NtBu)2Cl] (M = Mo, W) with aqueous HCl was reported by Sundermeyer [20]. The synthesis affords the final pentaoxo complex in high yield (98% yield for Mo or 81% yield for W ) but requires again a preliminary synthetic effort to access the required starting material (two steps from commercially available starting materials) and the sacrificial use of tBuNCO.

Our current exploration of Cp*Mo VI aqueous chemistry [21][22][23][24][25] has required the frequent preparation of compound [Cp*2Mo2O5] in substantial amounts. Until now, we have opted for the hydrolytic approach via the [Cp*MoCl4] compound, which we have recently improved [26]. The synthesis of this starting material is high yield, but requires three steps from commercially available [Mo(CO)6] [27,[START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] with isolation of the Cp*Mo(CO)3(CH3) intermediate. Furthermore, the oxidative hydrolysis step is delicate since the rates of oxidation and hydrolysis are similar; a complete hydrolysis before oxidation yields the dinuclear [Cp*2Mo2O4] by-product, which is relatively resistant to further oxidation and lowers the final yield [26]. This synthesis is summarized in Scheme 1. The considerable time investment required for this synthesis, in addition to the need for sacrificial reagents (MeI and PCl5 or PhICl2) has encouraged us to find a more direct and simpler preparative method.

Results

(a) Syntheses

We wondered whether compound Na[Cp*Mo(CO)3] could be transformed directly into Na[Cp*MoO3], therefore by-passing three steps and the isolation of two intermediates with respect to our traditional synthetic scheme outlined in Scheme 1. In addition, the Na salt of the trioxomolybdate(VI) complex, as obtained from the hydrolytic procedure from [Cp*MoCl4], can be converted to the final product by direct acidification of an aqueous solution without isolation. It is also possible to envisage a direct oxidation of the first intermediate, the sodium salt of the tricarbonylmolybdate(0) complex, without isolation. Therefore, the potential exists for a simple one-pot synthesis.

The new method that we report in this contribution has been inspired by the following considerations, partly extrapolated from previously published observations. The Cp* tricarbonyl anion may be oxidized monoelectronically by Fe2(SO4)3 (1 equivalent) to produce [Cp*2Mo2(CO)6] [START_REF] Birdwhistell | [END_REF]30], and by FeCl3 (2 equivalents) to afford the corresponding [Cp*Mo(CO)3Cl] [30]. Although a synthesis of the latter compound from [Cp*Mo(CO)3] - and Cl2 does not seem to have been described, the corresponding bromo compound has been prepared from [Cp*Mo(CO)3] -and Br2 (Equation 1, X = Br) [31]. The Mo IV compounds [Cp*Mo(CO)2X3] (X = Cl, Br) have been prepared from [Cp*2Mo2(CO)6] and 3 equivalents of X2 [32,33]. Thus, the stoichiometry of Equation 2 can be deduced, as well as a possible direct synthesis of [Cp*Mo(CO)2X3] from [Cp*Mo(CO)3] -. Further oxidation yields [Cp*MoX4] (Equation 3) [START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF]. An analogous process has also been described for the Cp system: stepwise generation of [CpMoCl4] from [Cp2Mo2(CO)6] via [CpMo(CO)3Cl] and [CpMo(CO)2Cl3] [34].

[Cp*Mo(CO)3] -+ X2  [Cp*Mo(CO)3X] + X - Equation 1 [Cp*Mo(CO)3X] + X2  [Cp*Mo(CO)2X3] + CO Equation 2 [Cp*Mo(CO)2X3]2 + X2  2 [Cp*MoX4] + 2 CO

Equation 3

The halogen nature determines the maximum allowed oxidation state of the [Cp*MoXn] system (n = 4 for X = Cl and Br; 3 for X = I in the anionic complex [Cp*MoI4] -) [33]. The corresponding fluoride system has not been investigated for molybdenum, but Cp*WF5 (prepared by aerial oxidation from Cp*WF4) is a stable compound [35], whereas the highest oxidation state attainable for X = Cl appears to be the same as for the Mo system, [Cp*WCl4] [27]. On the other hand, the maximum oxidation state (+6) is reached once again with oxo ligands, [Cp*MO3] -(M = Mo, W). Since an oxo ligand is readily obtained from two hydroxo ligands by elimination of H2O, we have imagined that a direct transformation of [Cp*M(CO)3] -to [Cp*MO3] -could be accomplished with X = OH, namely by using the easily available and inexpensive ROOH (R = tBu or H) compounds as oxidants. Further results suggesting that this idea has solid foundations are the good yield synthesis of [Cp*MoO2Cl] by oxidation of [Cp*Mo(CO)3Cl] with tBuOOH [5], as well as that of [Cp*MoO2(CH3)] and other similar cyclopentadienyl derivatives by oxidation of the corresponding tricarbonyl methyl precursors with the same oxidizing reagent [6].

A word of caution relates to a possible over-oxidation to afford peroxo derivatives. Indeed, excess tBuOOH converts [CpMo(CO)3(CH3)] to [CpMoO(O2)(CH3)], although the intermediacy of [CpMoO2(CH3)] in this reaction was established [6]. It has also been reported that the reaction of [Cp*MoO2Cl] with tBuOOH leads to the corresponding peroxo derivative, [Cp*MoO(O2)Cl] [4]. Another point to be clarified concerns the release of the three CO ligands as either CO or CO2, the two extreme cases requiring the use of either 3 or 6 equivalents of ROOH (R = tBu or H) according to the stoichiometries shown in Equation 4and Equation 5, respectively. Obviously, an intermediate stoichiometry may also be followed. Finally, the pH is also to be considered as a variable. At neutral pH, both starting material and reaction product are present in anionic form, whereas at lower pH the tricarbonyl anion is protonated to [Cp*Mo(CO)3H] (pKa = 17.1 in MeCN [36], from which the aqueous pKa may be estimated [37] as ~ 9), whereas the trioxo anion affords the unstable complex [Cp*MoO2(OH)] (pKa = 3.65) [38], followed by rearrangement to [Cp*2Mo2O5]. Although protons are not directly consumed during the oxidation process, the reaction may be catalyzed by acids or bases.

[Cp*Mo(CO)3] -+ 3 ROOH  [Cp*MoO3] -+ 3 ROH + 3 CO Equation 4 [Cp*Mo(CO)3] -+ 6 ROOH  [Cp*MoO3] -+ 3 ROH + 3 CO2

Equation 5

Because of the above mentioned precedents of the [Cp*Mo(CO)3X] oxidation (X = Cl, CH3) with tBuOOH [5,6], this oxidizing agent was selected for our initial synthetic studies.

The synthetic procedure was carried out several times at various tBuOOH/Mo ratios and pH values. The optimized conditions are given in the experimental section. First, we have observed the need of 6 equivalents of oxidant to complete the reaction. A MnO2 test on the final reaction mixture did not show any gas evolution, indicating the complete consumption of the oxidant. On the other hand, we did not observe any evidence for the formation of peroxo derivatives. It must be remarked that we have not tested the reaction with > 6 equivalents of oxidant and that a complex having the stoichiometry [Cp*MoO2(O2)] -has not been described.

The 1 H NMR spectrum of the crude reaction mixture only showed the resonance of the [Cp*MoO3] -ion when 6 equivalents of oxidant were used, whereas impure mixtures were obtained when using lower tBuOOH/Mo ratios. The reaction worked best (selective oxidation and cleaner product) under basic conditions, although the product was also formed upon working at neutral pH. It is interesting to note, however, that the oxidation of [Cp*2Mo2O4] (as a pure starting compound, prepared separately) with tBuOOH does not occur at a significant rate at neutral pH (no observable oxidation overnight at room temperature; slow oxidation upon heating). Oxidation to Na[Cp*MoO3], on the other hand, takes place under basic (NaOH) conditions (this is the predominant species after stirring overnight at room temperature). At acidic pH, compound [Cp*2Mo2O4] did react with tBuOOH, but the colour changed to orange-brown instead of the known [38] bright yellow colour of [Cp*MoO2(H2O)] + (the stable acidic form of "Cp*Mo VI ") and an 1 H NMR analysis indicated that a different product was obtained.

Using the optimized procedure, the final [Cp*2Mo2O5] product was recovered in high yields and in a spectroscopically pure form, notably without any sign of contamination by the tetraoxo Mo V derivative, [Cp*MoO2]2, which systematically accompanies our previously reported synthetic protocol [26]. After the tBuOOH addition, an immediate colour change occurs from orange to red, followed by slow bleaching to the final yellow colour of We have also tested the same synthetic protocol using the "greener" hydrogen peroxide as oxidant. Formation of the desired product was observed under the same conditions used for the tBuOOH oxidation (6 equivalents, high pH). In fact, the transformation is faster, the observed colour bleaching indicating that a complete conversion required only a few minutes, rather than 3 days. Conversely, the yields of isolated product were lower and less reproducible. The best yield (66.3%) is reported in the experimental section but much lower yields have also been occasionally obtained. We therefore recommend use of the tBuOOH oxidant for reliable results.

We have subsequently adapted the same procedure to the preparation of [Cp*2W2O5].

The oxidation step works much the same way as for the molybdenum analogue (6 equivalents of oxidant are required). However, in this case the best results were obtained by working at neutral pH, whereas the use of basic conditions led to less satisfactory and erratic results.

Dissolution of an authentic sample of [Cp*2W2O5] in basic MeOH-H2O (presumably yielding

[Cp*WO3] -) and monitoring by 1 H NMR against an internal standard showed a slow decomposition (ca. 50% over 2 days). This decomposition may be pH dependent, explaining the erratic results in synthesis, but we have not pursued a study of this decomposition process any further.

We should also underline two additional and subtle differences between the W and the Mo systems, not clearly described in the previous literature. The [Cp*2W2O5] compound has a non negligible solubility in water, whereas the Mo analogue is essentially insoluble. On the other hand, whereas [Cp*2Mo2O5] is relatively soluble in diethyl ether, the W analogue is not.

Therefore, extraction of the neutral precipitate with ether, which constitutes a convenient work-up procedure for the Mo compound, cannot be used for the W analogue. Another noticeable difference is the precipitation of the W product in the form of a very fine colloidal suspension, which does not settle even upon heating. Increasing the solution ionic strength did not show any apparent beneficial effect. Thus, the most effective procedure, though a bit more tedious, consists of dissolving the Na[Cp*WO3] residue into the minimum amount of water, followed by acidification and extraction of the resulting [Cp*2W2O5] product with CH2Cl2, in which the compound is readily soluble. Like for the synthesis of the Mo compound, the oxidation of Na[Cp*W(CO)3] also occurs when using 6 equivalents of H2O2, though once again the overall yield is less satisfactory and not always reproducible.

We would also like to briefly mention a few attempts to synthesize [Cp*2M2O5] from Na[Cp*M(CO)3] using bleach (ClO -) as oxidant. Although this reagent is not as "green" as H2O2, its ready availability and low cost made it an interesting candidate for our study. The Mo system was investigated more in detail. Use of just one equivalent of the reagent led rather selectively to Cp*Mo(CO)3Cl (as indicated by comparing the IR spectrum with the literature [39]; the compound was not isolated). Use of a higher amount of ClO -led (after acidification with CH3COOH as described above) to a mixture of the expected [Cp*2Mo2O5] product and significant amounts of [Cp*2Mo2O4] and [Cp*MoO2Cl] ( 1 H NMR signals at  2.02 [19], 1.99 [26] and 2.09 [19], respectively, in CDCl3), the chloro derivative being the major product. A separate procedure starting from pure [Cp*2Mo2O4] shows that this is oxidized by bleach in THF, but once again both [Cp*2Mo2O5] and [Cp*MoO2Cl] are obtained and the tetraoxo starting compound was not completely consumed, even when using a large excess of oxidant. Analogous results (several products according to the 1 H NMR spectrum) were obtained for the W system. Therefore, this reagent was abandoned in favour of The structure of compound [Cp*2W2O5] has already been reported, although this also contains an interstitial C6F5NH2 molecule [44]. Here, we describe a crystal structure that is devoid of any co-crystallized foreign molecule. This structure is isomorphous with that of [Cp*2Mo2O5] (polymorph II) described by Rheingold and Harper [16]. The relevant metric parameters (see Table 2) are close to those of the previously reported structure [44]: average W=O, 1.725(9) Å (cf. 1.722); average W-O(bridging), 1.886(7) Å (cf. 1.882(7) Å). These values are also close to those of the same polymorph of [Cp*2Mo2O5]: Mo=O, 1.708(9) Å and Mo-O(bridging), 1.882(12) Å. Although there is no imposed symmetry in this dinuclear molecule, the W-O-W moiety is close to linear (linearity is symmetry-imposed in the previously reported structure [44]). <Table 2 and Figure 2 here>

Conclusion

We have developed rapid, high yield, and simple one-pot synthetic procedures to access [Cp*2M2O5] (M = Mo, W) from commercially available [M(CO)6], needing only Cp*Na, tBuOOH (or H2O2), and CH3COOH reagents. With minor adaptations, taking solubility differences into account, these procedures should be applicable to other cyclopentadienyl derivatives, the only provision being that the compounds are stable in water. This is not the case, for instance, for compound [Cp2Mo2O5] [26]. We expect that these new syntheses will give further impulse to the development of high oxidation state organomolybdenum and -tungsten chemistry and catalysis in water.

Experimental Section

General procedures. Unless otherwise stated, all preparations and manipulations were carried out with Schlenk techniques under an oxygen-free argon atmosphere. All glassware was oven-dried at 120°C. Solvents were dried by standard procedures and distilled under dinitrogen prior to use. 1 The resulting solution was stirred at reflux during 24 h affording the deep orange solution of the corresponding Na[Cp*Mo(CO)3] salt (IR strong peaks at 1886, 1784, 1733 cm -1 ). To this solution, after cooling to room temperature, was added 40 mL of previously degassed 0.4 M NaOH, then 2.5 mL of a 70% aqueous tBuOOH solution (18 mmol), resulting in a colour change to red with vigorous gas evolution. The mixture was stirred at room temperature for 3 days, turning orange and then pale yellow, and finally evaporated to dryness to yield [Cp*MoO3]Na as a white solid. 1 H NMR (D2O): = 1.89 (Cp*). This solid was then extracted with 40 mL of distilled water and filtered. The pale yellow filtrate was acidified with diluted glacial acetic acid, causing precipitation of a yellow solid at pH 3.8-4.0. The acetic acid addition was stopped when the addition of one further drop did not cause any additional precipitation. The precipitate was extracted with 4 x 100 mL portions of ether, the last extract being colorless. The combined organic layers were washed with distilled water and dried over MgSO4 overnight. Evaporation of the solvent yielded Cp*2Mo2O5 as a yellow solid (0.742 g, 1.37 mmol, 91.2% yield). The 1 H NMR spectrum of this compound (singlet at = 2.01 in CDCl3) corresponds to that described in the literature [19].

In a separate procedure, the isolated white Na[Cp*MoO3] intermediate was recrystallized by slowly cooling a hot saturated THF solution to room temperature, yielding suitable single crystals for the X-ray analysis.

(a) By oxidation of [Cp*Mo(CO)3]Na with H2O2. This procedure is identical to that described above, except for the use of a 35% aqueous H2O2 solution (1.6 mL, 18 mmol) in place of the tBuOOH solution. The same color changes described above were observed.

However, these took place within a few minutes rather than over 3 days. Treatment of a small aliquot of this solution with MnO2 showed no gas evolution, indicating that complete consumption of H2O2 had taken place. The isolated white [Cp*MoO3]Na intermediate had the same 1 H NMR properties described above. Using this procedure, 0. The resulting solution was stirred at reflux during 24 h affording the dark orange solution of the corresponding Na[Cp*W(CO)3] salt (IR strong peaks at 1881, 1780, 1729 cm -1 ). To this solution, after cooling to room temperature, was added 40 mL of degassed water, then 2.5 mL of a 70% aqueous tBuOOH solution (18 mmol), resulting in a colour change to red with vigorous gas evolution. The solution was stirred at room temperature for 3 days, turning orange, then yellow, and finally colourless. The solution was then evaporated to dryness to yield [Cp*WO3]Na as a white solid. 1 H NMR (D2O): = 2.04 (Cp*). This solid was then extracted with the minimum amount of distilled water and then acidified with diluted glacial acetic acid, causing a colour change from colourless to yellow with precipitation of a very fine pale yellow solid (pH around 4), which did not settle. The acetic acid addition was alternated by extractions of the product into CH2Cl2, until further addition of one further drop did not cause any additional precipitation. The combined organic layers (total: 150 mL) were washed with distilled water and dried over MgSO4 overnight. Evaporation of the solvent yielded Cp*2W2O5 as a yellow solid (0.93 g, 1.30 mmol, 87% yield). The 1 H NMR spectrum of this compound (singlet at = 2.18 in CDCl3) corresponded to that described in the literature [19].

(b) By oxidation of [Cp*W(CO)3]Na with H2O2. This procedure is identical to that described above, except for the use of a 30% aqueous H2O2 solution (2 mL, 18 mmol) in place of the tBuOOH solution. The same color changes described above were observed. The isolated white [Cp*WO3]Na intermediate had the same 1 H NMR properties described above.

Using this procedure, 0.50 g of spectroscopically ( 1 H NMR) pure [Cp*2W2O5] (0.696 mmol; 46.4% yield) were recovered. Suitable single crystals were grown by layering a saturated CH2Cl2 solution with pentane at 0°C.

X-ray diffraction studies.

A single crystal of each compound was mounted under inert perfluoropolyether at the tip of a glass fibre and cooled in the cryostream of the Oxford-Diffraction XCALIBUR CCD diffractometer. Data were collected using the monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (SIR97) [45] and refined by least-squares procedures on F 2 using SHELXL-97 [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF]. All H atoms attached to carbon atoms were introduced in calculations in idealised positions and treated as riding models. The H atoms attached to oxygen atoms were located in the difference Fourier synthesis but they were treated as riding on their parent O atoms. The drawing of the molecules was realised with the help of ORTEP32 [START_REF] Farrugia | [END_REF]. Crystal data and refinement parameters are shown in Table 3. CCDC-644159 & 644160 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. <Table 3 here>

<Scheme 1 here>

 1 In this contribution, we report a substantial improvement of the synthetic method, affording both [Cp*2M2O5] (M = Mo, W) compounds in excellent yields from the corresponding [M(CO)6] starting material, in three simple steps without isolation of any intermediate and using inexpensive oxidants (tBuOOH or H2O2). We also describe the molecular structures of Na[Cp*MoO3]•5H2O and [Cp*2W2O5].

[

  Cp*MoO3] -over three days. The intermediate red colour is due to [Cp*Mo(CO)3]2, as verified by its isolation and spectroscopic (IR, 1 H NMR) characterization when the reaction was stopped at earlier conversions. During the entire synthetic procedure, no isolation of intermediates is required because water and tBuOOH can be added directly to the THF solution of Na[Cp*Mo(CO)3] that results from the first step. However, the solvents must be completely removed after the oxidation step and replaced with neat water in order to optimize the precipitation of [Cp*2Mo2O5].

  structures Compound Na[Cp*MoO3], obtained as a crude product from the oxidation procedure under basic conditions, could be crystallized from hot THF. The structure of the related [Cp*WO3] -ion has already been described in three different salts, [tBu(EtC{O}CH2CH2)-NH2][Cp*WO3], [tBuNH3][Cp*WO3]•1/2[tBuNH3]C1, and [(Ph3P)2N][Cp*WO3]•2H2O [19], but no salts of the [Cp*MoO3] -has previously been structurally characterized to the best of our knowledge. The compound crystallizes with two independent [Cp*MoO3] -ions in the asymmetric unit. The corresponding Na + ions are octahedrally surrounded by water molecules, three of which (defined by atoms O1, O3 and O5) are shared by the two Na atoms in a face-sharing bioctahedral (FSBO) arrangement. In addition, the sodium atoms Na1 and Na1 i of two different asymmetric units, related to each other by an inversion center, share two additional water molecules (those defined by O2 and O2 i ), forming a central edge-sharing bioctahedral (ESBO) unit. This arrangement gives rise to an overall tetraoctahedral [Na4(H2O)16] 4+ cluster, see Figure 1. Similar chains of aqua-bridged sodium cations have been described before [40-43], but a discrete tetranuclear unit with a FSBO-ESBO-FSBO arrangement, as observed in the present structure, is unprecedented according to the Cambridge Crystallographic Data Centre's database. This cluster is hydrogen bonded to additional, symmetry related Na4 clusters to form a one-dimensional infinite sodium chain, which is linked to other chains by additional hydrogen bonding to form two dimensional sheets (two views are provided in the Supporting Information). The two-dimensional sheets are separated from each other by two hydrophobic layers of organometallic anions. The anion layers are linked to the cation layers by hydrogen bonds, both directly (e.g. O11•••O8', O12•••O1', O22•••O5) and indirectly via two additional interstitial water molecules in each asymmetric unit, O9 and O10. All H atoms of the water molecules have been directly located from the difference Fourier map, unambiguously establishing the chemical composition as a sodium salt of the [Cp*MoO3] -ion, rather than a co-crystallized hydrated NaOH network and neutral Cp*MoO2(OH). Note that, although the crystallization solvent is neat THF, only water molecules from the oxidation reaction medium are retained for sodium coordination and for networking. <Figure 1 here> Significant distances and angles are shown in Table 1. The Mo-O distances average 1.752(12) and 1.747(30) Å for the two crystallographically independent anions (cf. 1.751, 1.746 and 1.758 Å in the three above-mentioned salts of [Cp*WO3] -). As discussed in that contribution [19], hydrogen bonding is probably responsible for the significant lengthening relative to a typical Mo-O double bond (for instance, 1.69(2) Å in [Cp*2Mo2O5], average over three different polymorphs [16]). The average O-Mo-O angles (106.0(2) and 105.3(25)°, respectively) are also close to those observed in the three [Cp*WO3] -salts (105.2, 104.6 and 105.3°) [19]. <Table 1 here>

  H NMR spectra were recorded on a Bruker AM 250, operating at 250 MHz. Chemical shifts are expressed in ppm downfield from Me4Si. Coupling constants are given in Hertz. Compound [M(CO)6] (Aldrich), a 30% aqueous solutions of tBuOOH (Aldrich) and 30% aqueous solutions of H2O2 (Fluka) were used as received. Pentamethylcyclopentadiene (Cp*H, Acros) was converted to the Na salt by reaction with NaNH2 in refluxing THF. Tetraoxo--oxobis(pentamethylcyclopentadienyl)dimolybdenum(VI), [Cp*2Mo2O5]. (a) By oxidation of [Cp*Mo(CO)3]Na with t BuOOH. In a flask containing NaCp* (0.521 g, 3.3 mmol) was added a solution of [Mo(CO)6] (0.792 g, 3 mmol) in THF (60 mL).

  539 g of spectroscopically ( 1 H NMR) pure [Cp*2Mo2O5] product (0.99 mmol; 66.3% yield) were recovered. Tetraoxo--oxobis(pentamethylcyclopentadienyl)ditungsten(VI), [Cp*2W2O5]. (a) By oxidation of [Cp*W(CO)3]Na with t BuOOH. In a flask containing NaCp* (0.521 g, 3.3 mmol) was added a solution of [W(CO)6] (1.053 g, 3 mmol) in THF (80 mL).

Figure 1 .

 1 Figure 1. ORTEP view showing two asymmetric units of the structure of Na[Cp*MoO3]•5H2O. The aqua ligand and interstitial water H atoms have been omitted for clarity.

Figure 2 .

 2 Figure 2. ORTEP view of compound [Cp*2W2O5].

Table 3 .

 3 Selected crystallographic and refinement parameters for compounds .

	Identification code	2{Na[Cp*MoO3]}•10H2O	[Cp*2W2O5]
	Empirical formula	C20 H50 Mo2 Na2 O16	C20 H30 O5 W2
	Formula weight	784.46	718.14
	Temperature,K	180(2)	160(2)
	Wavelength , Å	0.71073	0.71073
	Crystal system	Triclinic	Monoclinic
	Space group	P -1	P 21/n
	a, Å	8.7340(9)	9.3219(3)
	b, Å	12.5507(11)	16.7198(5)
	c, Å	15.5984(16)	13.8918(5)
	,°	78.673(8)	90.0
	,°	78.370(9)	98.568(3)
	,°	85.901(8)	90.0
	V, Å 3	1641.2(3)	2141.02(12)
	Z	4	4
	Dcalc, Mg/m 3	1.587	2.228
	, mm -1	0.855	10.763
	F(000)	808	1352
	Crystal size, mm 3	0.58 x 0.112 x 0.045	0.498 x 0.101 x 0.071
	°, range	2.90 to 26.31	2.85 to 28.28
	Reflections collected	12944	18790
	Independent reflections [Rint]	6702 [0.0371]	5299 [0.0504]
	Absorption correction	Multi-scan	Multi-scan
	Max. and min. transmission	0.9997 and 0.7370	1.00000 and 0.18984
	Refinement method	F 2	F 2
	Data / restraints / parameters	6702 / 0 / 371	5299 / 0 / 254
	GOF on F 2	1.050	1.053
	R1, wR2 [I>2(I)]	0.0586, 0.1485	0.0369, 0.0966
	R1, wR2 (all data)	0.0885, 0.1640	0.0583, 0.1072
	Largest diff. peak and hole	3.757 and -2.679	4.163 and -1.722

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). Supplemental support by a Bosphorus bilateral Programme of Integrated Actions, cosponsored by French Ministry of Foreign Affairs in France and by TUBITAK in Turkey (TBAG-U/142(105T265)) is also gratefully acknowledged.

Synopsis

A simple two-step, one-pot, high yield synthesis of compounds [Cp*2M2O5] (M = Mo, W) from [M(CO)6] is described.