Jenny Gun

Alexandre Modestov

Ovadia Lev

Dirk Saurenz

Mikhail A Vorotyntsev

Rinaldo Poli
email: poli@u-bouregogne.fr

Studies of the reduction of (C5Me5)2Mo2O5 in Methanol -Water -Acetate Solutions by On-Line Electrochemical Flowcell and Electrospray Mass Spectrometry

come

Introduction

The rapid development and eventual commercialization of the Electrospray Ionization Mass Spectrometers (ESI-MS) in the eighties and nineties has opened the door to the determination of nonvolatile inorganic, organic and organometallic compounds by mass spectrometry. 1,[START_REF] March | Quadrupole Storage Mass Spectrometry in Chemical analysis[END_REF] ESI-MS is an atmospheric pressure technique that uses an electrostatic sprayer to transfer ionic solutes from the sample solution to the gas phase. [START_REF] Vestal | [END_REF]4 The ESI-MS technique is gradually becoming a valuable tool for the investigation of organometallic compounds in protic as well as in aprotic solvents 4 and has proven especially useful in combination with the investigations of redox processes, particularly reduction processes.

Electrochemical methods are the method of choice for obtaining reduced organometallic compounds since potential dependent information can be readily obtained, reversibility is easily checked, and complexation by the chemical reducing agents or their byproducts is minimized. Bond et al. have made extensive use of ESI-MS for the understanding of complex electrochemical transformations of organometallic compounds such as the reductions of cis-(Et2-dcbpy)2RuX2 (X = Cl -, I -, NCS -, CN --) 5 and 17-electron rhenium(II) carbonyl phosphine complexes. 6 These works are based on the combination of preparative electrochemistry, followed by ESI-MS analysis. However, preparative electrochemistry suffers from several disadvantages. First, considerable time is needed in order to achieve full conversion, thereby limiting the resolution of potential dependent investigations. Second, the long reduction times often involve fouling of the electrode surface by polymeric by-products. Third, the reduced compounds have ample time to participate in unwanted subsequent chemical reactions among themselves or with the oxidized forms. Fourth, preparative electrochemistry followed by ESI-MS studies involves considerable sample handling which may lead to oxygen penetration and impurity introduction.

The importance of flow-through electrochemical cells connected to a mass spectrometer and the instrumentation involved have been recently reviewed. [START_REF] Bittins-Cattaneo | Electroanalytical Chemistry[END_REF] Cole et al., [START_REF] Xu | [END_REF]9 Bond et al., 10 and Van Berkel et al., 11,12 used different configurations of electrochemical cells coupled to on-line ESI mass spectrometers for the investigation of redox reactions. We devised an on-line concentric electrochemical cell connected to ESI-MS in which the counter electrode and its products are separated from the reacting stream reaching the ESI-MS in order to isolate and protect the reduced compounds from the oxidized substances generated at the counter electrode.

Oxygen-containing organomolybdenum complexes are especially intriguing since, in spite of their ability to attain a large number of oxidation states, relatively little is known about their speciation in protic solutions or about their electrochemistry. More information is available, on the other hand, on inorganic oxomolybdenum compounds, including ESI-MS speciation studies. Singly, doubly, and triply charged heteropoly and isopolymolybdate anions were studied in organic and aqueous organic solutions 13,14 though the reduction products of these compounds were not investigated by mass spectrometry. The subject of the present study is the identification of the electrochemical reduction products of complex Cp*2Mo2O5 in an aqueous-methanol solution. Cp*2Mo2O5 is a member of a relatively broad family of high oxidation organometallic compounds, some of which have shown remarkable stoichiometric and/or catalytic chemical transformations. Although some of these molecules are stable in an aqueous environment and are even synthesized in water, their aqueous chemistry has rarely been investigated. For the particular case of Cp*2Mo2O5, while most studies have been carried out in organic solvents, [15][16][17][18][19] other contributions have restricted the use of aqueous media to some aspects of the synthetic procedure. [20][21][22][23] Some of us have recently reported an improved aqueous synthesis 24 of Cp*2Mo2O5 and a detailed investigation of its speciation in a MeOH-H2O mixture by conductivity studies. 25 An additional interest in this molybdenum system is the redox-richness of the metal. Thus, exploring the redox behavior of Cp*2Mo2O5 as a prototype compound may open up avenues for the development of new stoichiometric or catalytic redox chemistry.

Results and Discussion

(a) Studies of Cp * 2Mo2O5 solutions and optimization of ESI-MS conditions

The speciation of Cp * 2Mo2O5 in water-methanol mixtures at various pH is known from recent kinetics and conductivity studies in one of our laboratories. 25 The use of the mixed water-methanol medium was dictated by the insolubility of Cp * 2Mo2O5 in pure water, while the hydrosoluble (but lipophobic) Cp*MoO3 -ion is obtained under basic conditions. As a brief summary of the speciation studies, compound Cp*2Mo2O5 was found to exist as a molecular dinuclear species in organic solvents, but self-ionization to Cp*MoO2 + and Cp*MoO3 -occurs to a larger and larger extent as the water content in the solvent mixture increases. In 20% MeOH-H2O, the compound behaves as a strong electrolyte affording, as the preponderant species, Cp*MoO2 + at pH < 2 and Cp*MoO3 -at pH > 5. Both species, plus a minor quantity of Cp*MoO2(OH), are present at intermediate pH values. 25 The first part of our investigation was devoted to ESI-MS studies of the reactant Cp * 2Mo2O5 in order to find the analytical conditions that will most closely reflect its speciation in solution. It is worth noting, in this respect, that ESI-MS provides a sensitivity advantage over the previous analyses, which may have missed some less dominant species.

On the other hand, even though ESI-MS is amongst the least intrusive mass spectrometric techniques, 26 the ionization process may nevertheless introduce fragmentation and rearrangement processes leading to the observation of peaks of species that are not present in the solution. Therefore, the experimental conditions must be carefully optimized in order to minimize these phenomena. It is also important to verify that no redox process takes place in the ESI chamber in the absence of an applied electrochemical potential. For practical reasons, the ESI-MS studies were conducted in a 1:1 MeOH-H2O mixture containing a 0.1 M CH3COONH4-CH3COOH buffer at pH 4. Under these conditions, a substantial amount of dimolybdenum species (either the neutral Cp*2Mo2O5 or the tight ion pair, Cp*MoO2 + ,Cp*MoO3 -) is also present in solution.

Figure 1 compares the ESI-MS analysis of a 0.1 mM solution of Cp * 2Mo2O5 in positive (a) and negative (b) detection modes. The positive mode ESI-MS spectrum exhibits four major isotopic patterns, corresponding to mono-, di-, tri-and tetra-nuclear species, respectively. The mononuclear region shows two major species (see left insert in Figure 1(a)), both exhibiting the characteristic, 7-peak isotopic pattern of a single Mo atom. The first species is characterized by an isotopic pattern at m/z = 259-267 and corresponds to the cation Cp * MoO2 + , while the second one starting 18 m/z units higher (m/z = 277 -285) is assigned to the protonated hydroxo species, Cp * MoO3H2 + . While the former is a solution-borne species, the latter results from the protonation of Cp*MoO2(OH) and Cp*MoO3 -in the ESI chamber (its pKa was estimated as ca. -2). Because of this protonation process, the ESI-MS fails to duplicate the known acid-base speciation. 25 ((Figure 1)) Two major species appear in the dimolybdenum region, both being characterized by the expected isotopic distributions for species containing two Mo centers (see second insert of Figure 1(a)). The most abundant isotopic pattern is at m/z = 535-551 which corresponds to the protonated form of Cp * 2Mo2O5. As stated above, this compound exists as a molecular species in organic solvents (including pure MeOH) but it partially dissociates in the presence of water. In a 1:1 MeOH-H2O mixture, as used in this study, the conductivity investigations showed that part of the dinuclear compound is undissociated. The observed peak can therefore be attributed to the combination of Cp * 2Mo2O5 with protons originating from the ammonium buffer. The less abundant and lighter pattern (m/z 517-533) in this region has 18 fewer mass units than the previous one and is therefore assigned to a dehydrated form. Probably, one of the Cp* protons is involved in the process leading to this species, thereby signaling ESI-MS fragmentation. 27,28 The isotope patterns at m/z 793 -815 and m/z 1090 -1117 fit very well the calculated distributions of Cp*3Mo3O7 + (slope = 1.0005, correlation coefficient, R 2 = 0.999) and Cp*4Mo4O11H3 + (slope = 1.008, R 2 = 0.9998), respectively. [START_REF] Winter | Sheffield Chemputer[END_REF] Thus, both complexes still feature the metal in the oxidation state +6. It should be noted that a deviation of a single atomic mass unit (AMU), corresponding to a 1-electron reduction process, can be easily distinguished by the isotope distribution fit. For example, curve fitting of the isotope distribution of a hypothetical Cp*4Mo4O11H4 + to the observed isotope pattern for the tetranuclear species gives a slope of 1.004 but a correlation coefficient, R 2 = 0.5451. Of course, the fitting of lower molecular weight molybdenum complexes to the observed patterns is even more sensitive to the assigned composition.

The negative mode spectrum of the same Cp * 2Mo2O5 solution is given in Figure 1(b). The major pattern at m/z = 275-283 corresponds to the deprotonated mononuclear species, Cp * MoO3 -(left insert). The second major distinguishable pattern (right insert, m/z = 415-431) is assigned to a species of formula Cp * Mo2O6 -, i.e. the product of the replacement of one Cp* -group with O 2-, a result of ESI-MS fragmentation. The negative mode spectrum also shows a much higher background current, relative to the positive mode spectrum, for m/z greater than 500. For these reasons, all further studies were conducted in the positive ion mode.

Two additional ESI-MS parameters were singled out for further optimization: The temperature of the heated capillary and the heated capillary voltage. Figure 2(a) shows the ESI-MS spectrum recorded under the same conditions of Figure 1(a), except that the heated capillary was held at 200°C rather than at 100°C. The differences between the two spectra confirm that the heated capillary temperature is a decisive factor in ESI-MS studies of our compounds. A comparison between the inserts of Figure 1(a) and Figure 2(a), reveals that CpMoO3H2 + almost completely disappears at high temperatures while the abundance of a new species with m/z = 273-281 (corresponding to CpMoO3H2 + -3H)evidently resulting from a fragmentation process -increases. Furthermore, the daughter product with m/z = 517-533 in the dinuclear region becomes the preponderant species, while the original isotope pattern of the protonated dinuclear species Cp * 2Mo2O5H + almost completely disappears. In addition, other fragmentation products are also formed in relatively large proportions. Evidently, Cp * 2Mo2O5 is cleaved at high temperatures.

((Figure 2))

The temperature dependence that was exemplified in Figure 2 Under increased temperature conditions the abundance of the parent peak of the protonated Cp * 2Mo2O5 decreases, while the abundance of the secondary peak of the dehydrated moiety goes through a maximum at 120-180°C and then also decreases. The abundance of the monomeric form Cp*MoO2 + increases monotonically with temperature indicating a somewhat surprising thermal stability for this ion, whereas its relative abundance at 100°C amounts to less than 10% of the dinuclear peak.

The relative abundances of the tri-and tetranuclear molybdenum species decrease steadily with the heated capillary temperature. This decrease suggests that the tri-and tetranuclear molybdenum species originate from the aqueous solution and that they were not generated by condensation reactions in the ESI chamber. Condensation reactions and dehydration should increase at higher temperatures contrary to the tendency that is exhibited in Figure 2(b). Thus, we believe that these compounds do exist in the solution although they were not observed before by any other technique. These results directed our subsequent experiments to minimal source temperatures, which was set at 100°C for detector sensitivity reasons.

The capillary voltage value was optimized (+25 V) during the automatic tuning process of our MS in order to give the highest sensitivity for the mass of Cp*2Mo2O5H + . Since prior studies pointed to the sensitivity of the observed spectra of organometallic complexes to the heated capillary voltage, [START_REF] Henderson | [END_REF] we have carried out a set of tests in order to check the influence of this parameter in our case. The experiments showed that the spectra were not significantly changed so long as the heated capillary voltage remained within the -25 to +100 V range, see Figure 2(c). It should be pointed out that no reduced molybdenum species were observed even when this potential was set as low as -100 V. Only for more negative cone voltages (< -110 V) did a reduction peak become observable (m/z = 777-799, assigned to Cp*3Mo3 V,V,VI O6 + ).

The concentration dependence of the peak intensities for the five major species are shown in Figure 3 ((Figure 3)) ((Scheme 1))

For the tetranuclear species, a similar association process may occur, although a simple breakdown into mononuclear charged components and a tentative structure for the assembled species cannot be as easily imagined as for the trinuclear species. It should be further remarked that these ionic association processes should be favored by the low polarity of the 50:50 water-methanol mixture. In the more polar 80:20 mixture where our previous speciation study was conducted, 25 no evidence for the presence of these ion was obtained, although the kinetics and conductivity may have been insensitive to the presence of small amounts of these species.

The choice of a buffer based on the ammonium cation, rather than on other metal cations, was dictated by the observation of complicating metal addition processes when the latter were used. For instance, Figure 4 shows the basic set of peaks of the dinuclear species in the presence of 1 M LiBF4 (acidified by acetic acid to pH 3.3). Competitive protonation and formation of adducts with metal cations, the latter being preponderant, take place and both adducts are discerned in the ESI-MS spectrum. The formation of metal ion adducts in preference to protonation in the ESI-MS chamber is a known phenomenon for neutral organometallic molecules. 31,32 ((Figure 4))

In addition to obtaining optimal ESI-MS parameters two important interim conclusions could be reached based on the open circuit studies of Cp*2Mo2O5 in methanol-water-acetate solutions. First, condensation reactions do not occur to a large extent in the ESI chamber; the tri-and tetranuclear molybdenum species appear to be already present in solution. Second, reduced molybdenum compounds are not generated to a noticeable extent in the ESI-MS. This condition is a prerequisite for our further on-line electrochemical reduction -ESI-MS studies. 2Mo2O5 is controlled by an interfacial step and it is not diffusion limited. A slope change at E = -0.65 V implies that an additional reduction step is switched on at this potential. It is quite obvious that the identification of the electroreduction products is impossible solely on the basis of the voltammetric experiment.

(b) On-line electrochemical reduction -ESI-MS study

((Figure 5))

Preparative electrolysis studies at -0.6, -0.7 and -0.8 V using a conventional 10 mm diameter glassy carbon electrode showed a gradual decrease of the current after prolonged polarization (2 hours) due to the formation of a thick oxide film on the electrode surface. This was accompanied by the accumulation of polymeric products in the solution, which increased the turbidity of the solution and could also be detected by ESI-MS studies. Thus, we preferred to direct the electrochemical studies to the flow-through configuration because the shorter time scale of the experiment and the constant renewal of the electrolyzed solution insured the absence of contamination from slower secondary reactions.

The electrochemical investigations were therefore pursued in a combined on-line electrochemical cell -ESI-MS experimental set up (Figure 6). The flow configuration and the characteristics of this setup are described in full details elsewhere. 33 Briefly, in this cell the feed stream reaches the sample electrode through the outer annulus of a concentric dual capillary arrangement. The feed stream split into an outer stream that diverge outward and washes away the cell solution and an axial stream that flows inward through a 20 micrometer gap between the glassy carbon sample and the flat end of the dual capillary arrangement. The feed converges into an inner capillary transfer line that leads to the mass spectrometer. We preferred this type of setup over a regular tabular flow cell in order to avoid chemical interferences due to the oxidation reactions on the counter electrode. This configuration also minimizes ohmic potential drops in the cell. Note that, unlike regular flow-through electrochemical detectors that operate under mass transport limited conditions, we target a kinetically controlled reaction. Thus, using conventional flow-through cells, oxidized compounds that are generated at the counter electrode will most likely escape electroreduction on the working electrode and will reach the transfer line to the mass spectrometer. This phenomenon may result either in the appearance of oxidized species in the MS or, even worse, in the occurrence of chemical reactions between the compounds generated at the counter and working electrodes. The flow cell of Figure 6 assures the confinement of the compounds being generated at the counter electrode away from the transfer line to the mass spectrometer.

((Figure 6))

During the coupled EC-ESI-MS experiments, mass spectra are continuously recorded and thus a threedimensional domain of signal intensities as a function of m/z and time is generated. Projections of this space onto a m/z axis for a given time range gives a time-integrated traditional mass spectrum, while the projection onto the time axis for a given m/z range gives the time evolution of the chosen species. Under potential ramp constraint a potential dependent generation or consumption of the specific compounds is obtained.

In order to translate the time dependent relative abundances into potential dependent curves it is necessary to measure the lag time in the transfer line from the electrochemical flow cell to the mass spectrometer and to estimate the degree of back-mixing (i.e. signal averaging) in the transfer line. These parameters were determined by the application of a potential step between -0.35 V and -0.9 V to the GC working electrode. According to the voltammetric curve (b)

shown in Figure 5, the Mo species in the solution are exclusively in their oxidized state at E = -0.35 V, while at E = -0.9 reduction process occurs. Figure 7 ((Figure 7)) is not significantly polluted by film deposition processes. It is to be noted that all species start to be reduced at approximately the same potential. This would seem consistent with the presence of rapid equilibria between the various complexes, as proposed above, and with the selective reduction of only one of these species. However, it cannot be excluded that all the different species, or at least some of them, are reduced independently at approximately the same potential. A clarification of this point needs additional experiments under different concentration and pH conditions, which are in progress and will be reported in due course.

((Figure 8))

The mass spectrum measured at the switching potential, see Figure 9, reveals a set of new peaks corresponding to the different products of electrochemical reduction. New isotopic patterns corresponding to the formation of reduced mononuclear, dinuclear, trinuclear and tetranuclear species evolved at reduced potentials.

((Figure 9))

The potential dependence of the MS intensities of the reduced molybdenum species are shown in Figure 10, Rakowski-Dubois and Pétillon. [34][35][36] No previous report of compounds where the Cp*2Mo2 n+ core is fully coordinated by hard, oxygen-based ligands seems to be available, the exception being made for the starting material Cp*2Mo2O5 and the corresponding Mo V dimer, Cp*2Mo2O4. 24,37,38 Related trinuclear Cp*Mo species having formula [Cp*3Mo3O6Hn] 2+

together
(n = 4, 5, 6), on the other hand, have been previously reported by Bottomley et al. 39 Amongst tetranuclear oxo clusters, the closest relationship is found with a Cp derivative, a red isomer of "CpMoO2", whose nuclearity was based solely on molecular weight determinations. 40 The more stable organge isomer has a dinuclear structure. A more reduced tetranuclear oxo cluster, Cp*4Mo4O7, formally a Mo 4.5+ species, has also been previously described. 41 No indication is obtained, however, that the latter species is obtained under our experimental conditions.

((Figure 10))

As shown in Figure 10 (central column), the isotopic envelopes of most identified species do not overlap with the other nearby isotopic patterns. An exception is the dinuclear complex Cp*2Mo2O4H + (m/z 519-535), which overlaps with another species having m/z = 517-533, a daughter ion obviously deriving from hydrogen loss from the Cp* rings.

The voltage dependent intensity of the Cp*2Mo2O4H + species is obtained by integrating a m/z region where the overlap with the minor species is minimal (i.e. m/z = 532-534). Complex Cp*MoO(OAc) + (m/z = 302-310) overlaps with another species having one fewer mass unit, indicating fragmentation with loss of hydrogen atoms. The envelope of the reduced trinuclear species is shifted exactly by 16 units to lower m/z from that of its precursor complex Cp*3Mo3O7 + , suggesting its formulation as a proton-free hexaoxo species.

The assignment of the chemical formulas for the reduced species is further supported by ion trap MS n fragmentation studies, all results being collected in Table 1. All MS n studies were conducted with isolation width corresponding to the whole range of the observed isotope patterns. The m/z = 302-310 monoacetate complex shows loss of acetic acid and formation of a methanol adduct in the MS [START_REF] March | Quadrupole Storage Mass Spectrometry in Chemical analysis[END_REF] . Similarly, the diacetate mononuclear complex (m/z 345-353) exhibits loss of an acetic acid unit. The low stability of these low-weight mononuclear fragments did not allow further MS [START_REF] Vestal | [END_REF] ((Table 1))

The MS n fragmentation of the trinuclear species is even more complicated. The primary species Cp*3Mo3O6 + (m/z = 777-799) produces four fragments in the MS [START_REF] March | Quadrupole Storage Mass Spectrometry in Chemical analysis[END_REF] . The first two correspond to loss of C5Me4CH2 and C5Me4CH2 + H2O, respectively, the third is a methanol adduct of the second and the fourth corresponds to Cp2Mo2O4 + which results from the loss of Cp*MoO2. It should be noted that the latter cleavage involves disruption of two oxo bridges and indeed it occurs at much higher collision energy compared to the first three processes. MS 3 fragmentation of the methanol adduct results in MeOH loss and a subsequent MS 4 fragmentation reveals four ions corresponding to Cp * loss plus formation of three solvent adducts of the resulting fragment. MS [START_REF] Vestal | [END_REF] and MS 4 fragmentation of Cp*2Mo2O4 + gives rise to subsequent dehydration processes. An MS n study of the reduced tetranuclear species could not be carried out because of the very low abundance of these peaks.

Table 2 presents the approximate potential values for the appearance of the various reduced species that may be measured on Figure 10. The analysis of these values enables us to derive a certain number of conclusions: (i) within a given nuclearity, the potential becomes more negative as the oxidation state decreases, as one would expect; (ii) when comparing species having the same formal oxidation state, the dinuclear compounds are always obtained at a less negative potential than the mononuclear ones, while the tetranuclear Cp*4Mo4O8H + species is obtained at approximately the same potential (-0.52 V) as the dinuclear Cp*2Mo2O4H + species -0.48 V); (iii) the latter complex is obtained at a less negative reduction potential than the other Mo V dinuclear species containing an acetate ligand; (iv) finally, this potential nearly matches the potential at which the peaks of the starting complex start to disappear (Figure 8). The large deviation between the threshold formation potentials and the mid-intensity potential, E1/2, points to kinetically controlled formation reactions for all of the reduced molybdenum species. This is unfortunate since it precludes the direct evaluation of the corresponding thermodynamic formation parameters for these new compounds.

((Table 2))

These combined observations may be interpreted as follows. The mononuclear products are likely to derive from the one-electron reduction of Cp*MoO2 + , affording a neutral radical. At more negative potentials, the latter species may be further reduced, possibly after a protonation stage. At each oxidation state level, two competing processes may occur, namely acetate/hydroxide exchange, and dimerization (accompanied by the establishment of a metal-metal bond). For instance, the neutral Mo V intermediate may dimerize to Cp*2Mo2O4, which is then detected by the ESI-MS in its protonated form, or it may be protonated and the resulting Cp*MoO(OH) + species may undergo a hydroxide-acetate exchange leading to the observed Cp*MoO(OAc) + . The driving force of the rapid and irreversible metal-metal bond formation would account for the less reducing potential which is needed to generate an oligonuclear species relative to a mononuclear one at the same oxidation state level. It is also possible, however, that the higher nuclearity products are obtained by direct reduction of the higher nuclearity species that are present in solution. It can be noted that each one-electron reduction for the mononuclear and dinuclear species is formally accompanied by the replacement of one oxygen atom with an acetato group (corresponding to a mass increase of 43 units). The only exception to this substitution rule is the observation of two different compounds having the Mo2 V,V core and related to each other by the formal replacement of an OH with an acetato group. The spectrum in Figure 9 does not show any mononuclear species with a mass greater that the 345-353 envelope, nor dinuclear species with a mass greater that the 647.0-664.0 envelope. This suggests that no Mo species in oxidation states lower than +4 are produced under the chosen experimental conditions.

The major trinuclear product obtained under reducing conditions is a Mo3 V,V,VI species. On the other hand, the same basic structure was reported for the dicationic Mo3 IV,IV,V , Mo3 IV,IV,IV , and Mo3 III,IV,IV complexes Cp*3Mo3O6Hn 2+ (n = 4, 5, 6). 39 Although the isotope envelope simulation indicate quite clearly the nature of the major product, minor amounts of products of further reduction cannot be excluded. We did not observe any signals attributable to dicationic species. It is possible that the reduction potential range and the conditions used in the present study do not allow access to the more reduced known trinuclear species, which were in fact previously obtained by zinc reduction of Cp*MoO2Cl under non aqueous conditions. 39 Concerning the tetranuclear reduction products, the [Cp*4Mo4O8H] + species may be related to the kinetic isomer previously identified for the Cp system when complex CpMoCl4 is hydrolyzed, namely Cp4Mo4O8. 40 It is believed that this complex does not contain metal-metal bonds, whereas the dinuclear Cp2Mo2O4 isomer does. 42 Therefore, the formation of this tetranuclear compound from precursors that are themselves devoid of metal-metal interactions is to be expected. The mixed-valence [Cp*4Mo4O10H2] + species does not have any equivalent in the literature. The shape of the abundance vs. potential curve indicates that a direct reduction to other species, possibly the tetranuclear Mo(V) cluster, becomes more favorable at more negative potentials.

As clearly shown by the cone voltage optimization study (Figure 2(c)), the experimental conditions of the positive mode electrospray process cannot induce, by themselves, an oxidation state decrease for the analyte. Therefore, the reduced molybdenum products shown in Figure 10 must originate from the electrochemical reactions in the electrochemical cell. The simple chemical composition of these species and the fact that they contain unfragmented acetate and pentamethylcyclopentadienyl ligands lead us to believe that they are all indeed solution borne species.

Conclusion

The coupled EC-ESI-MS technique has proven to be a powerful analytical tool for the investigation of complex electrochemical transformations. From the electroreducion of Cp*2Mo2O5, at least six previously unreported species have been identified and the potential required for the generation of each one of them has been assessed. These species may be described as oxo and/or acetate-stabilized Cp*Mo n+ , Cp*2Mo2 n+ Cp*3Mo3 n+ and Cp*4Mo4 n+ complexes. These species, as well as analogues with less coordinating anionic ligands, are expected to lead to new aqueous coordination chemistry, catalysis and electrocatalysis. Indeed, a few dinuclear and trinuclear species that are related to those observed by this EC-ESI-MS investigation have been synthesized and isolated in one of our laboratories. These investigations are described in separate publications. 43,44 Experimental Section General Remarks: The starting compound was prepared according to the literature procedure. 24 Unless otherwise stated, all solutions were prepared in H2O/MeOH (1:1) containing CH3COONH4 (0.25 M) and buffered to pH 4.0 by the addition of CH3COOH. A lower concentration of MeOH in the spray solution resulted in significant losses of relative abundances. All reagents were purchased from Aldrich. Methanol was purchased from Baker and was of HPLC grade.

The water was triple distilled.

Electrospray Mass Spectrometry: Finnigan (San Jose, USA) LCQ quadrupole ion trap mass spectrometer equipped with an electrospray ionization (ESI) interface was used for data acquisition. ESI was operated either in positive or in negative ion mode. In negative mode the tip of the capillary and the sampling cone were maintained at potentials of -3.5 kV (spray voltage) and -70 V (capillary voltage) respectively. In positive mode the spray voltage was at 3.6 kV and capillary voltage was 25 V. The source temperature was the subject of a separate study. Mass spectra were acquired by scanning the mass analyzer from m/z 100 to 2000 with 5 total microscans. Maximum injection time into ion trap was 50 ms. Sample injection flow rate was 5 uL*min -1 . In all experiments, helium was introduced at an estimated pressure of 1mTorr to improve the ion trapping efficiency. During MS n experiments the compounds were isolated in the ion trap with isolation width of isotope pattern and activated by using increased collision energy.

Electrochemical Investigations: PARC 273 potentiostat/galvanostat (Princeton, USA) was used. Three compartment electrochemical cell with a disk glassy carbon working electrode (Metrohm, Herisau, Switzerland), platinum counter electrode and Ag/ AgCl (sat) reference electrode (Metrohm) was used for the voltammetric studies.

Coupled Electrochemistry -Electrospray Mass Spectrometry: Figure 6 shows a scheme of the experimental set up used in the present study. Detailed description of the electrochemical set-up is given elsewhere. 33 The set up is comprised of a three-compartment electrochemical cell placed in a pressure chamber, a two coaxial capillary set, a micropositioning device, and a mass spectrometer equipped with an electrospray ionization (ESI) interface. The working glassy carbon electrode (1) was pressed to the bottom opening of a three compartments electrochemical cell (3). A Pt wire counter electrode and Ag/AgCl (KCl sat.) reference electrodes were placed in the side compartments of the cell. All reported potentials are given relative to this reference electrode. The electrolyte in the cell was 0.25 M CH3COONH4 in H2O/MeOH (1:1) buffered to pH 4.0 by CH3COOH. 1 mM Cp * 2Mo2O5 solution in the same electrolyte was pumped through the annulus of the capillary set to the electrode. A coaxial capillary set (4,5) was used to deliver the reagent solution to the electrode and to transfer the product stream to the interface of the mass spectrometer through the inner capillary. A syringe pump was used to deliver the feed stream to a T-junction (7) located outside the pressure chamber (6), from which the stream was directed through the annulus between the two capillaries to the electrode. The product stream (F0) was pumped, following the interaction with the electrode, upward through the inner capillary (4) by the excess pressure in the pressure vessel. All capillaries were fused silica from SGE (Austin, USA). The inner capillary was 25 m inner diameter, 285 m outer diameter, 15 cm long. The product stream was diluted with excess methanol delivered from a glass beaker (8). The liquid flow rates and dilution ratio (product stream/methanol) were controlled by setting the pressure in the pressure vessel and choice of capillary length and inner diameter . The cell pressure was 2600 kPa. After mixing with the auxiliary liquid, the sample stream was delivered to the ESI interface of the mass spectrometer by a 30 cm long, 50 m inner diameter capillary. The flow rate of the product stream was ca 0.7 l min -1 . The methanol flow in the auxiliary capillary was 14 l min -1 . The reagent flowrate (F1) was set to 4 l min - Experimental conditions are as specified in Figure 7. proposed chemical structure. Experimental conditions are as specified in Figure 7.

Captions for Figures

Graphical Abstract

An electrospray ionisation mass spectrometric analysis coupled on line to a flow through electrochemical cell reveals a rich aqueous reduction chemistry for compound Cp*2Mo2O5.

  figure shows the dependence of several of the most abundant positive ion peaks as a function of the heated capillary temperature. Peak heights were normalized to the highest peak observed at the given heated capillary temperature.

  . The relative abundances of the Cp*MoO3H2 + and Cp*MoO2 + species are virtually concentration independent, whereas those of all other species, including the dinuclear Cp*2Mo2O5H + , increase with concentration. Given the established equilibrium between the dinuclear Cp*2Mo2O5, on one side, and the separate ions Cp*MoO2 + and Cp*MoO3 -, on the other side, in MeOH-H2O solutions of various proportions, an increase of the relative abundance of dinuclear species with the concentration is expected. In principle, the observed increase in relative intensity with concentration for the trinuclear and tetranuclear peaks could be equally explained by invoking similar solution equilibria. Since we have shown that a water-rich environment favors the spontaneous ionization of Cp*2Mo2O5 into Cp*MoO2 + and Cp*MoO3 -, we propose that the trinuclear species derives from e.g. the combination of Cp*MoO3 -with two Cp*MoO2 + ions, as shown in Scheme 1.

Figure 5 (

 5 Figure 5(a) shows the background, 3 mV/sec voltammogram of a MeOH/H2O (1:1) solution (acetic buffer, pH 4) recorded in a regular three-compartment electrochemical cell with a glassy carbon working electrode. Curve (b) shows the voltammogram of a 1 mM solution of Cp * 2Mo2O5. The reduction wave starts at approximately -0.45 V vs saturated Ag/AgCl electrode. The currentpotential curves at the forward and backward scans nearly coincide and the height of the reduction wave is virtually independent of the scan rate for a rather wide range of scan rates (3 -100 mV/s). The last two observations indicate that the electrochemical reduction of Cp *2Mo2O5 is controlled by an interfacial step and it is not diffusion limited. A slope change at E = -0.65 V implies that an additional reduction step is switched on at this

  shows that the application of the potential step sharply decreases the concentration of Cp * 2Mo2O5H + . The intensity of the signal starts to decrease after a delay time of ~25 seconds. This seemingly corresponds to the retention time in the transfer line, in agreement with flow calculations. An additional 15 seconds interval is required to reach the half height of the concentration step change. The deviation of the MS response from a sharp step change is attributed to backmixing in the transfer line and to the time constant of the ESI interface.Thus, the "lag time" of the instrument is approximately 40 seconds. Therefore, the coupled EC-ESI-MS experiment with a continuous potential linear scan were recorded under quite slow scan rate conditions, i.e. 0.5 mV s -1 .

Figure 8

 8 Figure 8 shows the potential dependent abundance of all major species that are present in solution under open circuit conditions, i.e. the mononuclear Cp*MoO2 + and Cp*MoO3H2 + , the dinuclear Cp*2Mo2O5H + , the trinuclear Cp*3Mo3O7 + and the tetranuclear Cp*4Mo4O9(OH)3 + (see Figure 1(a)), during the electroreduction process. The recorded signal was obtained by summing the relative abundances of all the peaks in each respective m/z range covering the entire isotope envelope. The time axis of the product evolution curve was synchronized with the corresponding potential scale using 40 s delay time based on the potential step-change studies. During the backward, anodic scan, the concentration profiles essentially follow the same pattern observed in the cathodic scan, indicating that the experiment

  with their corresponding MS isotopic patterns and assigned structures. The latter are based on the fit of the observed MS isotope patterns to the calculated isotope distributions. It should be noted that, although the molecular weight and Mo nuclearity are unambiguous, the assigned structures are only tentative. Alternative possibilities exist; for instance, species Cp*2Mo2O2(OAc)2 + may display two terminal oxo and two bridging acetato ligands, or two bridging oxo and two terminal (chelating) acetato ligands, or all ligands may be bridging as shown in Figure 10. The chosen assignments are based on the comparison with related examples from the literature. The closest analogues to the dinuclear complexes are half-sandwich sulfido and alkylsulfido species, extensively investigated in the groups of

  fragmentation studies. The assignment of the m/z = 519 -535 to a Cp * 2Mo2O4H + is supported by MS 2 - MS 4 subsequent fragmentations showing a consecutive series of single dehydrations. In addition, a MS n study of separately synthesized Cp * 2Mo2O4 revealed an identical fragmentation pathway. The assignment of m/z = 561-577 to a Cp * 2Mo2O3(OAc) species is supported by MS 2 fragmentation showing neutral losses of an acetyl fragment or water. The isotope pattern at m/z = 604-620 is assigned to Cp * 2Mo2O2(OAc)2 + on the basis of the acetyl loss and dimer cleavage to give the Cp * MoO2(OAc) + and Cp * MoO(OAc)2 + cationic fragments in the MS 2 . The Cp * 2Mo2O(OAc)3 + complex (m/z = 647-663) gives rise to three different fragmentations at different collision energies, corresponding to loss of a one and two acetyl groups, and to dimer cleavage to give the mononuclear fragment Cp * Mo(OAc)2 + . Further MS 3 fragmentation of Cp * Mo(OAc)2 + reveals an additional acetyl loss. Finally, MS 4 fragmentation of Cp * Mo(OAc) + reveals one oxygen atom loss.

Figure 1 .

 1 Figure 1. Electrospray mass spectra of 0.1 mM solution of Cp*2Mo2O5 in H2O/MeOH (1:1) at pH 4. Heated capillary temperature = 100°C. (a) Positive mode. (b) Negative mode. The largest m/z peaks are marked in the figure and the first isotope in each isotopic pattern is marked in the inserts.

Figure 2 .Figure 3 .

 23 Figure 2. (a) Positive mode electrospray mass spectra of 0.1 mM solution of Cp*2Mo2O5 in H2O/MeOH (1:1) at pH 4. Heated capillary temperature = 200°C. (b) and (c) Evolution of the relative intensity of selected species ( : Cp*MoO2 + ; : Cp*MoO3H2 + ; : Cp*2Mo2O5H + -H2O; : Cp*2Mo2O5H + ; : Cp*3Mo3O7 + species; : Cp*4Mo4O11H3 + ) as a function of the heated capillary temperature (b) and heated capillary voltage (c)

Figure 4 .

 4 Figure 4. Detailed positive mode electrospray mass spectra for a 0.1 mM solution of Cp*2Mo2O5 in H2O/MeOH (1:1) in the presence 1 M LiBF4 (pH 3.3).

Figure 5 .

 5 Figure 5. Voltammetry in 0.25 M NH4Ac in H2O/MeOH (1:1) at pH 4 on a GC electrode. Scan rate: 3 mV/s. (a) blank solution; (b) 1 mM Cp*2Mo2O5.

Figure 6 . 3 -

 63 Figure 6. Scheme of the electrochemical cell used in this study.1working electrode; 2transfer line to the ESI/MS; 3 three compartments electrochemical glass cell; 4outer capillary; 5inner capillary 6pressure chamber; 7 -T-junction ; 8auxilliary fluid beaker; 9auxiliary capillary; 10pressure regulator; 11-Ztranslation stage; 12pressure chamber cover; 13rubber seal, 14steel rod Insert: A scheme of the miniature flow cell formed at the bottom of the coaxial capillary set up. F1 is the feed through the annulus of the coaxial capillary set up ; F0 is the product stream to the MS.

Figure 7 .

 7 Figure 7. Response of the EC-ESI-MS(positive mode) to a potential step change of the GC electrode. Feed stream: Cp*2Mo2O5 (1 mM) in 0.25 M NH4Ac H2O/MeOH (1:1) at pH 4. (a) potential step function. (b) time trace of the Cp*2Mo2O5H + signal intensity, integrated in the given m/z range.

Figure 8 .

 8 Figure 8. Mass spectra and potential dependent abundance of the initially present species during a linear potential sweep (scan rate 0.5 mV s -1 from -0.35 V to -1.0 V and back). Experimental conditions are as specified inFigure 7.

Figure 9 .

 9 Figure 9. Mass spectrum recorded at E = -1.0 V (switching potential) during the EC-ESI-MS study of Cp*2Mo2O5.

Figure 10 .

 10 Figure 10. Potential dependence of the relative abundance of selected species generated during the coupled electrochemistry -ESI-MS of Cp*2Mo2O5. Left: potential dependent evolution of the integrated signal. The m/z range used for signal integration is specified above each curve. Center: expanded isotopic pattern. Right:

  Figure 1

  Figure 3

  Figure 4

  Figure 5

  Figure 6

Table 1 .

 1 1 . 32 E. C. Kempen and J. S. Brodbelt, Anal. Chem., 1999, 71, 5493-5500. 33 A. D. Modestov, S. Srebnik, O. Lev, and J. Gun, Anal. Chem., 2001, 73, 4229-4240. F. Y. Pétillon, P. Schollhammer, J. Talarmin, and K. W. Muir, Coord. Chem. Rev., 1998, 178-179, 203-247. 37 H. Arzoumanian, A. Baldy, M. Pierrot, and M. Petrignani, J. Organometal. Chem., 1985, 294, 327-331. 38 E. de Jesus, A. Vazquez de Miguel, P. Royo, A. M. M. Lanfredi, and A. Tiripicchio, J. Chem. Soc., Dalton Trans., 1990, 2779-2784. Assigned formulas and MS n studies of the reduction products formed during EC -ESI-MS studies of Cp*2Mo2O5.

	34	M. Rakowski DuBois, Chem. Rev., 1989, 89, 1-9.
	35	M. Rakowski DuBois, Polyhedron, 1997, 16, 3089-3098.
	39	F. Bottomley, J. Chen, K. F. Preston, and R. C. Thompson, J. Am. Chem. Soc., 1994, 116, 7989-7995.
	40	M. Cousins and M. L. H. Green, J. Chem. Soc., 1964, 1567-1572.
	41	F. Bottomley, V. Sanchez, R. C. Thompson, O. O. Womiloju, and Z. Q. Xu, Can. J. Chem., 2000, 78, 383-394.
	42	C. Couldwell and K. Prout, Acta Crystallogr., 1978, B34, 933-934.
	43	F. Demirhan, J. Gun, O. Lev, A. Modestov, R. Poli, and P. Richard, J. Chem. Soc, Dalton Trans., 2002, 2109-
		2111.
	44	F. Demirhan, P. Richard, and R. Poli, Inorg. Chim. Acta, in press.

36

Table 2 .

 2 Threshold potential (Ein) and half-wave potential (E1/2) corresponding to the formation of reduced compounds during the EC-ESI-MS study of Cp*2Mo2O5 in Methanol-Water-Acetate solutions. a Experimental conditions are described in the captions of Figure 9.

	Compound	m/z	Ein, V	E1/2, V

Acknowledgments

RP and MV thank the CNRS and the Ministery of Research for funding. DS thanks the Conseil Régional de Bourgogne for a post-doctoral fellowship. ADM and OL acknowledge the financial support of the Israel Science Foundation. JG thanks the U. Bourgogne for a visiting professorship. We are grateful to the French Embassy in Israel for an Arc-en-Ciel/Keshet travel grant and to Prof. G. Tsirlina for useful discussions.


John C Gordon

Garth R Giesbrecht

David L Clark

P Jeffrey Hay

D Webster Keogh

Rinaldo Poli

Brian L Scott

John G Watkin

à la diffusion de documents scientifiques de niveau

recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The chemistry of metal-nitrogen multiply-bonded complexes has witnessed a dramatic surge in interest in recent years [3][4][5][6][7][8][START_REF] Nugent | Metal-Ligand Multiple Bonds[END_REF] due to the ability of the M=N functionality to undergo a wide range of reactivity, including metathesis of imines, aldehydes and carbodiimides, metallacycle formation with alkenes and alkynes, dealkylation and C-H bond activation. [START_REF] Zuckerman | [END_REF][11][12][13][14][15][16][17] Imidocontaining complexes have been documented for the majority of the transition metals, and also the actinides. [18][19][20][21][22][23][24][25] Notable by their absence, however, are examples of terminal imido complexes of scandium, yttrium or the lanthanide elements. There have been two previous reports of structurally-characterized complexes which contain imido ligands bridging multiple lanthanide metal centers: (i) two 3-phenylimido ligands cap the tetranuclear core of the complex [Yb4(- 2 : 2 -Ph2N2)4(3-NPh)2(THF)4], 26 and (ii) a 4-NH ligand lies at the center of the tetranuclear complex [{( 5 -2-C9H6SiMe2NH)Ln}2(3-Cl)(THF)]2(4-NH)(THF). 27 Following a report by Evans et al. that trialkylaluminum reagents were capable of deprotonating anilido ligands, 28 we chose to examine whether the use of a sterically encumbered anilido ligand on a lanthanide metal center would allow the trialkylaluminum-mediated formation of an imido functionality. As a result, we describe here the isolation and characterization of a dimeric samarium amido-imido compound which provides clear structural insight into the behavior of an imido functionality on a lanthanide metal center. [START_REF] Chan | After submission of this paper, the synthesis and characterization of two related ytterbium imido complexes were reported[END_REF]

Results and Discussion

Reaction of Sm[N(SiMe3)2]3 30 with three equivalents of 2,6-di-iso-propylaniline in toluene produces the brick-red complex [Sm(NHAr)3]2 (1) (Ar = 2,6-i-Pr2C6H3) in moderate yield (eq. 1). Compound 1 is only sparingly soluble in toluene or hexane. The poor solubility of 1 in nonpolar solvents and its high reactivity towards donor solvents, coupled with the paramagnetism of Sm(III), did not facilitate characterization by NMR spectroscopy. Slow evaporation of a saturated hexane solution of 1 yielded orange plates that were suitable for X-ray diffraction. The molecular structure of compound 1 is presented in Figure 1. A list of relevant bond lengths and angles is available in Table 1; complete details of the structural analyses of compounds 1 and 2 are listed in Table 3. Complex 1 possesses a -arene-bridged dimeric structure analogous to that reported for the yttrium analog; however, in that case, the Xray crystal structure was not of sufficient quality to allow for a detailed discussion of bond lengths and angles. [START_REF] Chan | After submission of this paper, the synthesis and characterization of two related ytterbium imido complexes were reported[END_REF] In the solid state structure of 1, each samarium atom is ligated by three nitrogen atoms. Additionally, each metal center engages in an  6 -arene interaction with the aryl ring of an amide ligand attached to an adjacent samarium. The three nitrogen atoms and the aryl ring centroid support a tetrahedral geometry around the metal atom, with the angles anchored by the samarium atom ranging from 92.6(3)° to 117.6°. The distances between the metal center and the six carbon atoms of the aryl ring range from 2.838(10) to 3.094 (10) Å with an average distance of 2.952(10) Å (Sm(1)-arene(cent) = 2.596 Å). This average Ln-C bond distance is similar to those reported for the related aryloxide species [Sm(O-2,6-i-Pr2C6H3)3]2 31 (2.986 (8) and 3.016(8) Å for the two independent molecules in the asymmetric unit) as well as other known examples of compounds containing trivalent 4f element--arene interactions (e.g. average Ln-C distances of 2.89(3) and 2.91(6) Å have been reported for ( 6 -C6Me6)Sm(AlCl4)3 32 and ( 6 -C6H6)Sm(AlCl4)3, 33 respectively, while the intramolecular -arene interaction in Yb(O-2,6-Ph2-C6H3)3 exhibits an average Ln-C distance of 2.978(6) Å. 34 ). The Ln-C distances determined for compound 1 are significantly longer than the Gd-C(ave) distance of 2.630(4) Å found for the zerovalent complex, Gd( 6 -t-Bu3C6H3)2 35 and are slightly longer than the distances found in {[PhP(CH2SiMe2NSiMe2CH2)2PPh]Ln}2{ 6 : 6 -(C6H5)2} (Ln = Y, 36 Ho 37 ), in which the lanthanide centers interact with the phenyl rings of a dianionic bridging biphenyl unit. The bonds between samarium and the two nitrogens of the terminal amido ligands in 1 (2.241(8) and 2.273(11) Å) are slightly shorter than the Sm-Nbridging distance (2.351(10) Å) and are unremarkable for samarium-amido bonds. 38 The infrared spectrum of [Sm(NHAr)3]2 (KBr plates, Nujol) exhibits two distinct (C=C) stretching modes in the aromatic region (1590 and 1575 cm -1 ), consistent with two different arene environments in the solid state. Similar features have been observed in the solid state and solution infrared spectra of [Sm(O-2,6-i-Pr2C6H3)3]2 (1586 and 1572 cm -1 ). 31 The solution IR spectrum of 1 (benzene solution) also displays two (C=C) stretching modes in the aromatic region (1590 and 1572 cm -1 ), suggestive of the fact that the solid state  6 -bridged arene structure is maintained in solution. Although a dimeric structure involving anilido ligands bridging through nitrogen is also possible, the (C=C) stretching frequencies in this case might be expected to be so similar that they would not be resolved as two distinct bands. 39,[START_REF]We have also synthesized and characterized the lanthanum derivative, [La(NHAr)3]2; Xray crystallography shows the same -arene bridged structure in the solid state. 1 H and 13 C{ 1 H} NMR spectroscopy are consistent with a dimeric structure being maintained in solution[END_REF] Treatment of 1 with four equivalents of trimethylaluminum in toluene, followed by crystallization from hexane, leads to isolation of the trialkylaluminum adduct [(-ArN)Sm(-NHAr)(-Me)AlMe2]2 (2) in moderate yield (eq. 2). Compound 2 is extremely soluble in hydrocarbon solvents such as hexane and hexamethyldisiloxane. However, the similar solubility of the aluminum-containing byproduct [Me2Al(NHAr)]2 hampers the isolation of 2 in pure form. Multiple recrystallizations from hexane failed to separate complex 2 from the aluminum dimer. 1 H NMR spectroscopy revealed a series of sharp resonances assignable to the aluminum dimer and several broad, unassignable resonances, presumably due to complex 2. Attempts to purify 2 by extraction into hexamethyldisiloxane resulted in a deep red solution and a colorless precipitate of [Me2Al(NHAr)]2, which was removed by filtration. Removal of the solvent and re-examination by NMR spectroscopy still revealed the presence of the aluminum complex. Multiple iterations of the extraction/filtration procedure proved unsuccessful in the isolation of pure 2. At this point, we suspected that the residual aluminum complex was being generated in solution as a result of the decomposition of 2. We have recently observed similar redistribution reactions in related 4f-element complexes supported by phenoxide ligation. 41 However, thermolysis experiments did not indicate any significant decomposition, suggesting that difficulties in obtaining pure samples of 2 are due to the similar solubility properties of [Me2Al(NHAr)]2 and [START_REF]Attempts to prepare the lanthanum analog [(-ArN)La(-NHAr)(-Me)AlMe2]the reaction of [La(NHAr)3]2 with trimethylaluminum were not successful[END_REF] Red crystals of 2 that were suitable for an X-ray structure determination were obtained from hexane. An ORTEP representation of compound 2 is available in Figure 2; selected bond lengths and angles are presented in Table 2. The X-ray diffraction study of 2 revealed a dimeric samarium amido-imido complex, lying on a crystallographic inversion center, in which the two samarium metal centers are asymmetrically bridged by two 2-imido ligands. Each metal center also bears a terminal -NHAr ligand, the nitrogen lone pair of which is coordinated to a trimethylaluminum molecule. Sm-N bond lengths within the central Sm2N2 core are unusually short, as expected for bridging imido (NR 2-) as opposed to bridging amido (NR2 -) linkages. The two unique Sm-N distances within the Sm2N2 core are 2.152(8) and 2.271(7) Å. In comparison, typical Sm-N distances for bridging amido ligands are much longer, as exemplified by the distances of 2.468 (7) and 2.614 (7) [START_REF] Chan | After submission of this paper, the synthesis and characterization of two related ytterbium imido complexes were reported[END_REF] In fact, the bridging Sm-N distances in 2 are comparable to, if not shorter than, those found for typical terminal samarium amido ligands, for example 2.284 (7) 44 and 2.218(5) and 2.202(5) Å in {(i-Pr2N)2SmCl3[Li(TMEDA)]2}. 38 The asymmetry of the Sm2N2 core is further compounded by the presence of a significant interaction between the ipso carbon of the bridging -NAr ligand and one of the samarium metal centers (Sm(1)-C(1) = 2.666(8) Å). This results in an acute Sm(1)-N(1)-C(1) angle of 90.2(5)°, and a correspondingly obtuse Sm(1*)-N(1)-C(1) angle of 168.1(8)°. The angles about each bridging nitrogen atom sum to 357.2°, indicating near-planarity of the substituents, and hence an almost perfect T-shape coordination geometry about nitrogen. Similar interactions with phenyl rings have been observed previously in bis(2-imido) complexes of uranium 22,25,45,46 and zirconium. 20 In contrast to the very short Sm-N bridging distances, the terminal amido Sm(1)-N(2) distance is extremely long (2.535(8) Å), presumably as a result of the nitrogen-coordinated AlMe3 molecule preventing -donation of the lone pair to the metal center. We also note the structural similarity of 2 to the bismuth amido-imido complex [Bi(NHAr)(-NAr)]2, 47 although in the case of the bismuth complex no interaction with the bridging phenyl ring was observed.

Sm

[(-ArN)Sm(-NHAr)(-Me)AlMe2]2 (2).
Å in [K(THF)6]2[Sm(-NHAr)(NHAr)3]2 (Ar = 2,6- Me2C6H3).
Å in Sm[N(SiMe3)2]3, 43 2.331(3) Å in ( 5 -C5Me5)2Sm(NHPh)(THF),
Compound 2 also maintains a short Sm-C distance (i.e. aluminum bound methyl group) of 2.649( 14) Å, which is similar to the Sm-C(alkyl group) distances previously determined for (5) and 2.632(5) Å); 48 Et (2.627(4) and 2.649(4) Å). 41 This distance is slightly shorter than typical Ln-C distances found in complexes containing bridging methyl groups [49][50][51][52] and suggests the presence of an agostic interaction between the aluminum methyl group(s) and the metal center. There is also a distinct elongation of the Al-C bond associated with the "agostic" methyl group (Al(1)-C(25) (2.094(12) Å) vs. Al(1)-C (26) (1.939(12) Å) and Al(1)-C(27) (1.971(11) Å)). DFT calculations, however, suggest that this interaction may be more accurately described as a bridging methyl group, based on electronic considerations (see later).

(ArO)Sm[-OAr)(-R)AlR2]2 (R = Me (2.620

Theoretical Studies on Complex 2

Structural results using realistic ligands. Since, to the best of our knowledge, complex 2 represents the first structurally characterized example of a discrete imido complex of a 4f-element, we were interested in obtaining a more detailed picture of the bonding interactions that stabilize the Sm2N2 core within this molecule. DFT calculations employing the B3LYP functional were carried out on the dimeric model species, [(-PhN)Sm(-NHPh)(-Me)AlMe2]2 species (Fig. 3), in which the isopropyl substituents on the bridging and terminal phenyl groups were replaced by hydrogen atoms. The structure was optimized using the basis sets and effective core potentials (ECPs) on Sm described in the Experimental Section. The partially occupied 4f 5 shell was incorporated into the "large core" ECP, leaving the 5d and 6s orbitals as the most important valence orbitals. Recent studies on Ln(NH2)3 and Ln[N(SiH3)2]3 (Ln = La through Lu) model complexes have shown that accurate results may be obtained without the inclusion of the f electrons in the valence shell. 53,54 We have also found that "large core" provide results comparable to those of calculations which explicitly included the 4f 5 shell for larger complexes such as Ln[CH(SiMe3)2]3. 55 These calculations also effectively describe the agostic interactions present in molecules such as the aforementioned Ln[N(SiMe3)2]3 55 and Sm[CH(SiMe3)2]3. 56 Comparing some of the key geometrical parameters between the optimized structure and the experimental structure (Table 4) we see that the Sm-N bridging bond lengths are calculated to be 2.220 Å (Sm=N) and 2.299 Å (Sm-N) (differing by 0.08 Å), as compared to the experimental values of 2.153 Å (Sm=N) and 2.271 Å (Sm-N) (differing by 0.11 Å). Overall the calculations appear to be giving a reasonable description of the multiple bond character in the Sm-imido linkage. In addition, the calculated bond length of the longer Sm-N terminal bond (2.55 Å) agrees well with the observed bond length (2.535 Å).

The results of the calculations illuminate the nature of the Sm-(-CH3)-Al interactions, since the accurate locations of the methyl hydrogen atoms are not available from the experimental X-ray structure. The coordination geometry at the bridging carbon atom can be described as distorted trigonal bipyramidal, with the Sm atom and one H atom occupying axial positions, while the Al atom and the two remaining H atoms take up the equatorial positions (see Figure 3). The Al-CH3 moiety is therefore arranged in a staggered conformation relative to the Al•••Sm axis. In addition, relative to the Al-C axis, the methyl group is titled away from the Sm atom, as evidenced by the smaller than tetrahedral Al-C-H angle to the axial H atom and the larger ones to the two equatorial H atoms. Thus, the -CH3 ligand is best described as a bridging methyl group, similar to the situation observed in Al2Me6, and not as an Al-CH3 group establishing agostic interactions with the Sm atom. In other words, both C-Al and C-Sm interactions are established with the same carbon electrons.

To probe some of the factors affecting the multiple bonding of the imido fragments within the dimer, three additional sets of calculations were performed where each structure was re-optimized to compare to the original complex: (a) the AlMe3 groups were removed entirely;

(b) the original dimer was calculated without 5d basis functions on the Sm atoms and; (c) the full dimer was maintained but the Ph substituents on the bridging and terminal ligands were replaced by methyl groups. These results are also summarized in Table 4. Removing the Lewis acidic AlMe3 groups has surprisingly little effect on the bond lengths in the bridging Sm2N2 unit, with the bond alternation actually modestly enhanced. The Sm-N terminal bond becomes 0.2 Å shorter when the nitrogen is no longer complexed to the aluminum center, as one would expect since more electron density is now available on the nitrogen for bonding to the samarium center in the absence of the Lewis acid. This result indicates that the trimethylaluminum groups do not directly affect the electronic structure of the Sm2N2 core, but rather have a steric function, with the -NH(AlMe3)Ph groups acting as bidentate, uninegative ligands.

In contrast, replacing the N-Ph groups with N-Me groups results in a nearly symmetric Sm-N-Sm linkage, with the two Sm-N bond lengths differing by less than 0.01 Å. The Sm-Nbr-C angle (153° in the case of N-Ph) changes to 131° for the bridging N-Me group, suggesting that overlap with the phenyl ring plays an important part in maintaining the double-bond character in the Sm=N-Ph bridging bonds. These results are consistent with those reported by Andersen et al. for the uranium complexes, (MeC5H4)4U2(-NR)2 which exhibit symmetric (R = SiMe3) or asymmetric (R = Ph) bridging imido groups depending on the nature of the organic constituent. 45 In that case, the asymmetry in the structure of (MeC5H4)4U2(-NPh)2 was ascribed largely to the contributions of an  3 -azabenzylic resonance structure in which negative charge was delocalized to the ortho and para positions of the phenyl ring, although the presence of C-N -bonding was not conclusively established.

The net charge on the Sm centers from the population analysis in the most realistic model is +1.14, (i.e. much less ionic than the formal oxidation state of +3). There is considerable donation of electron density into the 5d orbitals as evidenced by the overall d population of 1.28 electrons. The remaining valence population of 0.56 electrons involves the 6s and 6p orbitals.

The particular importance of d orbital participation is illustrated in the results of Table 4. In the absence of 5d functions on samarium, a lengthening of 0.09 and 0.11 Å in the calculated Sm=N and Sm-N bond distances is observed. This represents an overall increase in bond distances of approximately 7% relative to the metric data garnished by the X-ray study. These results show that in their role as -acids, the 5d orbitals play an important role in the stabilization of the Sm=N fragment (see later). The population analysis also suggests that it is more accurate to view the bonds involving the samarium center as covalent bonds, in contrast to the commonly held view of bonds involving the lanthanides as being predominantly ionic in nature.

Bonding analysis and results from model calculations. In considering the bonding in

the Sm dimers it is useful to consider the frontier MOs of the bridging [N-Ph] 2-ligand (Scheme 1). The nitrogen atom can be thought to have three electron pairs: the  lone pair (n and two p lone pairs, one conjugated with the  electrons of the phenyl group (), and one parallel to the ring (||). We would expect these MOs on each bridging nitrogen atom to play an important role in the Sm-N bonding. Indeed, we find the highest two occupied MOs correspond to the symmetric and anti-symmetric combinations of the Np orbitals interacting with the Sm centers. Contour plots of these orbitals are shown in Fig. 4 (designated as MOs 147 and 148). A bonding analysis of these MOs (Table 5) shows approximately 3 percent 5d orbital contribution from each Sm center for the HOMO (MO 148) and about 8 percent 5d contribution from each Sm center for the next highest MO (MO 147). Therefore, while the composition of these MO's are mainly ligand based in nature, the overall metal orbital contribution is essentially dominated by the 5d orbitals on each metal center (roughly 75% of the metal based contribution from Sma and Smb is 5d in nature in MO 148, while in MO 147 it is closer to 90%).

At slightly lower energies one finds the symmetric and antisymmetric combinations of the Np|| ligand orbitals interacting with the Sm centers (MOs 143 and 146), which also have significant 5d orbital participation (6 and 7 percent, respectively). Together these four MOs account for 0.54 electrons of the 1.28 5d electrons from the population analysis. One finds orbitals involving the nligand combinations at much lower binding energies. Finally, some of the MOs involved in the bonding of the terminal NHPh groups to the Sm ions are also found, denoted as Sm-Nt porbitals in Table 5, at similar energies to the orbitals described above.

To probe the bonding interactions more clearly, calculations on simpler model systems were performed in which all amido and aluminum substituents (except for the methyl group which bridges the aluminum and samarium atoms) were replaced with hydrogen atoms. The N-Ph bridging ligand was maintained. The optimized structure of this simplified model yielded similar geometrical parameters to the more realistic models (Table 4), and the structures are compared in Figure 5. The simpler model resulted in a lengthening of the Sm-Cbridging methyl bond length from 2.71 Å to 2.87 Å and in a shortening of the Sm-Nt bond length from 2.55 Å to 2.44 Å, leading to a shift of the Al atoms to the opposite side of the Sm2(-N)2 plane, as seen in Figure 5(b). The configuration adopted by the simpler model probably reflects the actual electronic preference of the molecule, since the four bonds to each samarium atom (namely the Sm-Nt, the Sm-CH3, and the two Sm-Nb bonds) are closer to the ideal tetrahedral configuration.

The distortion observed in the experimental structure as well in the optimized larger model is attributable to the aryl-aryl repulsive interactions between the terminal amido and the bridging imido groups. This repulsion is also reflected in the longer Sm-Nt bond for the larger model.

To gain a better understanding of the factors which favor asymmetric over symmetric bridging imido groups, an MO analysis of the simplified structure, presented in Figure 5(b), was undertaken. Figure 6 compares the results of the lower energy asymmetric structure (Figure 6a) with the symmetric structure (Figure 6b), where where the molecule was constrained in Cs symmetry. The two highest occupied MOs in this model are the same Sm-N p orbitals discussed previously in the full model (see Figure 4, MOs 147 and 148). The next highest MOs are the Sm-N p|| orbitals, as in the previous picture, while the MOs involving the terminal NHPh ligand, which has been simplified to NH2, are not present. It is instructive to compare the changes in orbital energies of the highest MOs in Figure 6 for the two different structures, although a rigorous correspondence with the total energy of the molecule in DFT calculations is problematic. One sees that the HOMO shows only a slight change. The next occupied orbital is substantially lower in energy in the asymmetric structure, where it not only maintains a strong Sma=NPh bond interaction but also shows bonding interactions with Smb. Similarly a much lower orbital (bottom of Fig. 6) involving the C p orbitals of the phenyl ring shows some Cipso-Smb bonding character. Thus, it appears that when N-Ph groups are employed, an asymmetric structure is favored due to (a) the overlap of the Np orbital with the Sma 5d orbital and (b) the additional stabilization afforded by the formation of a weak Cipso-Smb bond. In the case of imido groups such as N-SiMe3, these interactions are not possible and a symmetric structure is observed.

Conclusions

Reaction of the -arene bridged anilido dimer [Sm(NHAr)3]2 (1) (Ar = 2,6-i-Pr2C6H3) with 4 equivalents of AlMe3 leads to the formation of the bis-2-imido complex [(-ArN)Sm(-NHAr)(-Me)AlMe2]2 (2). To the best of our knowledge this represents the first example of a structurally characterized 4f-element complex containing a discrete imido functionality.

Although the isolation of compound 2 appears to be somewhat problematic, the structural elucidation of this molecule clearly demonstrates that the synthesis of species containing lanthanide main-group multiple bonds is feasible. DFT calculations indicate that the metal (acceptor) 5d orbitals play a significant role in stabilizing -donation from the imido groups within the Sm2N2 core. We are currently developing new approaches toward the isolation of further novel examples of 4f-element complexes containing Ln=X multiple bonds (X = CR2, NR, O). We envisage that these systems will not only provide us with new and interesting issues in structure and bonding, but may also provide some potentially reactive synthons with which to carry out a variety of small molecule transformations.

Experimental Section

General Considerations. All manipulations were carried out under an inert atmosphere of oxygen-free UHP grade argon using standard Schlenk techniques or under oxygen-free helium in a Vacuum Atmospheres glovebox. AlMe3 (2.0 M hexane solution) was purchased from Aldrich and used as received. 2,6-di-iso-propylaniline was purchased from Aldrich, dried over molecular sieves and distilled prior to use. Sm[N(SiMe3)2]3 was prepared according to a literature procedure. 56 Hexane and toluene were de-oxygenated by passage through a column of supported copper redox catalyst (Cu-0226 S) and dried by passing through a second column of activated alumina. Hexamethyldisiloxane was distilled over sodium benzophenone and degassed prior to use. C6D6 was degassed, dried over Na-K alloy, and trap-to-trap distilled before use. 1 H NMR spectra were recorded on a Bruker AMX 500 spectrometer at ambient temperature. Infrared spectra were recorded on a Nicolet Avatar 360 FT-IR spectrometer as Nujol mulls between KBr plates. Elemental analyses were performed on a Perkin-Elmer 2400 CHN analyzer. Elemental analysis samples were prepared and sealed in tin capsules in the glovebox prior to combustion.

[Sm(-NHC6H3 i Pr2-2,6)(NHC6H3 i Pr2-2,6)2]2 (1). 2,6-di-iso-propylaniline (2.00 g, 11.0 mmol) in 10 mL toluene was added to a pale yellow toluene solution (50 mL) of Sm[N(SiMe3)2]3 (2.30 g, 3.70 mmol). This caused the solution to turn red-orange. The reaction mixture was stirred at room temperature for 15 minutes, and then heated to reflux for 30 minutes. Upon cooling, a brick-red solid precipitated. The solvent was then removed under vacuum and the solid washed with hexane to remove HN(SiMe3)2. Slow evaporation of a saturated hexane solution yielded dark orange plates of 1 (1.30 g, 52% yield). The paramagnetism and low solubility of 1 in noncoordinating solvents did not allow for characterization by 1 H or 13

The

  First Example of a µ 2 -Imido Functionality Bound to a Lanthanide Metal Center: X-ray Crystal Structure and DFT Study of [(µ-ArN)Sm(µ-NHAr)(µ-Me)AlMe2]2 (Ar = 2,6-iPr2C6H3)

  C{ 1 H} NMR spectroscopy. IR (Nujol, cm -1 ): 1590 (m), 1575 (m), 1423 (s), 1358 (m), 1340 (m), 1324 (m), 1300 (m), 1250 (s), 1215 (m), 1193 (w), 1148 (w), 1138 (w), 1105 (m), 881 (m), 838 (m), 802 (s), 767 (s), 746 (s). Anal. Calcd. for C72H108N6Sm2: C, 63.66; H, 8.01; N, 6.19. Found: C, 63.02; H, 8.25; . 0.60 mL of a 2.0 M solution of AlMe3 in hexane (1.20 mmol) was added to an orange slurry of 1 (400 mg, 0.300 mmol) in toluene (50 mL). Within a few minutes the slurry dissolved and the solution became deep red.

Figure 1 .

 1 Figure 1. ORTEP view of [Sm(-NHC6H3 i Pr2-2,6)(NHC6H3 i Pr2-2,6)2]2 (1) drawn with 30%

Figure 2 .

 2 Figure 2. ORTEP view of [(-NC6H3 i Pr2-2,6)Sm(-NHC6H3 i Pr2-2,6)(-Me)AlMe2]2 (2) drawn

Figure 3 .

 3 Figure 3. View of the fully optimized [(-PhN)Sm(-NHPh)(-Me)AlMe2]2 structure from

Figure 4 .

 4 Figure 4. Contour plots (left) and ChemDraw representations (right) of the highest occupied

Figure 5 .

 5 Figure 5. Views of the fully optimized [(-PhN)Sm(-NHR)(-Me)AlR'2]2 models.

Figure 6 .

 6 Figure 6. MO diagram and contour plots of selected orbitals for the model compound

Table 3 . Crystallographic Data

 3

	Compound	1	2
	Formula	C72H108N6Sm2	C54H88Al2N4Sm
	Molecular Weight	1358.34	1105.91
	Temperature, K	203(2)	203(2)
	Crystal System	monoclinic	monoclinic
	Space Group	P21/n	C2/c
	Crystal Size, mm	0.21 x 0.21 x 0.21 0.06 x 0.12 x 0.12
	a, Å	19.363(2)	15.912(1)
	b, Å	9.4027(9)	18.635(1)
	c, Å	20.063(2)	18.941(1)
	, o	90	90
	 o	110.193(2)	90.709(2)
	, o	90	90
	V, Å 3	3428.3(6)	5616.0(7)
	Z	2	4
	Dcalc, g/mL	1.316	1.308
	Absorption coeffiecient, mm -1 1.739	2.133
	F(000)	1412	2276
	Theta range, o	1.26 to 23.33	1.7 to 23.4
	Total reflections	10114	7377
	Independent reflections	4711	3709

Table 4 . Comparison of calculated and experimental structural parameters for [(-ArN)Sm(-NHAr)(-Me)AlMe2]2 (Ar = 2,6-i-Pr2C6H3) and related model compounds

 4

	Bond lengths (Å)

Acknowledgments

We acknowledge support from Laboratory Directed Research and Development at Los Alamos and from the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy. Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy under Contract W-7405-ENG-36.

The solvent was removed under vacuum to produce a dark brown tacky solid. Hexane (10 mL) was added and the mixture pumped dry, again yielding a tacky solid. This procedure was then repeated. Following this, hexanes (20 mL) was added, the mixture filtered through a Celite pad, and the resulting filtrate concentrated to approximately 10 mL. This filtrate was then allowed to slowly evaporate under the glovebox atmosphere. Once at dryness, an oily orange-red solid resulted. This was washed with a minimal amount of cold hexanes (-10 °C) and pumped to dryness (yield 0.170 g). A 0.100 g sample of this solid was redissolved in hexanes (5 mL) and filtered through a Celite pad, and the resulting orange solution was allowed to slowly evaporate under the glovebox atmosphere. Once at dryness, an orange-red waxy crystalline solid resulted.

A single-crystal X-ray diffraction study was carried out on a portion of this material, after which further vacuum was pulled on the solid to ensure complete dryness (yield 0.050 g). Anal. Calcd. Crystallographic Studies. Crystals of 1 and 2 were attached to a glass fiber using a spot of silicone grease. The crystals were mounted from a matrix of mineral oil under argon flow. The crystals were immediately placed on a Bruker P4/CCD/PC diffractometer, and cooled to 203 K using a Bruker LT-2 temperature device. The data were collected using a sealed, graphite monochromatized MoK X-ray source. A hemisphere of data was collected using a combination of  and  scans, with 30 second frame exposures and 0.3º frame widths. Data collection and initial indexing and cell refinement was handled using SMART 58 software. Frame integration and final cell parameter calculations were carried out using SAINT 59 software. The final cell parameters were determined using a least-squares fit to 4402 reflections for 1. The data were corrected for absorption using the SADABS 60 program. Decay of reflection intensity was not observed.

The structures were solved in the space groups P21/n for 1 and C2/c for 2 using Direct methods and difference Fourier techniques. The initial solutions revealed the samarium and the majority of all non-hydrogen atom positions. The remaining atomic positions were determined from subsequent Fourier synthesis. The amide hydrogen atom positions were found on the difference maps, and refined with the isotropic temperature factor set to 0.08 Å 2 . All other hydrogen atom positions were idealized (C-H = 0.98 Å for methine, 0.97 Å for methylene, 0.96 Å for methyl, and 0.93 Å for aromatic). The idealized hydrogen atoms were refined using the riding model, with isotropic temperature factors fixed to 1.2 times the equivalent isotropic U of the carbon atom they were bound to. The final refinement 61 included anisotropic temperature factors on all non-hydrogen atoms. Structure solution, refinement, graphics, and preparation of publication materials were performed using SHELXTL NT. 62 Additional details of data collection and structure refinement are listed in Table 3.

Details of Theoretical Calculations.

All calculations were carried out using the B3LYP functional [START_REF] Becke | [END_REF]64 by using a "large core" effective potential 65;66 on the samarium atom including shells up to the filled 4s, 4p and 4d shells in the core. In addition, the partially filled 4f 5 shell, corresponding to the configuration of Sm(III), was also included in the core for a total of 51 electrons. The remaining 11 electrons were treated explicitly which includes the "outer core" 5s 2 5p 6 electrons and the valence 6s 2 5d 1 electrons. A valence [5s, 4p, 3d] basis set was employed.

6-31G basis sets were used for C, N, Al and H atoms. All calculations were carried out with Gaussian 98. 67

Supporting Information Available

Listings of fractional atomic coordinates, bond lengths and angles, anisotropic thermal parameters and hydrogen atom coordinates for complexes 1 and 2, and coordinates of the optimized model structure [(-PhN)Sm(-NHPh)(-Me)AlMe2]2 from DFT calculations (xx pages). This material is available free of charge via the Internet at http://pubs.acs.org.