Thibault Terencio

Jana Roithová

Stéphane Brandès

Yoann Rousselin

Marie-José Penouilh

Michel Meyer

A Comparative IRMPD and DFT Study of Fe 3+ and UO2 2+ Complexation with N-Methylacetohydroxamic Acid

Keywords: hydroxamate, siderophore, DFT, IRMPD spectroscopy, uranyl, iron

Iron(III) and uranyl complexes of N-methylacetohydroxamic acid (NMAH) have been investigated by mass spectrometry, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. Comparison between IRMPD and theoretical IR spectra enabled to probe the structures for some selected complexes detected in the gas phase. The results show that the coordination of Fe 3+ and UO2 2+ by the hydroxamic acid is of a very similar nature. Natural Bond Orbitals (NBO) analysis suggests that the bonding in the uranyl complexes possesses a slightly stronger ionic character than in the iron complexes. Collision induced dissociation (CID), IRMPD, and 18 O-labelling experiments unambiguously revealed a rare example of the U=O bond activation concomitant with elimination of a water molecule from the gaseous [UO2(NMA)(NMAH)2] + complex. The U=O bond activation is observed only for complexes with one monodentate NMAH molecule forming a hydrogen bond towards one "yl" oxygen atom, as found by DFT calculations. This reactivity might explain oxygen exchange observed for uranyl complexes.

INTRODUCTION

Hydroxamic acids with a general formula R 1 C(=O)N(OH)R 2 were first reported in 1869 by Lossen. [START_REF] Lossen | Uber die Oxalohydroxamsäure[END_REF] They are usually classified as primary if R 2 = H or as secondary if R 2 is an alkyl or aryl group. Among the primary hydroxamic acids, formo-(R 1 = R 2 = H) and acetohydroxamic acids (R 1 = CH3, R 2 = H) are the most studied ones. They exist as a mixture of different conformers (Z or E) and tautomers (H-migration). 2 Tautomerism involves the nitrogen-bound hydrogen atom, which can migrate to the carbonyl oxygen atom. This rearrangement can be prevented if an alkyl or aryl group is introduced to the nitrogen atom instead of the hydrogen atom. As the simplest representative of the secondary hydroxamic acids, we are focusing herein on N-methylacetohydroxamic acid (NMAH, R 1 = R 2 = Me), which prevails in the E form in aqueous solutions regardless of its protonation state. [START_REF] Brandès | Conformational and structural studies of Nmethylacetohydroxamic acid and of its mono-and bis-chelated uranium(VI) complexes[END_REF] Hydroxamic acid moieties are present in a large number of siderophores. Their high affinity for iron(III) makes them particularly suitable for biological applications, and they are key compounds involved in Fe(III) transporting. [START_REF] Miethke | Siderophore-based iron acquisition and pathogen control[END_REF] It was shown that siderophores can also bind to other metal ions, 5 thus highlighting their advantage in solving environmental issues such as water pollution by heavy metals. [START_REF] Ahmed | Siderophores in environmental research: Roles and applications[END_REF] Most scientific effort was devoted until now to acetohydroxamic acid binding to transition metals. It was shown that this ligand, after deprotonation, acts as a bidentate ligand through its two oxygen atoms. Hydroxamates are strong oxodonor groups which can form stable complexes with hard Lewis acids. [START_REF] Harris | Coordination chemistry of microbial iron transport compounds. 16. Isolation, characterization, and formation constants of ferric aerobactin[END_REF] We and others have shown that coordination of the primary hydroxamates to alkali-metal ions can lead to their Lossen rearrangement and therefore to their degradation, provided that sufficient activation energy is supplied (i.e. heating to about 90 °C). 5c,8 The rearrangement is associated with the migration of the hydrogen atom from the nitrogen to the terminal oxygen atom. [START_REF] Jašíková | Metal-assisted lossen rearrangement[END_REF] The harder the metal ion is, the more is this rearrangement favored.

Actinides are known to establish strong bonds with oxygen atoms, thus, it has been suggested that siderophores can favorably bind to actinides. 10 Among actinides, uranium is particularly wide-spread because of its use in nuclear industry. Uranium can also contaminate sea water, soils, or rivers from abandoned mines. 11 Related to this problem, complexation of uranium by hydroxamic acids gained increasing interest in order to separate uranium, from neptunium and plutonium, during treatment of nuclear fuel. 12 While complexation of transition metals with hydroxamates was quite extensively studied, 5b,13 structural and physico-chemical properties of the uranium complexes with hydroxamates are still not fully understood. 12a, [START_REF] Groenewold | Collision-induced dissociation tandem mass spectrometry of desferrioxamine siderophore complexes from electrospray ionization of UO2 2+ , Fe 3+ and Ca 2+ solutions[END_REF] Recently, we described the first crystal structures of two mono-and bischelated uranyl (UO2 2+ ) complexes incorporating the NMA - ligand, having [UO2(NMA)(NO3)(H2O)2] and [UO2(NMA)2(H2O)] compositions. [START_REF] Brandès | Conformational and structural studies of Nmethylacetohydroxamic acid and of its mono-and bis-chelated uranium(VI) complexes[END_REF] The uranyl cations in these complexes are typically surrounded by five oxygen atoms located in the essentially flat equatorial plane, ruling out steric interactions between the bidentate NMA -and the monodentate (nitrato and/or water) ligands. Owing to differences in the equatorial ligand field strengths, mono-and bischelation can be readily distinguished from each other by vibrational spectroscopy. Raman and infrared active vibration modes of uranyl undergo marked bathochromic shifts by increasing the number of bound hydroxamate anions. The main objective of this paper is to show to which extent the complexation of uranyl and iron with the model N-methylacetohydroxamate is similar. In order to gain a deeper insight into the interactions between the metals and NMA -in the gas phase, we report the results of theoretical calculations (DFT) combined with experimental infrared multiphoton dissociation spectroscopic data (IRMPD) and other experiments.

RESULTS

Crystal Structure of [Fe(NMA)3]H2O. The title complex was readily obtained by treating 3.4 equiv of NMAH with one equiv of Fe(acac)3 in methanol, followed by recrystallization of the isolated solid. Dark-red single crystals of [Fe(NMA)3]H2O having a plate-like shape were grown by diffusion of cyclohexane into a dichloromethane solution of the complex. Crystallographic parameters, data collection, and refinement information in tabular form are available in the Supporting Information. The complex crystallizes in the centrosymmetric space group P21/n with two independent molecular units and two water solvent molecules (O7 and O8) per asymmetric unit (Figure 1). The independent units are interconnected to each other in an alternate fashion by each water molecule via hydrogen bonds involving the hydroxamic oxygen atoms, as O5 and O5A interact with both protons attached to O7, while O3 and O3A are bound to the O8 water molecule (Table S8 and Figure S1 in Supporting Information). Owing to the nonsymmetrical structure of the NMA -ligand, the trichelated [Fe(NMA)3] complex is supposed to form a statistical 1:3 mixture of fac and mer isomers in solution. Interestingly, this ratio is reversed in the crystal state. Indeed, the crystallographically independent molecule labeled A contained in the asymmetric unit possesses a fac arrangement, while the carbon and nitrogen atoms of one NMA -ligand bound to Fe1 are split over two positions (N1/C2B, C1/C1B, C2/N1B, C3/C3B) with equal site occupation factors (Figure 1 and Table S9 in Supporting Information). Hence, half of the Fe1 molecular units correspond also to the fac isomer for which all chelates adopt the same directionality, while for the other half one chelate among the three is coordinated in the reversed sense, giving rise to the mer isomer. . IRMPD spectroscopy provides IR spectra of mass-selected ions. 16 The absorption of photons induces fragmentation of the mass-selected ions that is detected by mass spectrometry. Dependence of the fragmentation yield on the photon energy provides the IRMPD spectrum. The IRMPD spectra are multiphotonic (typically many photons have to be absorbed in order to induce the fragmentation of the given ion) and the peak intensities therefore do not linearly correlate with the intensities expected in the single-photon spectra (classical IR spectra, theoretical IR spectra). It can be difficult or impossible to obtain IRMPD spectra for ions with high-energy fragmentation thresholds. This is the case for the [Fe(NMA)2] + monomer, therefore only the dimer has been investigated here. S13 in Supporting Information for the full assignment).

Gas

The IRMPD spectrum of [Fe2(NMA)5] + contains four dominant composite bands at 976, 1183, 1496, and 1605 cm -1 (Figure 2). We have assigned the bands based on comparison with theoretical IR spectra (B3LYP/SDD/cc-pVTZ) of relevant complexes. In the first approximation, we modelled a simple mononuclear complex [Fe(NMA)2] + . Two coordination modes are possible around Fe 3+ , either a square-planar (Femono1) or a tetrahedral (Femono2)

arrangement. The B3LYP method predicts the quartet (S = 3/2) and the sextet (S = 5/2) ground state for 4 Femono1 and 6 Femono2, respectively (Figure 3). In comparison with neutral Nmethylacetohydroxamic acid and its anion (Table S17 in Supporting Information), the geometries of the acetohydroxamate ligands suggest much larger delocalization of the nonbonding electrons from the nitrogen atom towards the carbonyl carbon atom (i.e. larger contribution of the third resonance structure in Figure 3c). This leads to a contraction of the central C-N bond and an elongation of the carbonyl C=O bond (see the green values in Figure 3). The changes are slightly more pronounced for the 4 Femono1. The calculated IR spectra corresponding to 4 Femono1 and 6 Femono2 are almost identical with the exception of the (C-N) stretching mode (Figure 2 and Table S13 in Supporting Information). It is found at 1603 cm -1 for the square-planar isomer 4 Femono1 and at 1572 cm -1 for the tetrahedral isomer 6 Femono2. The higher frequency of the C-N stretching mode in 4 Femono1 compared to 6 Femono2 is in accord with larger geometry changes in this isomer resulting in the shorter C-N bond.

The bands at 1496 and 1605 cm -1 in the IRMPD spectrum can be thus assigned to the carbonyl stretching and the C-N stretching, respectively. Compared to the spectrum of NMA - (Figures S8-S9 and Table S12 in Supporting Information), the coordinated ligands reveal a considerable red-shift of the C=O vibration ( = 138 cm -1 ) and a blue shift of the C-N vibration. This is associated with the electron delocalization and geometry changes described above. The tentative assignment of the other bands is shown in Figure 2, the detailed discussion can be found in the in the Supporting Information.

The comparison with the results expected for the mononuclear complex helps in the assignment of the dominant bands, but cannot account for the whole IRMPD spectrum of [Fe2(NMA)5] + . Therefore, we refined the study by computing the structures and IR spectra of binuclear complexes [Fe2(NMA)5] + . The most stable structures possess always an octahedral coordination sphere around both iron centers and we assumed only high-spin complexes (other spin state combinations led to severe problems with the convergence of the calculations). However, the bridging mode between the two iron(III) atoms can differ. Here, we present four structures with different bridging modes (Fedi1, Fedi2, Fedi3, Fedi4 in Figure 4a). In Fedi1 and Fedi2, two hydroxamate ligands are bridging the iron centers via a single oxygen (1,1 binding) having different ligand orientations. In Fedi4, one additional NMA - ligand connects both irons through two oxygen atoms in a 1,4 mode. Fedi3 was calculated with Fedi1 as the starting structure, but the 1,1-bridging oxygen (in the back) is linked to the carbon instead of the nitrogen atom. S14 in Supporting Information for the full assignment).

The relative energies found for complexes Fedi1, Fedi2, Fedi3, and Fedi4 are 0, +4, +6, +28 kJ mol -1 , respectively. Energetically, the complexes having only two 1,1-oxygen bridges between both iron atoms (Fedi1-3) are more stable than the complex containing one additional ligand bridging the two metal centers (Fedi4) in a 1,4 fashion. The comparison of Fedi1 and Fedi2, shows that the bridging-ligand orientation affects the overall stability of the complex only little (4 kJ mol -1 ). Bridging by the oxygen atom linked to the nitrogen atom as in Fedi1

provides a more stable assembly than the bridging by the carbonyl oxygen atom as in Fedi3

(E = 6 kJ mol -1 ).

The calculated IR spectra for all four isomers are rather similar, as all evidence four composite bands that are also present in the experimental IRMPD spectrum (Figure 4b).

Considering the experimental spectral resolution (ca. 20 cm -1 ) and the rather high resemblance of the theoretical spectra, the gas-phase structure of the [Fe2(NMA)5] + cation cannot be unambiguously assigned. Keeping in mind that the calculated energy difference between the most stable Fedi1 and Fedi2 isomers (E = 4 kJ mol -1 ) is below the accuracy of the theoretical calculations, the occurrence of both species in equilibrium is highly probable.

Furthermore, the experimental trace in Figure 4b evidences an additional band at higher wavenumbers (1620 cm -1 ), suggesting that some species possess a bridging carbonyl oxygen atom such as found in structure Fedi3, which lies only 6 kJ mol -1 in energy above Fedi1.

Therefore, the feature at 1620 cm -1 can originate from a contribution of Fedi3. The highlystrained 1,4-bridged Fedi4 structure also evidences a carbonyl stretch around 1620 cm -1 , but its experimental detection is unlikely because of its high energy (E = +22 and + 28 kJ mol -1 with respect to Fedi1 and Fedi3, respectively). The experimental spectrum can thus be best described as a superposition of individual components arising from several isomers (i.e. Fedi1-

Fedi3).

The composite experimental band at 1605 cm -1 is nicely reproduced by theory, suggesting different binding modes of the hydroxamate ligand (either analogous to the one found in the square-planar monomer Femono1, the tetrahedral monomer Femono2, or as bridging both iron-centers). Similarly to the theoretical spectra of monomers, the intensity of the CH3 deformation modes centered at 1450 cm -1 is much larger than the experimentally recorded one. This enhancement can be either a consequence of the mixing with the C=O stretching, but the experimental intensities can also be underestimated due to the multiphoton nature of the IRMPD method. The IRMPD spectrum of the mass-selected [UO2(NMA)(NMAH)2] + cation (m/z = 536; Figure 5b) looks at first sight very similar to the spectrum of the iron complex discussed above. It suggests a similar effect of the UO2 2+ core on hydroxamate ligands as was found for Fe 3+ . The spectrum can be divided into four different regions: 9001000, 11501250, 14001500, and 15501650 cm 1 . Each range contains several overlapping bands reflecting binding modes of each ligand. The detailed assignment can been found in Table S15 in Supporting Information. S15 in Supporting Information for the full assignment).

Gas Phase Structures of Uranyl

The linear UO2 2+ cation (the "yl" oxygen atoms define the axial positions) typically interacts with four to six oxygen-bound ligands in the equatorial plane. [START_REF] Silver | Why Is Uranyl Formohydroxamate Red?[END_REF] In the most stable structure U1 (Figure 5a), the OH group of one N-methylacetohydroxamic acid makes a H-bond with one "yl" oxygen atom of the UO2 2+ core, while the second hydroxyl group interacts with the neighboring NOH oxygen atom. In the second most favored structure (U2), both monodentate NMAH molecules are hydrogen-bonded to the opposite uranyl oxygen atoms.

The various bending modes of the CH3 substituents are spread over the 9001500 cm 1 region and are, as expected, not very sensitive to the structural differences between U1 and U2. The situation is markedly different in the 15001700 cm 1 range, since U1 gives rise to three different CN stretching frequencies (1623, 1604, and 1587 cm 1 ), owing to the differences in the binding mode and protonation state of the three ligands. Both bands at 1604

and 1623 cm 1 correspond to the two protonated NMAH ligands, while the lower-energy band at 1587 cm 1 is assigned to the stretching mode for the deprotonated anionic hydroxamate ligand. The same order is observed for the C=O stretching bands: those at 1498 and 1505 cm 1 are associated to the NMAH molecules and that centered at 1486 cm 1 to the bidentate NMA  anion. Experimentally, only poorly resolved overlapping bands are observed instead of the three distinct peaks.

The IRMPD spectrum of [UO2(NMA)(NMAH)2] + corresponds rather well to the IR spectrum calculated for U1 (Figure 5b). Especially in the ranges around 950 and 1600 cm 1

the spectrum is well reproduced by the theoretical IR spectrum of U1, whereas the prediction for U2 is not matching. The O=U=O antisymmetric stretching mode (as) of the uranyl cation is observed at 944 cm 1 by IRMPD (a comparison with literature data can be found in the Supporting Information). Remarkably, the wavenumber computed for U1 (933 cm -1 , see Table S15 in Supporting Information) agrees fairly well with the experimental value, which is consistent with a hydrogen bond being formed to only one of the oxygen atoms. In turn, hydrogen bonding to both uranyl oxygen atoms as found in U2 gives rise to an absorption band located near 900 cm -1 (Figure 5b), inconsistent with the experimental findings. A scaling factor of 0.99 was applied to all harmonic frequencies. The panel on the top shows the peak attribution derived from the normal-mode analysis. The thin blue line is a zoom of the experimental spectrum magnified 8.75 times, the corresponding scale appearing on the right side.

Water

Possible structures of the dehydrated complex were oprimized by DFT computations.

The three arrangements of lowest energy are shown in Figure 6a. These calculations suggest that the most stable species (U3) contains indeed a monooxo uranium cation. Moreover, the theoretical IR spectrum of U3 is perfectly consistent with the experimental IRMPD spectrum (Figure 6). The other possibilities demonstrate that elimination of water from two in-plane hydroxamic ligands (U4 and U5) is disfavored by energetics and the products have IR spectra inconsistent with the experimental results.

We have further probed, whether fragmentations of uranyl complexes incorporating on In the case of the [UO2(NMA)(NMAH)2] + (U1) cationic species, average axial U=O (1.78 Å) and equatorial UO (2.37 Å) bond lengths are similar to those found by X-ray diffractometry for [UO2(NMA)(NO3)(H2O)2] (U=O = 1.770(1) Å, UON = 2.387(3) Å, U-OC = 2.396(3) Å). [START_REF] Brandès | Conformational and structural studies of Nmethylacetohydroxamic acid and of its mono-and bis-chelated uranium(VI) complexes[END_REF] Overall, binding of NMA  to iron or uranium produces similar changes: C=O, NO, and NCMe bonds are getting longer, while CN and CCMe bonds are shortened when compared to the free hydroxamate ligand (Figures 345). These expected trends are also consistent with the shifts undergone by the corresponding vibration frequencies. The only noticeable difference between the iron and uranium complexes is the longer metal-oxygen distance for the latter species as stressed above (2.37 vs. 1.942.06 Å), which is consistent with the larger radius of the uranium atom (ri = 0.73 vs. 0. 645 Å for hexacoordinated uranium(VI) and high-spin iron(III), respectively). 20

Comparison of the Bond Nature in the Iron and Uranyl Complexes. Both optimized structures of the Femono2 and U1 complexes were subjected to the Natural Bond Orbitals (NBO) analysis to gauge the covalent character of the metal-ligand interactions occurring in the gas phase (Table S18 in Supporting Information). The electronic natural configurations determined by the NBO analysis are [core]4s 0.25 3d 5.85 4p 0.37 4d 0.01 with a charge of 1.51 at the iron(III) in complex Femono2 and [core]7s 0.15 5f 2.60 6d 1.36 7p 0.32 with a charge of 1.72 at the uranium in the uranyl complex U1. The number of d-electrons on Fe(III), close to 6, can be explained by a ligand-to-metal charge transfer, which is a well-known characteristic of iron-hydroxamate complexes responsible for their orange-red color. [START_REF] Edwards | Experimental and theoretical vibrational spectroscopy studies of acetohydroxamic acid and desferrioxamine B in aqueous solution: Effects of pH and iron complexation[END_REF] The electronic configuration of uranium is similar to the one reported before for an uranium(VI) phosphopeptide complex (2.60 and 1.40 respectively for 5f and 6d orbitals). [START_REF] Wu | Interactions between uranium(VI) and phosphopeptide: experimental and theoretical investigations[END_REF] The similarity of the NBO charges in both complexes suggests that the bonding of the ligands should be of similar nature. Barros et al. proposed that the ratio between the NBO charge and the oxidation number of the metal should be a good descriptor of a covalent or ionic nature of the metal-ligand interactions in a complexed species. [START_REF] Barros | Single but Stronger UO, Double but Weaker UNMe Bonds: The Tale Told by Cp2UO and Cp2UNR[END_REF] The lower this ratio, the more covalent are the bonds, due to a greater sharing of electrons. The calculated ratios for Femono2 and U1 are 0.50 and 0.29, respectively, suggesting that bonds to uranium(VI) are more covalent than those to iron(III). It is important to note that the Barros ratio reflects the sum of all ionic and covalent contributions of all ligands around the metal. In the case of U1, NBO analysis reveals that both U=Oyl bonds are the main contribution to the overall high covalence, whereas the equatorial oxygen atoms show partially ionic interactions (Table S18 in Supporting Information). Hence, while the polarization of the U=Oyl bonds is 79% for O and 21% for U, we found an average value of about 89% for O and 11% for U for the equatorial bonds to the hydroxamates. The NBO analysis reveals multiple character of the U=Oyl bond, whereas there is just a single bond between U and the oxygen atoms of the hydroxamates.

In the case of Femono2, the NBO analysis reveals a similar character of all FeO bonds with an average electron density polarization of the Fe-O bonds of 85% at oxygen and 15% at iron (Table S18 in Supporting Information). This finding agrees with the study of Domagal-Goldman et al., [START_REF] Domagal-Goldman | Quantum chemical study of the Fe(III)-desferrioxamine B siderophore complex-Electronic structure, vibrational frequencies, and equilibrium Fe-isotope fractionation[END_REF] who assigned the high stability of the iron(III) complex with desferrioxamine B (Fe(DFB)H = 10 30.6 ) at least to some extent to the covalent contribution.

Interestingly, their NBO analysis showed no indication of covalence for the analogous iron(II)

or aluminum(III) complexes.

From these findings, it can be concluded that Fe III O and U VI Oeq bonds constituted by hydroxamate anions and hydroxamic acids are highly-polarized covalent bonds, as typically expected for hard acid-hard base complexes. [START_REF] Lemire | Antimicrobial activity of metals: mechanisms, molecular targets and applications[END_REF] The MO bonds in the uranium complex [UO2(NMA)(NMAH)2] + (U1) are slightly more polarized than those occurring in [Fe(NMA)2] + (Femono2). It most likely originates from the differences in orbital shapes and overlapping with the ligand orbitals (uranium 4f-orbitals overlap less well with the s and p orbitals of the ligand than the 3d orbitals of iron do). In that context, our present findings provide an additional example for U=O bound activation, most likely promoted by hydrogen bond interactions involving the abstracted "yl" oxygen atom in the parent ion. In the case of [UO2 (3,4,3-LI-CAM)H5] -, the OH groups from the catechol units bound to uranyl in the equatorial plane, as well as those remaining unbound but located in close vicinity to the coordination center, play a prominent role in the activation process. Transfer of exchangeable protons from neighboring donor groups is an essential step in the formation of water, as this process is expected to lower the energy barrier of the U=O bond rupture. Here, [UO2(NMA)(NMAH)2] + provides the ever first case of bond activation through water elimination that is favored by a direct hydrogen bond between the apical Oyl atom and one hydroxyl group from a coordinated ligand.

U=O Bond

These findings have also some relevance to the chemical properties of uranyl in the condensed phase. While its oxygen atoms are reputed inert in the ground state, albeit exchange occurs rapidly in the excited state upon UV irradiation, dramatic enhancement of the lability of the Oyl atoms in aqueous solutions has been observed by Szabó and Grenthe for the [(UO2)2(-OH)2(H2O)6] 2+ species. [START_REF] Szabó | Reactivity of the "yl"-Bond in Uranyl(VI) Complexes. 1. Rates and Mechanisms for the Exchange between the trans-dioxo Oxygen Atoms in (UO2)2(OH)2 2+ and Mononuclear UO2(OH)n 2-n Complexes with Solvent Water[END_REF] These authors evidenced a fast exchange with the oxygen atoms provided by bulk water for that particular hydrolyzed species, with a half-life of 0.13 s and an activation enthalpy of 80( 14) kJ/mol for the rate determining step. Conversely, no appreciable oxygen exchange could be detected by [START_REF] Silver | Why Is Uranyl Formohydroxamate Red?[END_REF] O NMR spectroscopy for UO2 2+ , UO2(OH) + , UO2(OH)4 2-, UO2(OH)5 3-, and for the ternary complex [(UO2)2(-OH)2F2(oxalate)2] 4-, suggesting that exchange only takes place in polynuclear hydroxide complexes, provided water molecules are present in the first coordination shell. The suggested mechanism involves a proton transfer from water molecules bound in the equatorial plane to the "yl"-oxygen that weakens the U=O interaction, followed by a rapid exchange of the resulting OH group with the bulk water. As a matter of fact, the U=O bond reactivity in the gas-phase and in solution shares some common features, namely the pivotal role of the hydroxyl group and the presence of an additional proton donor.

CONCLUSION

Complexation of Fe 3+ and UO2 2+ with N-methylacetohydroxamate are qualitatively of the same nature, as evidenced by the bond distances and the IR spectra. The M-O bonds between the metal (M = U or Fe) and the oxygen atom of hydroxamate can be described as highly polarized covalent bonds. The interaction of the hydroxamate ligands with the uranyl results in slightly more polarized M-O bonds that the interaction with iron(III). The hydroxamate moiety is affected in the same way upon binding to either iron or uranyl: the carbonyl bond elongates, owing to the extended conjugation, while the CN bond shortens as it gains a double-bond character. The geometry changes are evidenced by the red-and blue shifts of the corresponding IR absorption bands, respectively. The gas-phase uranium complex [UO2(NMA)(NMAH)2] + easily eliminates a water molecule upon collisional activation.

IRMPD spectroscopy, DFT computations, and MS-MS measurements performed with natural and [START_REF] Politzer | Some Intrinsic Features of Hydroxylamines, Oximes and Hydroxamic Acids: Integration of Theory and Experiment[END_REF] O-enriched uranyl samples suggest that the fragmentation involves the uranyl oxo ligand. The fragment ions thus have only one oxo ligand and three bidentally coordinated hydroxamates. In the context of depollution and complexation of uranium, these results show that a siderophore can bind also strongly to uranium(VI), but specific reactivity can arise in the gas phase from the oxygen of the "yl" moiety.

EXPERIMENTAL SECTION

CAUTION! Uranium (primary isotope 238 U) is a weak -emitter (4.197 MeV) with a half-life of 4.47 × 10 9 years. All complexes were synthesized in monitored fume hoods, in a laboratory equipped with and -counting equipment.

Materials and Methods.

All solvents and analytical-grade chemicals were obtained from commercial suppliers and used without further purification. N-methylacetohydroxamic acid (NMAH) was prepared following the improved procedure described recently by Brandès et al. [START_REF] Brandès | Conformational and structural studies of Nmethylacetohydroxamic acid and of its mono-and bis-chelated uranium(VI) complexes[END_REF] The sample used herein was taken from the same batch for which analytical data ( 1 H and [START_REF] Kurzak | Hydroxamic and aminohydroxamic acids and their complexes with metal ions[END_REF] C NMR, CHN contents) have been reported elsewhere. [START_REF] Brandès | Conformational and structural studies of Nmethylacetohydroxamic acid and of its mono-and bis-chelated uranium(VI) complexes[END_REF] An aqueous stock solution of in the mother solution.

Fourier-transform mid-infrared (400-4000 cm -1 ) spectra (FTMIR) were recorded at 4 cm -1 resolution on a Bruker VERTEX 70v spectrometer fitted with an A225 diamond attenuated total reflection (ATR) accessory (Bruker) and a DTGS (deuterated triglycine sulfate) detector (350-4000 cm -1 ). Raman spectra were collected with a Renishaw inVia spectrometer equipped with a 632.8 nm He-Ne laser excitation source, a 1800 grooves/mm grating, and a microscope fitted with a either a 50 (solid samples) or 20 (liquid samples)

objective. Solids were deposited on a glass slide, while solutions were introduced in a fluorescence cuvette of 1 cm path length (Hellma). Wavenumbers were calibrated with respect to the silicon scattering line at 520(1) cm -1 of an internal standard. Elemental analyzes were performed on a Flash EA 1112 (Thermo Scientific) CHNS analyzer at the "Plateforme Crystallography. Clear dark-red plate-shaped single crystals of [Fe(NMA)3]H2O suitable for X-ray crystallography were obtained by slow diffusion of cyclohexane (10 mL) into a dichloromethane solution (2 mL) of the isolated complex. A specimen with dimensions of 0.15  0.12  0.02 mm 3 was selected and mounted on a MITIGEN holder oil on a Bruker D8 VENTURE diffractometer equipped with an Oxford Cryosystems low-temperature device operating at T = 100(2) K. Data were measured using  and  scans of 1.00° per frame for 120.00 s using the Cu K radiation from a sealed X-ray tube. Cell parameter refinement and data reduction were performed using the SAINT software, which corrects for Lorentz polarization. [START_REF]SAINT: Area-Detector Integration Sofware, v. 8.37A[END_REF] The structure was solved in the space group P21/n by direct methods using the ShelXT software [START_REF] Sheldrick | SHELXT -Integrated space-group and crystal-structure determination[END_REF] and then refined by full-matrix least-squares on F 2 using ShelXL (release 2014/7), [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF] both routines being implemented in the Olex2 environment. [START_REF] Dolomanov | OLEX2: a complete structure solution, refinement and analysis program[END_REF] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed at calculated positions and refined using a riding model. Solutions were injected with a syringe pump at a flow rate of 5 L/min. Spectra were collected using the following settings: heather temperature: 50 °C, spray voltage: +4 kV, capillary temperature: 275 °C, capillary voltage: 0 V, tube lens: 20 V, sheath gas pressure: 10 psi. The resolution was set to 60 000 at m/z = 200 amu. CID experiments were carried out with helium as collision gas. The analyzed ion was selected with an isolation width of m/z = 1 amu. The normalized collision energy, as defined by the constructor, was varied step by step between 0 and 18%. The other instrumental parameters were set to 0.25 for the activation Q (a parameter that defines the frequency of the applied radiofrequency potential) and 30 ms for the activation time.

Crystal

Infrared multiphoton dissociation (IRMPD) spectra were collected with a Bruker Esquire 3000 ion trap mass spectrometer. The IR photons were provided by free electron laser CLIO (Centre Laser Infrarouge Orsay, Orsay, France). [START_REF] Ortega | Extension in far-infrared of the CLIO free-electron laser[END_REF] The ions were generated in the same way as above, mass selected, and stored in the ion trap. The mass-selected ions were irradiated by several laser macropulses that induced a fragmentation (the wavenumber power dependence is shown in Figure S14; irradiation time was 150 ms). The dependence of the fragmentation yield on the photon wavenumber provides the experimental IRMPD spectrum with a resolution of 1520 cm 1 . 35

DFT Calculations. Theoretical calculations were conducted using DFT methods with Gaussian 09 package rev. D. [START_REF] Frisch | Gaussian 09 revision 01[END_REF] Specifically, we used the unrestricted B3LYP method, [START_REF] Stephens | Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force-Fields[END_REF] comprised of Becke exchange (B3) [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF] and Lee, Yang and Parr's gradient-corrected correlation functional (LYP). [START_REF] Lee | Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density[END_REF] Structures were optimized using B3LYP functional and cc-pVTZ basis set (for organic atoms H, C, N and O) and Stuttgart pseudo-potential (SDD) basis set (for U and Fe). For each optimized structure, the frequency calculation was performed in order to validate the minimum on the potential energy surface, calculate the zero-point energy (ZPE), and obtain the infrared spectrum. The predicted gas spectra were broadened using a Gaussian function with a full width at half-maximum of 10 cm -1 . For all theoretical spectra, the scaling factor was chosen in order to obtain the best agreement with the experimental data. Scaling factors of 0.99 and 0.985 were used for the iron and uranium complexes of NMA  , respectively. A factor of 0.98 was applied for the free ligand. In order to gain more insights about the bonding character, we have used Natural Bond Orbitals (NBO) analysis, as implemented in Gaussian 09, i.e. NBO 3.0. 40

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at

Table of Contents

Highly polarized covalent bonds of similar nature have been evidenced by DFT calculations and IRMPD spectroscopy for N-methylacetohydroxamate (NMA -) complexes of Fe 3+ and UO2 2+ , albeit with slightly stronger ionic character for the later. Upon collisional activation, the gas phase cation [UO2(NMA)(NMAH)2] + easily eliminates a water molecule through abstraction of one uranyl oxo ligand. This unusual dissociation pattern highlights a specific reactivity of the "yl" oxygen atom bound to uranium(VI).

Figure 1 .

 1 Figure 1. (a) ORTEP view of the asymmetric unit found in the crystal structure of [Fe(NMA)3]H2O. Thermal ellipsoids are drawn at the 50% probability level and all protons are omitted for clarity. Disordered atoms were refined isotropically with site occupation

  Phase Structures of Iron Complexes. Electrospray ionization of a methanolic solution of a genuine sample of [Fe(NMA)3]0.6H2O produces [Fen(NMA)3n-1] + (n = 1 and 2) cluster ions (Figure S4 in Supporting Information). We have investigated the dinuclear cluster [Fe2(NMA)5] + (m/z = 552). The collision-induced dissociation of this cation leads to the elimination of one or two N-methylacetohydroxamate radicals (Figure S5 in Supporting Information). The structure of the [Fe2(NMA)5] + cation was further probed by infrared multiphoton dissociation spectroscopy (IRMPD)

Figure 2 .

 2 Figure 2. Comparison of the IRMPD spectrum of [Fe2(NMA)5] + (m/z 552; fragments with m/z 464 and 376 were monitored during IRMPD) and the theoretical IR spectra calculated for 4 Femono1 and 6 Femono2. The harmonic frequencies were scaled by a factor of 0.99. The panel on the top shows the peak attribution derived from the normal-mode analysis (see TableS13in Supporting Information for the full assignment).

Figure 3 .

 3 Figure 3. B3LYP optimized structures of (a) N-methylacetohydroxate and Nmethylacetohydroxamic acid, and (b) the square-planar ( 4 Femono1) and tetrahedral ( 6 Femono2) isomers of the [Fe(NMA)2] + complex together with their relative energies. Numbers in green are bond distances (in red for Fe-O distances) in Angströms (Å), those in black are the Mulliken charges and the ones in blue are the Mulliken spin distributions. (c) Resonance structures of (Z)-NMA -.

Figure 4 .

 4 Figure 4. (a) Structure of the four possible isomers of [Fe2(NMA)5] + (Fedi1, Fedi2, Fedi3, and Fedi4, optimized geometries can be found in the SI). Numbers in green correspond to selected bonding distances in Angströms (Å), while those in red refer to the Fe-O distances. (b) IRMPD spectrum of the [Fe2(NMA)5] + ion (identical to Figure2) and theoretical spectra of Fedi1, Fedi2, Fedi3, and Fedi4. The harmonic frequencies were scaled by a factor of 0.99. The panel on the top shows the peak assignment derived from the normal-mode analysis (see TableS14in Supporting Information for the full assignment).

  Complexes. A mass spectrum of a methanolic solution of [UO2(NMA)2(H2O)] shows formation of a variety of [UO2 2+ (NMA)(L)n] complexes with L = NMAH, H2O, or CH3OH and n = 0-2 (Figure S8 in Supporting Information). Assignments given in Table S11 were confirmed by (+)-HR-ESI-MS spectroscopy (Figure S9 in Supporting Information) together with simulations of the isotopic patterns. We have further investigated only complex [UO2(NMA)(NMAH)2] + bearing one hydroxamate and two neutral hydroxamic acids as ligands.

Figure 5 .

 5 Figure 5. (a) Structure of possible isomers U1 and U2 of [UO2(NMA)(NMAH)2] + (optimized geometries can be found in the SI). Numbers in green refer to selected bonding distances in Angströms (Å), while those in red refer to the U-O distances. (b) IRMPD spectrum of the [UO2(NMA)(NMAH)2] + cation (m/z = 536; fragments with m/z 518 and 447 and its water adduct with m/z 465 were monitored during IRMPD) and theoretical spectra of U1 and U2.A scaling factor of 0.985 was applied to all harmonic frequencies. The panel on the top shows the peak assignments derived from the normal-mode analysis. The thin blue line is a zoom of the experimental spectrum magnified 4 times, the corresponding scale appearing on the right side (see TableS15in Supporting Information for the full assignment).

  All our attempts in optimizing the structure of the triply ligated complex [UO2(NMA)(NMAH)2] + resulted in octahedral coordination with just four U-O bonds in the equatorial plane. All attempts to put five or more U-O bonds in the equatorial plane failed and the calculations invariably converged to stable structures with only four equatorial oxygen atoms. Two of these donor atoms are provided by the bidentate N-methylacetohydroxamate (NMA  ) ligand. The two remaining oxygens are the carbonyl oxygen atoms of both neutral N-methylacetohydroxamic acids (NMAH). The NOH groups of the monodentate NMAH molecules are available for intramolecular hydrogen bonds.

  Elimination from [UO2(NMA)(NMAH)2] + . The collision induced dissociation (CID) experiment with [UO2(NMA)(NMAH)2] + (m/z = 536.172) shows that the complex easily loses a molecule of water (m/z = 518.162) or one NMAH ligand (m/z = 447.125) (Figure S10 in Supporting Information). The elimination of NMAH can be followed by secondary reactions with background gases, H2O and CH3OH, yielding [UO2(NMA)(NMAH)(H2O)] + and [UO2(NMA)(NMAH)(CH3OH)] + . The eliminated water molecule can contain either the oxygen atom from a coordinated ligand or from one U=O bond of the uranyl cation. The activation of the U=O bond and thus elimination of the uranyl oxygen atom was proved by CID of isotopically labelled [U 18 O2(NMA)(NMAH)2] + . The CID clearly shows exclusive elimination of H2 18 O (Figure S11 in Supporting Information). Further, the structure of the dehydrated complex (m/z = 518)was investigated by IRMPD spectroscopy (blue curves in Figure6). Contrary to the parent ion, the IRMPD spectrum of the dehydrated complex does not contain the intense band at about 940 cm -1 corresponding to the O=U=O antisymmetric stretching. The bands corresponding to the CN and C=O stretchings are broad, overlapping, and slightly redshifted. The disappearance of the uranyl vibration mode suggests formation of a UO 4+ complexed species.

Figure 6 .

 6 Figure 6. (a) Structure of the possible isomers U3, U4, and U5 formed by water elimination from [UO2(NMA)(NMAH)2] + (optimized geometries can be found in the SI). Numbers in green refer to selected bond distances in Angströms (Å), while those in red refer to the U-O

  one or two NMA -/NMAH ligands would also lead to the U=O bond activation. The CID spectrum of the isotopically labeled [U 18 O2(NMA)(NMAH)] + ion (m/z = 451.136) shows elimination of CH3N 16 O (Mr = 45.021) and NMAH (Mr = 89.047), but no sign of U= 18 O activation (Figure S12 in Supporting Information). Similarly, CID of [U 18 O2(NMA)(CH3OH)] + (m/z = 394.114) does not reveal any process involving the U= 18 O bond (Figure S13 in Supporting Information). Accordingly, the loss of the uranyl oxygen atom for the UO2 2+ /NMA -system is associated with the structure of [UO2(NMA)(NMAH)2] + . The postulated apical U=OHO-N hydrogen bond in U1 provides an opportunity for a facile hydrogen transfer from the ligand to the uranyl oxygen atom. The formation of the hydroxyl group will be associated with bidentate coordination of the formed anionic ligand. This process will lower the energy barrier for the second proton transfer, which ultimately leads to the elimination of water. DISCUSSION Similarities and Differences of Iron and Uranyl Complexes. Good agreement between experimental IRMPD and theoretical IR spectra enables us to discuss in more details relevant structural properties. The following analysis encompasses the selected ligand NMA  and its protonated form NMAH, as well as the Femono1, Fedi1, and U1 complexes. Relevant bond distances retrieved from each DFT-optimized structure are reported in

  Activation. Fragmentation experiments on several monovalent cations produced by electrospray ionization of methanolic solutions of the [UO2(NMA)2(H2O)] complex revealed an unexpected behavior specific to the [UO2(NMA)(NMAH)2] + gas-phase species, which translates into abstraction of an "yl" oxygen atom from the uranyl core and the concomitant elimination of a water molecule. Reputed as particularly inert, both U=O bonds in UO2 2+ are extremely difficult to activate and typically undergo reduction upon silylation or reaction with strong Lewis acids. Direct bond cleavage initiated by ESI-MS CID of coordinated uranyl cations has only been achieved very recently for a few number of systems, although none incorporated a chelating ligand nor eliminated a molecule of water. 26 CID of [UO2(N3)Cl2] -produces the nitrosyl complex [UO(NO)Cl2] -together with N2, [UO2(NCO)Cl2] -decomposes into [UONCl2] -and CO2, while [UO2(CH3CN)] 2+ generates the intermediate cation [UO2(NC)] + that fragments into UON + and CO. Most remarkably, the ever-first example of double activation for an uranyl complex in the gas phase has been reported in fall 2017 by Abergel et al. 27 MS-MS of the tetracatecholamide [UO2(3,4,3-LI-CAM)H5] -anion produced a heptacoordinated U 6+ chelate of [U(3,4,3-LI-CAM)H] -formula upon simultaneous release of two water molecules, as ascertained by IRMPD and 18 O-labeling experiments.

  [START_REF] Politzer | Some Intrinsic Features of Hydroxylamines, Oximes and Hydroxamic Acids: Integration of Theory and Experiment[END_REF] O-labeled uranyl (0.185 mol/kg) was prepared by dissolving 52.13 mg of [UO2(NO3)2(H2O)2]4H2O with 561.39 mg of H2 18 O (Sigma-Aldrich, 97% 18 O) directly in a stoppered fluorescence quartz cell of 1 cm path length. The cuvette was exposed for ca. 180 h to UV radiations in the center of a Luzchem ring illuminator equipped with two 22 W mercury UVA lamps, after which the solution was acidified to p[H] ~ 1.0 by adding 3 L of 69% HNO3 (~15.4 M, Fischer, Trace Analysis). The enrichment level was monitored as a function of irradiation time by (+)-HR-ESI-MS after diluting 500 times the stock solution in analytical-grade methanol. Integration of the isotopic pattern of the UO2 + signal gave 80% of U 18 O2 + (m/z = 274.04), 17% of U 16 O 18 O + (m/z = 272.04), and 3% of residual U 16 O2 + (m/z = 270.04). The almost complete oxygen exchange was confirmed by Raman spectroscopy with the total disappearance of the signal at 870 cm -1 (s(U= 16 O)) and the occurrence of a strong band at 824 cm -1 (s(U= 18 O)), assigned to the fully labeled U( 18 O)2 + cation. The weak foothump at ~850 cm -1 agrees with the presence of a small amount of partially labeled U 16 O 18 O 2+

  To an aqueous solution (5 mL) of NMAH (100 mg, 1.122 mmol) at pH 4.9 was added in one portion 141 mg (0.281 mmol) of [UO2(NO3)2(H2O)2]4H2O. After dissolution, the pH dropped to 1.7 and the reaction mixture became orange, indicating complex formation. The pH was then adjusted to 5.1 with ca. 5 mL of a 0.1 M N(CH3)4OH solution. Red-orange crystals deposited upon partial slow evaporation of the mother liquor. X-ray diffraction studies evidenced unambiguously the [UO2(NMA)2(H2O)] formula.[START_REF] Brandès | Conformational and structural studies of Nmethylacetohydroxamic acid and of its mono-and bis-chelated uranium(VI) complexes[END_REF] The remaining crystalline solid was recovered by filtration, washed with a minimum amount of water, and dried under vacuum. Yield: 80 mg (0.172 mmol, 61%). IR (ATR, cm -1 ):  ~ = 2938 (v br, w), 1718 (w), 1597 (s), 1475 (s), 1419 (s), 1374 (m), 1217 (m), 1164 (s), 1034 (w), 972 (m), 896 (as(U=O), s), 830 (s(U=O), w), 752 (s), 612 (s), 595 (m), 486 (s). Raman (cm -1 ):  ~ = 2939, 1622, 1603, 828 (s(U=O), vs), 757, 221. Anal. calcd. (%) for C6H14N2O7U (464.21 g/mol): C 15.52, H 3.04, N 6.03; found: C 15.61, H 3.06, N 6.09. Synthesis of [U 18 O2(NMA)2(H2O)]. To an aqueous solution (3 mL) of NMAH (7 mg, 0.0785 mmol) was added 0.5 mL of a 0.1 M N(CH3)4OH solution followed by 0.1 mL (0.02 mmol) of an ~0.2 M U 18 O2(NO3)2 solution acidified with HNO3 (~0.09 M). The reaction mixture became orange and the pH was adjusted to 5.3 with ca. 0.1 mL of a 0.1 M N(CH3)4OH solution. The solution was then evaporated to dryness under vacuum and analyzed without further purification. IR (ATR, cm -1 ):  ~ = 2939 (v br, w), 1599 (m), 1491 (s), 1415 (w), 1331 (br, s), 1215 (w), 1162 (w), 1033 (w), 968 (w), 947 (s), 889 (w), 877 (w), 850 (as(U= 18 O), s), 832 (w), 752 (m), 612 (m), 482 (m), 459 (w). Raman (cm -1 ):  ~ = (s(U=O), vs). Isotopic exchange on the uranyl center in the bischelated complex was ascertained by the bathochromic shift experienced both by the Raman and IR active symmetric and antisymmetric U=O stretching modes, respectively. The wavenumbers found for the labeled complex, s(U=O) = 785 and as(U=O) = 850 cm -1 , are in perfect agreement with the values predicted by the Hooke's law (U= 18 O) = (U= 16 O) × (U= 16 O/U= 18 O) 1/2 = 0.947, where  designates the reduced mass of the harmonic vibrator ( = MUMO/MU + MO).

  Data for [Fe(NMA)3]H2O. C9H20FeN3O7, Mr = 338.13 g/mol, monoclinic space group P21/n (No. 14), a = 10.4087(6) Å, b = 10.4063(6) Å, c = 27.484(2) Å,  = 91.156(3)°, V = 2976.3(3) Å 3 , Z = 8, Z' = 2, calcd = 1.509 g/cm 3 , T = 100(2) K, (Cu K) = 8.458 mm 1 ( = 1.54184 Å), 30590 reflections measured (6.432° ≤ 2Θ ≤ 133.37°), 5184 unique (Rint = 0.0544, Rsigma = 0.0369), 4460 used with I > 2(I), 370 parameters and 16restraints. Final R1 = 0.0375 (I >2(I)), wR2 = 0.1006 (all data), min./max. residual electron density = 0.282/0.648 e/Å 3 . Some disorder (50%) was found in the crystal structure and some geometric parameters of disordered components in each group were restrained by using EADP constraints or RIGU restraints. See the Supporting Information for more details.Mass Spectrometry Measurements.Mass spectrometry experiments were conducted with a Finnigan LCQ instrument equipped with an electrospray ionization source operated in a positive charge mode.33 Methanolic solutions of the complexes were introduced to the electrospray ionization source through a fused-silica capillary using a syringe-pump. The operating conditions were set as following: spray voltage to 4.55 kV, capillary voltage to 0 V, and the temperature of the capillary to 150 °C. Collision-induced dissociation of massselected ions was achieved by excitation of the ions at their secular frequency and collision with the helium buffer gas.Alternatively, high-resolution (+)-ESI-MS spectra of[START_REF] Politzer | Some Intrinsic Features of Hydroxylamines, Oximes and Hydroxamic Acids: Integration of Theory and Experiment[END_REF] O enriched uranyl samples dissolved in methanol were acquired at the PACSMUB platform on a LTQ OrbitrapXL (Thermo Scientific) spectrometer equipped with a Ion Max source and HESI-II probe.

  DOI: 10.1021/acs.inorgchem.XXX. Crystallographic data for [Fe(NMA)3]H2O; statistics on bond distances for iron hydroxamate complexes; TGA, FTIR, and ESI-MS data for [Fe(NMA)3]0.6H2O; IRMPD data for [Fe2(NMA)5] + ; FTIR and ESI-MS data for [UO2(NMA)2(H2O)]; CID spectra for selected ions generated from [UO2(NMA)2(H2O)] and [U 18 O2(NMA)2(H2O)]; laser power during IRMPD measurements; detailed assignment of the IR spectra of NMAH, NMA -, Femono1, Femono2, Fedi1, and U1; Mulliken charge distribution for NMAH, NMA -, Femono2; DFT-calculated bond distances for NMAH, NMA  , Femono2, Fedi1, and U1; NBO analysis of Femono2, U1, and Fedi1 (pdf).

Table S17 (

 S17

Supporting Information) and shown in Figures

3-5

. Calculated C=O

(1.23 Å)

and CN

(1.35 Å)

bonds distances for NMAH are in perfect agreement with the data reported in the literature for hydroxamic acids (1.23 and 1.341.40 Å, respectively).

[START_REF] Politzer | Some Intrinsic Features of Hydroxylamines, Oximes and Hydroxamic Acids: Integration of Theory and Experiment[END_REF]

Structural parameters obtained in this work for the diferric [Fe2(NMA)5] + (Fedi1) complex (1.942.06 Å for FeO, 1.27 Å for C=O, 1.361.38 Å for NO, and 1.32 Å for CN) are also reproducing well the crystallographic distances determined herein for the neutral [Fe(NMA)3] complex (2.01(2) Å for FeO, 1.295(5) Å for C=O, 1.360(5) Å for NO, and 1.306(4) Å for CN) and those reported by Dhungana et al. for the ferrioxamine B complex (1.981 Å for FeON, 2.037 Å for FeOC, 1.282 Å for C=O, 1.377 Å for NO, 1.317 Å for CN).

[START_REF] Dhungana | Crystal structure of ferrioxamine B: a comparative analysis and implications for molecular recognition[END_REF]

  CCDC-1538337 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033. the funding from the European Union's Seventh Framework Program (FP7/2007-2013) under the grant agreement No. 226716. The CLIO staff, particularly Vincent Steinmetz, is acknowledged for their help and assistance.

	Accession Codes

ACKNOWLEDGEMENTS

The Czech Science Foundation (14-20077S), the Centre National de la Recherche Scientifique (CNRS), the Conseil Régional de Bourgogne through the "Plan d'Actions Régional pour l'Innovation" (program PARI II CDEA), the European Regional Development Fund (FEDER), the French-Czech exchange program "Partenariat Hubert Curien (PHC) Barrande", and the program "Défi NEEDS Environnement" (project ACTISOL) are gratefully acknowledged for their financial support. Measurements at CLIO were performed owing to

AUTHOR INFORMATION

Corresponding Authors

* E-mail: roithova@natur.cuni.cz. * E-mail: michel.meyer@u-bourgogne.fr.

ORCID

Jana Roithova: 0000-0001-5144-0688 Michel Meyer: 0000-0003-2295-7826 Stéphane Brandès: 0000-0001-6923-1630

Notes

The authors declare no competing financial interest.


Michaël Rochoy

Emmanuel Chazard

Sophie Gautier

Régis Bordet

Vascular dementia encoding in the French nationwide discharge summary database (PMSI): variability over the 2007-2017 period

Keywords: dementia, vascular, database, clinical coding, diagnosis Mots-clés : démence vasculaire, base de données, codage clinique, diagnostic

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

Introduction

In France, dementia affected 1 to 1.2 million people directly and 3 million relatives indirectly in the early 2010's [START_REF]L'état de santé de la population en France[END_REF][START_REF] Helmer | Évolution temporelle des démences : état des lieux en France et à l'internationale[END_REF]. Several predictive scenarios have been carried out, estimating a prevalence of 1.75 million in 2030 [START_REF] Jacqmin-Gadda | 20-Year prevalence projections for dementia and impact of preventive policy about risk factors[END_REF] and 1.81 million in 2050 [START_REF] Mura | How many dementia cases in France and Europe? Alternative projections and scenarios 2010-2050[END_REF]. Cerebrovascular disease are the most common causes of dementia after Alzheimer's disease: vascular dementia (VaD) is estimated to be about 15% of dementias [START_REF] O'brien | Vascular dementia[END_REF]. The management of vascular dementia is based on the effective control of cardiovascular risk factors: hypertension, smoking, ischaemic heart disease, atrial fibrillation, diabetes, obesity, etc. [START_REF] O'brien | Vascular dementia[END_REF] In 1993, the National Institute of Neurological Disorders and Stroke with support from the Association Internationale pour la Recherche et l'Enseignement en Neurosciences (NINDS-AIREN) proposed criteria for the diagnosis of VaD [START_REF] Román | Vascular dementia: diagnostic criteria for research studies. Report of the NINDS-AIREN International Workshop[END_REF]. In 2000, Erkinjuntti et al. proposed a modification of the NINDS-AIREN criteria as a new research criteria for subcortical VaD [START_REF] Erkinjuntti | Research criteria for subcortical vascular dementia in clinical trials[END_REF]. In 2005, two reviews highlighted the importance of poststroke VaD [START_REF] Leys | Poststroke dementia[END_REF][START_REF] Hénon | Poststroke dementia[END_REF]. Advances in brain imaging, genetics and the evolution of the concept of dementia have led to the individualization of different subtypes of VaD in ICD-10: multiple infarctions, subcortical VaD, acute onset VaD, mixed types. Recently, the VASCOG statement proposed new criteria for vascular cognitive disorders [START_REF] Sachdev | Diagnostic criteria for vascular cognitive disorders: a VASCOG statement[END_REF]. The application of these different criteria to diagnose VaD in France is not known. The French Programme de Médicalisation du Système d'Information (PMSI) database gives access to the evolution of diagnoses over all hospital stay. Our aim was to analyze the evolution of VaD ICD-10 coding in France between 2007 and 2017.

Methods

Study design

This study consists of a retrospective cohort, based on secondary use of the PMSI database (presented below). We included all the inpatient stays from 2007 to 2017, from the website ScanSanté (Technical agency for Hospital Information).

Data source

The PMSI database is the French nationwide exhaustive hospital discharge database [START_REF] Boudemaghe | Data Resource Profile: The French National Uniform Hospital Discharge Data Set Database (PMSI)[END_REF]. The database used in this study comprehends all the inpatient stays, from nonprofit and for-profit acute care hospitals (medicine, surgery and obstetrics), excluding psychiatric hospitals and rehabilitation care centers. This database includes administrative data (admission and discharge dates and modes), demographic data (age, gender, geographic area), diagnoses encoded in the International Classification of Diseases and Related Health Problems, 10 th edition (ICD-10) [START_REF]WHO | International Classification of Diseases (ICD)[END_REF] and other pieces of information [START_REF] Bezin | The national healthcare system claims databases in France, SNIIRAM and EGB: Powerful tools for pharmacoepidemiology[END_REF]. This information is anonymized and can be reused for research purposes [START_REF] Schwarzinger | The neglected role of alcohol use disorders in earlyonset dementia: a nationwide study from France[END_REF].

Inclusion criteria

Dementia and related diseases encoding rules have been defined in 2006 [START_REF]Agence technique de l'information sur l'hospitalisation (ATIH)[END_REF]. In accordance with those rules, we included inpatient stays having one of the following ICD-10 codes as principal diagnosis: acute onset vascular dementia (F010*, 21 codes), multiple infarctions (F011*, 21 codes), subcortical vascular dementia (F012*, 21 codes), mixed vascular dementia cortical and subcortical (F013*, 21 codes), other vascular dementia (F018*, 21 codes), unspecified vascular dementia (F019*, 21 codes). In comparison, we noted the number of organic mental disorders, including symptomatic disorders: dementia of Alzheimer's disease, VaD, dementia during other diseases classified elsewhere, unspecified dementia, organic amnesia syndrome, non-induced delirium, other mental disorders, personality and organic behaviour disorders, organic mental disorder (codings from F00 to F09).

Results

Evolution of VaD

From 2007 to 2017, the number of inpatient stays with a principal diagnosis of organic mental disorders has followed the increase in the number of stays: 70,145 in 2007 (0,301% of inpatient stays) and 86,421 in 2017 (0,300%). Over the same period, the number of inpatient stays with a principal diagnosis of VaD remained stable: 10,204 in 2007 (0,0437% of inpatient stays) and 11,654 in 2017 (0,0404% of inpatient stays). Among inpatient stays for organic mental disorders, VaD accounted for 14,5% in 2007 and 13,5% in 2017 (Table 1).

Evolution of subtypes of vascular dementias

Inpatient stays for other VaD increased over 11 years (+85%), while inpatient stays for unspecified VaD remained stable (+3%) (Table 1). Among inpatient stays with specific codes in 11 years, there were fewer multiple infarctions (-50%) and a globally unchanged number of inpatient stays for mixed VaD (+9%). Number of inpatient stays for acute onset VaD increased between 2007 and 2013 (+228%) and then decreased between 2013 and 2017 (-13%). Number of inpatient stays for subcortical VaD followed the same pattern: + 77% until 2012 and then -11% until 2017 (Figure 1). In 2017, inpatient stays for mixed VaD represented 34.6% of inpatient stays for VaD, unspecified VaD 27.2%, subcortical VaD 13.6%, other VaD 11.9%, acute onset VaD 6.6% and multiple infarctions 6.1%.

Discussion

Between 2007 and 2017, we show a slightly relative decrease in the number of hospital stays for VaD compared to all hospital stays (0.0437% to 0.0404%). This result can be explained by a lower identification of vascular dementias, a decrease in hospital stays for vascular dementia or a better control of cardiovascular risk factors [START_REF] Birkhead | Improving care for patients with acute coronary syndromes: initial results from the National Audit of Myocardial Infarction Project (MINAP)[END_REF][START_REF] Arveiler | Trends in coronary heart disease in France during the second half of the 1990s[END_REF][START_REF] Setoguchi | Improvements in Long-Term Mortality After Myocardial Infarction and Increased Use of Cardiovascular Drugs After Discharge: A 10-Year Trend Analysis[END_REF][START_REF] Harmsen | Stroke Incidence and Mortality Rates 1987 to 2006 Related to Secular Trends of Cardiovascular Risk Factors in Gothenburg[END_REF][START_REF] Tuppin | Frequency of cardiovascular diseases and risk factors treated in France according to social deprivation and residence in an overseas territory[END_REF]. In our study, vascular dementias accounted for about 14% of hospital stays for organic disorders; in the literature, VaD is estimated to account for about 15% of dementias [START_REF] O'brien | Vascular dementia[END_REF]. About subtypes of VaD, we describe two trends: first, the increase in "other VaD" diagnoses; second, a shift from multiple infarct inpatient stays to acute onset VaD and subcortical VaD inpatient stays, particularly between 2007 and 2012. The respective proportions of different VaD subtypes remained stable between 2013 and 2017. Over 10 years, it is unlikely that diagnostic rates could have changed as much; rather, our results reflect changes in coding patterns between 2007 and 2012.

Dementia diagnosis is a clinical syndromic diagnosis, which requires fulfilling diagnostic criteria. Such criteria exist for Alzheimer's disease [START_REF] Mckhann | Clinical diagnosis of Alzheimer's disease: report of the NINCDS-ADRDA Work Group under the auspices of Department of Health and Human Services Task Force on Alzheimer's Disease[END_REF][START_REF] Dubois | Research criteria for the diagnosis of Alzheimer's disease: revising the NINCDS-ADRDA criteria[END_REF], frontotemporal dementia [START_REF] Neary | Frontotemporal dementia[END_REF] and dementia with Lewy bodies [START_REF] Mckeith | Consensus guidelines for the clinical and pathologic diagnosis of dementia with Lewy bodies (DLB): report of the consortium on DLB international workshop[END_REF][START_REF] Mckeith | Diagnosis and management of dementia with Lewy bodies: Fourth consensus report of the DLB Consortium[END_REF]. About VaD, it was initially thought that cerebrovascular disease caused dementia when there were many large cortical infarcts [START_REF] Hachinski | Multi-infarct dementia. A cause of mental deterioration in the elderly[END_REF][START_REF] Mckay | Multi-Infarct Dementia: A Historical Perspective[END_REF]. Fourth revision of Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) and ICD-10 used this concept of multiple infarctions to define VaD [START_REF] O'brien | Vascular dementia[END_REF][START_REF] Perneczky | Is the time ripe for new diagnostic criteria of cognitive impairment due to cerebrovascular disease? Consensus report of the International Congress on Vascular Dementia working group[END_REF]. In 1992 and 1993, Alzheimer's Disease Diagnostic and Treatment Centers (ADDTC) and then the NINDS-AIREN proposed criteria for the diagnosis of vascular dementia (VaD) [START_REF] Román | Vascular dementia: diagnostic criteria for research studies. Report of the NINDS-AIREN International Workshop[END_REF][START_REF] Chui | Criteria for the diagnosis of ischemic vascular dementia proposed by the State of California Alzheimer's Disease Diagnostic and Treatment Centers[END_REF]. These criteria were compared: because of their high specificity, the NINDS-AIREN criteria have been preferred in most relevant studies [START_REF] O'brien | Vascular dementia[END_REF][START_REF] Pohjasvaara | Comparison of different clinical criteria (DSM-III, ADDTC, ICD-10, NINDS-AIREN, DSM-IV) for the diagnosis of vascular dementia. National Institute of Neurological Disorders and Stroke-Association Internationale pour la Recherche et l'Enseignement en Neurosciences[END_REF]. They also expanded the idea to include not only multiple cortical and/or subcortical infarcts, but also strategic single infarcts, non-infarction white matter lesions, hemorrhages, and hypoperfusion as possible causes of VaD. Thus, in our study, analysis of the subtypes shows a trend in line with the literature and diagnostic criteria: VaD by multiple infarction accounted for 13.8% of vascular dementias in 2007 and only 6.1% in 2017. Also, the increase in "other VaD" suggests a lack of precision among ICD-10 proposals. We do not observe any change in the distribution of the various subtypes of VaD after 2014, which could be linked to the VASCOG statement [START_REF] Sachdev | Diagnostic criteria for vascular cognitive disorders: a VASCOG statement[END_REF]. Principal strength of our study is to study all hospital stays in France over 11 consecutive years, for a total of 285,748,938 stays. Our study has potential limitations. The inpatient stays ICD10 codes were registered for billing purposes, and we did not confirm them versus clinical records. The use of the PMSI for activity-based payment of hospital services can lead to under-coding diseases that have no financial impact. However, the coding as the main diagnosis of vascular dementia (regardless of the precision) or Alzheimer's dementia is associated with the same homogeneous group of stay, and with the same pricing for stays from 3 to 20 days. The pricing of the stay will increase by coding the associated diagnoses (undernutrition, decubitus ulcer, bedridden patient, etc.); thus, there is no financial inducement to use precisely one code of dementia rather than the other. Since 2012, government agency in charge of the PMSI database has been working on the Ovalide tool, a PMSI data validation tool, which will make it possible to identify low quality in PMSI data. Use of hospital coding to define the diagnosis is a recognized way of counting dementia [START_REF] Launer | Counting dementia: There is no one "best" way[END_REF]. We study here the number of hospital stays; some patients may have had several stays. Besides, we interested in the evolution of coding, and there is no reason to believe that the annual number of stays of patients with VaD may have changed between 2007 and 2017. Defining and identifying dementias in the major French administrative databases is an issue of interest: a recent review proposed several algorithms, two of which have been validated [START_REF] Gallini | Acute onset vascular dementia Multiple infarctions Subcortical vascular dementia Mixed vascular dementia (cortical and subcortical) Other vascular dementia Unspecified vascular dementia[END_REF].

Conclusion

The number of hospital stays for vascular dementia has remained stable over 11 years. This represents a slight relative decrease in proportion to the total number of hospital stays. One possible explanation is the improved control of cardiovascular risk factors. These data can be useful to encourage good control in patients at risk. We also showed a decrease in hospital stays for imprecise diagnoses of "multiple infarction", associated with an increase in the number of diagnoses of "subcortical or mixed VaD", "acute onset VaD" and "other VaD". Finally, these data suggest an adequacy with the evolution of diagnostic criteria. Improvement in the coding quality of VaD is engaging for future studies with the PMSI database. VaD: Vascular dementia

Figure 1 :

 1 Figure 1: Evolution of subtypes of vascular dementia among the principal diagnosis of inpatient stays in France between 2007 and 2017

Figure 1 :

 1 Figure 1: Evolution of subtypes of vascular dementia among the principal diagnosis of inpatient stays in France between 2007 and 2017

Table 1 :

 1 Number of inpatient stays for vascular dementia in France

Table 1 :

 1 Number of inpatient stays for vascular dementia in France

	Principal diagnosis	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017
	Acute onset VaD	272	342	546	760	827	802	892	785	789	731	773
	Multiple infarctions	1,410	1,367	1,226	1,104	1,035	932	791	712	767	734	709
	Subcortical VaD	1,006	1,155	1,304	1,597	1,635	1,776	1,723	1,701	1,565	1,548	1,584
	Mixed VaD
	(cortical and	3,686	4,219	4,090	3,788	3,801	3,719	4,033	3,773	3,882	4,118	4,026
	subcortical)
	Other VaD	749	831	953	1,044	1,099	1,057	1,053	1,094	1,237	1,305	1,387
	Unspecified VaD	3,081	3,031	2,655	2,419	2,540	2,853	2,749	2,863	2,853	3,073	3,175
	Total (F01*)	10,204	10,945	10,774	10,712	10,937	11,139	11,241	10,928	11,093	11,509	11,654
	Organic
	mental disorders	70,145	75,685	76,021	78,464	82,176	83,417	83,728	83,079	84,193	87,628	86,421
	(F00*-F09*)

Total inpatient stays 23

  ,339,533 23,781,314 24,297,929 24,709,773 25,259,627 25,828,027 26,476,016 27,087,492 27,600,941 28,538,316 28,829,970

Acknowledgments

MR thanked Dr. Laure Belmont for his insights into the coding.

Conflicts

The authors do not have any conflict of interest to declare.


C Furgeaud

F Brenet

J Nicolai
email: julien.nicolai@univ-poitiers.fr

Multi-scale study of T i 3 S iC 2 thin film growth mechanisms obtained by magnetron sputtering

Keywords: MAX phase, thin films, epitaxy, TEM, magnetron sputtering

In the present work we focus on the mechanisms involved in T i 3 S iC 2 MAX phase thin-film formation and on the impact of stoichiometry and thickness/element quantity correlation. T iAl x thin-films, with x = 0 -2, 100 -300 nm thick, were deposited by magnetron sputtering on a S iC -4H (0001) substrate. Samples were annealed at 1000 • C for 30 minutes and analysed by XRD, AFM, SEM and TEM. The MAX phase T i 3 S iC 2 was formed at 1000 • C in accordance with thermodynamic considerations. Moreover, an epitaxial relation between the film and the substrate occurred as follows (0001) MAX //(0001) S iC and [12 10] MAX //[12 10] S iC due to the formation of an earlier T iC phase by species interdiffusion. The catalytic role of Al has been demonstrated and explained by the formation of T i 2 AlC as precursor for T i 3 AlC 2 and for T i 3 S iC 2 . Finally, second phases and continuity of films have been controlled by x and thickness factors. Indeed, the MAX phase film continuity is obtained for Al richer films, whereas the purest MAX phase have been synthesized for the thinnest layer of T iAl x deposited.

Introduction

M n+1 AX n phases (n = 1 -3) consist in a large class of nanolaminated materials where M is an early transition metal element, A is an A-group element and X is either C or N [START_REF] Eklund | The Mn + 1AXnphases: Materials science and thinfilm processing[END_REF][START_REF] Barsoum | MAX Phases: Properties of Machinable Ternary Carbides and Nitrides[END_REF][START_REF] Barsoum | The MN+1AXN Phases: A New Class of Solids[END_REF][START_REF] Sun | Progress in research and development on MAX phases: a family of layered ternary compounds[END_REF][START_REF] Lapauw | Synthesis of the new MAX phase Zr2AlC[END_REF][START_REF] Lapauw | Synthesis of the novel Zr3AlC2 MAX phase[END_REF][START_REF] Gonzalez-Julian | Novel Cr2AlC MAX-phase/SiC fiber composites: Synthesis, processing and tribological response[END_REF]. For n = 1, 2, 3 the MAX phases are called 211, 312 and 413 respectively due to the periodic arrangement of their structure: M 6 X octahedrons separated by A element layers in a hexagonal structure. This particular arrangement gives to the MAX phases a unique combination of metal and ceramic properties, opening the way to a large field of applications [START_REF] Barsoum | Elastic and Mechanical Properties of the MAX Phases[END_REF][START_REF] Whittle | Radiation tolerance of Mn+1AXn phases, Ti3AlC2 and Ti3SiC2[END_REF][START_REF] Lapauw | Nanolaminated ternary carbide (MAX phase) materials for high temperature applications[END_REF].

The Three Miles Island (1979, USA) and Fukushima Daiichi (2011, Japan) nuclear powerplant accidents demonstrated the importance to improve nuclear reactor safety. Thus, the concept of Accident Tolerant Fuel (ATF) has been introduced in order to improve the properties (oxidation, irradiation and corrosion resistance) of the Zircalloy full cladding during a cooling failure. In that way, some MAX phases are good candidates, indeed the T i 3 S iC 2 and T i 2 AlC MAX phases have been extensively studied due to their excellent properties, including irradiation resistance [START_REF] Wang | Effects of He irradiation on Ti3AlC2: Damage evolution and behavior of He bubbles[END_REF][START_REF] Patel | Effect of helium irradiation on Ti3AlC2 at 500C[END_REF][START_REF] Yang | The structural transitions of Ti3AlC2induced by ion irradiation[END_REF][START_REF] Zhang | The damage evolution of He irradiation on Ti3SiC2as a function of annealing temperature[END_REF][START_REF] Shen | Effects of Xe+irradiation on Ti3SiC2at RT and 500C[END_REF], oxidation resistance [START_REF] Sun | High temperature oxidation behavior of Ti3SiC2-based material in air[END_REF] and very specific mechanical properties [START_REF] Higashi | Anisotropic plastic deformation of single crystals of the MAX phase compound Ti3SiC2 investigated by micropillar compression[END_REF]. The use of thin-film MAX-phase on SiC as cladding for nuclear fuel is one promising way. Moreover, T i 3 S iC 2 MAX phase thin-film on S iC can be used as ohmic contacts due to the very low contact resistance [START_REF] Drevin-Bazin | Epitaxial growth of Ti3SiC2 thin films with basal planes parallel or orthogonal to the surface on α-SiC[END_REF][START_REF] Drevin-Bazin | Investigation of Ti3SiC2 MAX Phase Formation onto N-Type 4H-SiC[END_REF][START_REF] Tsukimoto | Growth and Microstructure of Epitaxial Ti3SiC2 Contact Layers on SiC[END_REF]. Several different techniques have been used to synthesize bulk MAX phases. Among these techniques, the most widespread is hot isostatic pressing (HIP) [START_REF] Lapauw | Synthesis of the new MAX phase Zr2AlC[END_REF]; however, various techniques have been developed for the growth of MAX phase thin films using magnetron sputtering technology, either from elemental targets or from compound targets on various substrates [START_REF] Su | Deposition and characterization of Ti2AlC MAX phase and Ti3AlC thin films by magnetron sputtering[END_REF][START_REF] Grieseler | Nanostructured plasma etched, magnetron sputtered nanolaminar Cr 2AlC MAX phase thin films[END_REF][START_REF] Feng | Preparation of Ti2AlC MAX Phase Coating by DC Magnetron Sputtering Deposition and Vacuum Heat Treatment[END_REF][START_REF] Shu | One-step synthesis of polycrystalline V2AlC thin films on amorphous substrates by magnetron co-sputtering[END_REF]. As shown on our recent papers, the co-deposition of T i and Al on S iC -4H substrates leads, depending on annealing conditions, to the formation of either T i 3 S iC 2 or T i 2 AlC. Since mechanisms leading to each MAX phase remain a source of debate, it's difficult to predict product characteristics: MAX phase nature and microstructure [START_REF] Drevin-Bazin | Epitaxial growth of Ti3SiC2 thin films with basal planes parallel or orthogonal to the surface on α-SiC[END_REF][START_REF] Drevin-Bazin | Module de puissance à base SiC fonctionnant à haute température Aspect matériaux[END_REF][START_REF] Nicolaï | Formation mechanisms of Ti2AlC MAX phase on SiC-4H using magnetron sputtering and post-annealing[END_REF]. The use of Al seems to be helpful to the synthesis of T i 3 S iC 2 , although the mechanism is not well known [START_REF] Abi-Tannous | Parametric investigation of the formation of epitaxial Ti3SiC2 on 4H-SiC from Al-Ti annealing[END_REF][START_REF] Tsukimoto | Growth and Microstructure of Epitaxial Ti3SiC2 Contact Layers on SiC[END_REF]. In the present study, we focus on the mechanisms involved in MAX phase formation at a high tem-perature especially on the catalytic role of Al. We study the growth mechanisms of the T i 3 S iC 2 MAX phase depending on both, the stoichiometry and thickness of the fresh-deposited T iAl x layer. T i 3 S iC 2 formation was studied by XRD, AFM, SEM, STEM-HAADF, EDS and HRTEM in order to obtain relevant microstructure, chemistry and morphologies characterization, properties. The aluminium effect on the MAX phase thin film properties will be discussed by considering thermodynamic and kinetic effects.

Experimental Methods and Protocol

S iC -4H substrates, single crystal (0001), n-type, supplied by TANKEBLUE, were used in this study. Al and T i were co-deposited at room temperature onto S iC substrates by magnetron sputtering using pure Al (99.999%) and T i (99.995%) targets in a high vacuum system (residual pressure < 2.10 -7 Pa, Ar working pressure 0.3 Pa). Deposition parameters are summarized in table 1. Before deposition, the substrate was in situ cleaned by an etching at 60V during 600 s. Composition and thin film thickness have been checked using X-ray reflectometry (3000 Seifert) and SEM-EDS technique (JEOL 7001 TTLS) ( The samples were annealed in a vacuum lamp furnace (heating rate close to 20 • C.s -1 ). Homogeneity of the samples (composition and thin film thickness) has been checked using X-ray reflectometry and SEM techniques. The structural investigations were performed by using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Diffraction experiments were conducted on a D8 Brucker AXS diffractometer operating in the Bragg-Brentano geometry under atmosphere environment. Diffractometer operates with a Cu tube and the K β radiation is absorbed by a Ni filter in order to obtain a pure Cu K α radiation (λ = 0.15418 nm). The LynxEye detector is used for ω -2θ scans with a slight offset to avoid S iC reflections (ω -θ = 0.2 • ). Rocking curves have been achieved to investigate mosaicity of thin films with a classic scintillator detector. Conventional, High-Resolution TEM and Scanning Transmission Electron Microscopy (STEM) images were acquired using a JEOL 2200 FS (Schottky-FEG, 200 kV). The diffraction patterns and HRTEM micrographs were studied using JEMS software [START_REF] Stadelmann | EMS -a software package for electron diffraction analysis and HREM image simulation in materials science[END_REF]. Using an Energy Dispersive x-ray Spectroscopy (EDS) detector (Bruker Quantax X flash 6) and an analytic single tilt-holder, Al, T i and S i atomic composition were studied. Due to the carbon contamination induced by STEM, the carbon was detected but not quantified. TEM samples were prepared by Focus Ion Beam (FIB) using an FEI-HELIOS dual-beam using the lift-out classical method [START_REF] Langford | In situ lift-out using a FIB-SEM system[END_REF]. The surface morphology of the films was imaged after deposition and measured using a Nanoscope III Multimode Digital Instruments atomic force microscope (AFM) operating in tapping mode under air environment. All images were post-processed with WSxM software [START_REF] Horcas | WSXM: A software for scanning probe microscopy and a tool for nanotechnology[END_REF]. The surface roughness and lateral grain size were determined from Z-profile acquisitions. Surface analysis, topology and chemistry were achieved using SEM JEOL-7001F TTLS equipped with an EDS detector. SEM images were achieved at 5 keV for 100 nm sample thick and at 10 keV for 300 nm sample thick, while EDS analysis were acquired at 5keV considering L series of T i and K series of Al, S i and C elements in order to limit bulk contribution.

Results

Figure 1: ω -2θ XRD diffractograms of T i (100 nm), T iAl and T iAl 2 (both100 nm and 300 nm thick) onto 4H -S iC annealed at 1000 • C during 30 min. Black, blue and red dashed lines correspond to theoretical diffraction peak position for a pure T i 3 S iC 2 , T i 5 S i 3 and T iC powder respectively. Fig. 1 shows X-ray diffractograms of T iAl -T iAl 2 , 100 -300 nm thick and T i -100 nm thick on S iC annealed at 1000 • C for 30 min. Diffractogram of T i deposited on S iC presents an intense peak at 35.9 • attributed to (111) reflection of T iC structure. In order to obtain a more readable figure we show only the base of the peak allowing correct observations of other peaks present. Moreover, two weak peaks observed at 10.0 • and 40.9 • could be respectively attributed to (0002) and (0008) reflections of the T i 3 S iC 2 MAX phase. The four other diffractograms exhibit four peaks at 10.0 • , 20.1 • , 30.4 • and 40.9 • corresponding, respectively, to (0002), ( 0004),( 0006) and (0008) reflections of the T i 3 S iC 2 MAX phase structure. Thus, all samples annealed, whatever thickness or stoichiometry, lead to formation of MAX phases. However, secondary peaks could be observed and are attributed to secondary phases. For T iAl 100 nm, two very weak peaks are detected at 13.14 • and 40.08 • . These peaks could correspond to a slightly strained T i 2 AlC structure. Considering T iAl 300 nm the secondary phase is T iC with the (111) reflection. In the case of T iAl 2 , 300 nm, two reflections of T i 5 S i 3 are detected at 42.16 • and 42.69 • attributed to (300) and (112) planes respectively. Moreover the (111) reflection of T iS i 2 -C54 structure [START_REF] Inui | Defect and electronic structures in TiSi2 thin films produced by co-sputtering. Part 1: Defect analysis by transmission electron microscopy[END_REF] is also observed at 27.74 • . Finally, we can identify a broader peak for (0008) T i 3 S iC 2 reflection often explained by Al substitutions or interstitials. Moreover, the rocking curves achieved on the (0002) T i 3 S iC 2 peaks for all samples show a full width at half-maximum (FWHM) of 0.3 • indicating a weak misorientation of the films as shown Fig. 2 for the example case of T iAl 2 -300 nm. Fig. 3 shows the SEM micrographs of the top-viewed microstructure of samples after annealing at 1000 • C during 30 minutes. EDS analyses reveal nude S iC areas without films on 100 nm samples. Statistical study reveals a MAX phase covering rate close to 75% and 60% in the case of T iAl 100 nm and 300 nm respectively, 95% and 100% in the case of T iAl 2 100 nm and 300 nm respectively. Moreover, for the T iAl 2 samples SEM micrographs reveal grain boundary contrasts attesting to a polycrystalline film. Grains constituting films are faceted exhibiting hexagonal shape. More complex topography is observed on T iAl sample without clear grain boundary contrasts. Discontinued films present many terraces on the surface but seem monocrystalline. Global and local EDS analyses have been achieved on these four samples and summarized in Table 2. As shown in this table the initial deposited ratio T i/Al is not conserved. Indeed, the global T i/Al ratio is much higher than ten, indicating a loss of aluminium during annealing. Local EDS analyses have been done on many areas in order to accumulate statistical data. We only show here average results per sample respectively named from A to K. As shown on Fig. 3 andTable.2 the points A and G only probe S iC substrate. All the others points show the presence of Al, T i, S i, and C elements.

As shown on Fig. 4 the T iAl samples exhibit some aggregates which are not present on the T iAl 2 samples. For the four specimens the RMS is close to 2 nm for clean surfaces and close to 12 nm for areas containing aggregates. The average height of terraces is close to 80 nm, 170 nm, 70 nm and 70 nm respectively measured on Fig. 4a, 4b, 4c and 4d with a WsXM software plugin [START_REF] Horcas | WSXM: A software for scanning probe microscopy and a tool for nanotechnology[END_REF]. The highest point is respectively at 220 nm, 370 nm, 170 nm and 450 nm. The average thickness of the thin film is close to 130 nm, 220 nm, 50 nm and 100 nm for the samples A, B, C and D respectively. The film is very flat (Fig. 6) and exhibits a very good crystallinity, however some areas without thin film on S iC -4H are also observed on samples A and C as expected from SEM images. EDS analyses Fig. 7, done on samples, show that the composition of the film is close to 75%T i for 25%S i which corresponds to T i 3 S iC 2 . Moreover, the Al presence at the interface between S iC and the T i 3 S iC 2 is observed for the samples B, C and D. The aluminium profile is spread over ten nanometers with a maximum value close to the S iC corresponding to 60%, 5% and 5% respectively for the samples B, C and D.

Al(%) T i(%) S i(%) C(%) O(%) T i/S i ratio S i/C ratio

Discussion

Aluminium effect on T i 3 S iC 2 thin film formation

The results obtained on the T i -100 nm sample show that it is very difficult to form T i 3 S iC 2 using only T i thin film even though all the elements are present. Moreover, the very intense peak of (111) TiC indicates that the T i can easily interact with the S iC to form T iC by the following reaction:

S iC + T i → T iC + S i (1)
as indicated by the negative free Gibbs energy ∆G 0 = -107 kJ [START_REF] Gotman | Interaction between SiC and Ti powder[END_REF]. It seems that the formation of T iC structure in a silicon-rich environment is not enough to obtain T i 3 S iC 2 structure. As shown, the addition of Al strongly increases the amount of T i 3 S iC 2 obtained with the same annealing conditions. We assume that the role of aluminium in this mechanism is to obtain the 211 T i 2 AlC MAX phase by the following reaction:

2T iC + Al → T i 2 AlC + C (2)
Which is in a good agreement with DFT simulations [START_REF] Ward | Crystallographic evolution of MAX phases in proton irradiating environments[END_REF]. Since the T i n+1 S iC n system does not have the 211 phase, the silicon cannot be incorporated in the T i 2 AlC MAX phase [START_REF] Keast | Prediction of the stability of the Mn+1AXn phases from first principles[END_REF]. We assume that the silicon will segregate above the T i 2 AlC. In a second time, the T i 2 AlC structure turns into a T i 3 AlC 2 structure. This reaction can be considered as a simple re-arrangement of the T i 2 AlC structure by releasing aluminium. At high temperature, it is well known that the 312 MAX phase is more stable than the equivalent 211 MAX phase [START_REF] Eklund | The Mn + 1AXnphases: Materials science and thinfilm processing[END_REF]. Thus, the next step of the reaction is:

2T i 2 AlC → T i 3 AlC 2 + (T i, Al) (3)
Once the T i 3 AlC 2 phase has been created, silicon above the T i 3 AlC 2 thin film diffuses through it, leading to substitution between Al and S i to form T i 3 S iC 2 MAX phase. The exchange of the A element of MAX phase has already been observed to form T i 3 AuC 2 starting from T i 3 S iC 2 [START_REF] Fashandi | Synthesis of Ti3AuC2, Ti3Au2C2 and Ti3IrC2 by noble metal substitution reaction in Ti3SiC2 for hightemperature-stable Ohmic contacts to SiC[END_REF]. By this way we assume that, in our conditions, the T i 3 S iC 2 structure is more stable than the T i 3 AlC 2 structure.

T i 3 AlC 2 + S i → T i 3 S iC 2 + Al (4)
The formation of the T i 3 S iC 2 structure is therefore highly enhanced by the Al presence due to the formation of 211 and 312 T i n+1 AlC n MAX phases as summarized Fig. 8.

Effect of aluminium stoichiometry on thin film

growth mode As shown by SEM and AFM on Fig. 3 and Fig. 4 the T iAl x stoichiometry has an important effect on the thin film growth mode. Indeed, for a defined thickness, the x-value of T iAl x tailors the surface morphology. In fact, for x = 2 the MAX phase layer appears continuous with terraces while for x = 1 some surfaces stay uncovered by the MAX phase. This observation might mean low Al concentration leading to higher surface diffusivity of both elements. Moreover, the uncovered areas have a different chemical composition depending on the thickness. Indeed, for the 100 nm sample the uncovered areas consist in S iC while for the 300 nm sample, they consist in T iC according to EDS-SEM and TEM analyses (Table 2 and Fig. 6). A brief observation of the T i-Al phase diagram reveals that stoichiometry conditions to obtain T iAl or T iAl 2 are a little bit different. Indeed, tolerance of Al concentration to obtain T iAl 2 is less drastic than for T iAl formation. In other words, in the case of T i depletion for the T iAl phase, the structure will be easily accommodated due to the large stoichiometry range of T iAl stability in its diagram. However, the T iAl 2 phase is stable on a fine stoichiometry range of the diagram, thus depletion of T i or Al could lead to phase changing subsequently leading to more complex diffusion mechanisms for Al and T i. Thus, diffusion mechanisms of both elements and subsequent film growth morphologies depend on Al stoichiometry.

Effect of titanium quantity on MAX phase thickness

The main difference between T iAl and T iAl 2 consists in the formation of secondary phases containing silicon or not. Moreover, the thickness of T i 3 S iC 2 thin film grown is higher for T iAl than T iAl 2 . In our magnetron sputtering chamber the amorphous deposited thin film has a density close to the theoretical one of the equivalent bulk crystal (ρ T iAl 2 = 3.53 g.cm -3 and ρ T iAl = 4 g.cm -3 ). As such, for a similar thickness, T iAl is two times titanium-rich than T iAl 2 . For a similar T iAl x fresh-deposited film thickness, the synthesized MAX phase is thicker for T iAl than T iAl 2 . Considering AFM/SEM results and TEM observations we assume that the total quantity of MAX phase produced is higher for T iAl than T iAl 2 . As shown by TEM experiments the MAX phase layer is two times thicker for T iAl than for T iAl 2 (Fig. 7). Moreover, statistical studies achieved using SEM experiments showed that the MAX phase covering rate is higher than 50% for T iAl.

Effect of T iAl x thickness

T iAl stoichiometry leads to a very pure T i 3 S iC 2 MAX phase for the 100 nm sample and to T i 3 S iC 2 plus T iC for the 300 nm sample. TEM observations, corresponding to a D region of Fig. 3, have shown that the T iC has grown on S iC surface (Fig. 6), which is in a good agreement with our assumptions about the first steps of MAX phase formation. Moreover, the presence of this phase could be explained by the lack of Al, not allowing the formation of T i 2 AlC and then the T i 3 S iC 2 phase.

T iAl 2 stoichiometry leads to the formation of a very pure and flat T i 3 S iC 2 MAX phase with a high coverage rate. For the 300 nm sample we also observe secondary phases containing titanium and silicon like T iS i 2 -C54 or T i 5 S i 3 above the MAX phase corresponding to K region on the Fig. 3.

The impact of thickness reagents has been already studied for a T i -S i -C system [START_REF] Yang | Thickness-dependent phase evolution and bonding strength of SiC ceramics joints with active Ti interlayer[END_REF]. In this case the Al lack prevents the fast formation of MAX phase below 1 µm of T i. However, the increase of the thickness seems to help the increasing of T i 3 S iC 2 formation volume as observed on our systems.

Al presence after annealing

Surprisingly, only few traces of Al are detected on all samples by global and local SEM-EDS and by TEM/STEM -EDS techniques. Aluminium disappears from the bulk and the surface of thin films, indeed, it's not detected anymore either in MAX phases or in secondary products such as T i 5 S i 3 and T iS i 2 -C54. However, a poor quantity of aluminium was detected at the interface for three different samples: T iAl-300 nm, T iAl 2 -100 and 300 nm. The aluminium and silicon diffusion mechanisms toward a MAX phase T i 3 S iC 2 is the last step of our model. We assume that this process occurs from the surface to the interface. Thus, if the diffusion length is too long and the annealing time too short, it's not possible for all the aluminum sites to be substituted by silicon atoms. In this case, a very low quantity of Al is still trapped at the interface, not allowing formation of T i 3 S iC 2 but rather the conservation of T i 3 AlC 2 . The conservation of this one could be considered as Al substitution and interstitial defects in the T i 3 S iC 2 MAX phase. Defects presence has been already suspected with XRD analysis with the broadening of the diffraction peak corresponding to (0008) plane of T i 3 S iC 2 for the sample T iAl 2 -300 nm (Fig. 1).

In the case of T iAl -100 nm we can admit that the thickness of the MAX phase is thin enough for silicon and aluminum co-diffusion, moreover, many lateral free surfaces exist (as shown on SEM image Fig. 3) creating more diffusion pathways. For the other three cases the MAX phase is thick enough to trap Al at the interface (h f = 200 nm for T iAl x -300 nm) or not possess free lateral surfaces due to the continuity of the film (T iAl 2 -100 nm).

Moreover, we noticed at each annealing that the crucible containing the sample was covered with aluminium, indicating that the aluminium was coming out of the sample.

Conclusions

In the present work a two-step method to obtain epitaxial T i 3 S iC 2 thin film onto S iC -4H using magnetron sputtering deposition of T iAl x was studied for x = 0, 1 and 2. It was demonstrated that the addition of Al strongly increases the amount of T i 3 S iC 2 obtained by the formation of T i 2 AlC as a precursor for T i 3 S iC 2 . Moreover, increase the amount of aluminum modifies the growth of T i 3 S iC 2 thin film and subsequent surface morphology from a discontinuous to a continuous layer. For x = 1 discontinuous films are monocrystalline and a lot of terraces emerge at the surface while x = 2 stoichiometry leads to continuous and polycrystalline films. Finally, the formation of secondary phases was observed for the 300 nm samples, especially T iC for x = 1 and T iS i 2 -C54 plus T i 5 S i 3 for x = 2.

It seems that the stoichiometry is not the only relevant parameter: thickness seems to play a major role in MAX phase formation. The interdependence between stoichiometry and thickness on the T i and Al quantity makes it possible to tailor the morphology, thickness and microstructure of the final MAX phase films. Moreover, the adjustment of these two interdependent parameters will be necessary in order to avoid secondary phase formation and obtain a pure and high quality MAX phase.

Figure 2 :

 2 Figure 2: ω -scan at 2θ = 10.0 • corresponding to the (0002) T i 3 S iC 2 peak. The full width at half maximum (FWHM) is 0.3 • for the four samples.

Figure 3 :

 3 Figure 3: SEM micrographs of a) T iAl-100 nm, b) 300 nm, c) T iAl 2 -100 nm and d) 300 nm annealed 1000 • C -1x30min. EDS analysed points are marked by letters from A to K.

Figure 4 :

 4 Figure 4: Surface morphology images of T iAl a) 100 nm, b) 300 nm and T iAl 2 c) 100 nm, d) 300 nm annealed at 1000 • C -1x30min.

Fig. 5

 5 Fig.5 shows the thin films obtained on S iC -4H. The diffraction patterns obtained and the analysis of HRTEM micrographs show that the thin films correspond to 312 MAX phase structure. Moreover the MAX phase grows in epitaxy with S iC-4H following the relation (0001) MAX //(0001) S iC and [12 10] MAX //[12 10] S iC . The average thickness of the thin film is close to 130 nm, 220 nm, 50 nm and 100 nm for the samples A, B, C and

Figure 5 :

 5 Figure 5: HRTEM micrograph of T iAl -100 nm annealed 1000 • C -1x30min a) and associated diffraction pattern b). The diffraction pattern can be indexed using S iC -4H and T i 3 S iC 2 structures on [11 20] zone axis.

Figure 6 :

 6 Figure 6: a) STEM micrograph of T iAl 2 -300 nm annealed 1000 • C -1x30min. b) BF-TEM micrograph of T iAl -300 nm annealed 1000 • C -1x30min.

Figure 7 :

 7 Figure 7: EDS line scan along growth direction for TiAl a) 100 nm, b) 300 nm and T iAl 2 c) 100 nm, d) 300 nm annealed at 1000 • C -1x30 min.

Figure 8 :

 8 Figure 8: Schematic representation of T i 3 S iC 2 MAX phase thin-film formation.

Table

Table 1 : Growth parameters used during magnetron sputtering depo- sition of all samples.

 1 ).

	Phases T i	Al	Working	Thickness
		power	power	pressure	(nm)
		(W DC)	(W DC)	(Pa)
	T i	144	-	0.3	100
	T iAl	300	158	0.3	100 / 300
	T iAl 2 144	166	0.3	100 / 300

Table 2 :

 2 Local EDS results, bold letters correspond to the average composition of the entire sample and regular letters correspond to precise composition at the point letter.

Aknowledgements

The authors would like to acknowledge P. Guerin for his technical support concerning magnetron sputtering and H. Bahsoun for the preparation of the TEM samples. The authors would like to specially thank E. Chasson for his help. This work partially pertains to the French Government program "Investissements d'Avenir" (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01). This work has been partially supported by " Nouvelle Aquitaine " Region and by European Structural and Investment Funds (ERDF reference: P-2016-BAFE-94/95).