Dr Sizhe Li

Prof. Dr Justyna Jaszczyk

Prof. Dr Xavier Pannecoucke

Prof. Dr Thomas Poisson

Olivier A J Martin

Cyril Nicolas
email: cyril.nicolas@univ-orleans.fr

Olivier R Martin

O R Martin

Stereospecific Synthesis of Glycoside Mimics Through Migita-Kosugi-Stille Cross-Coupling Reactions of Chemically and Configurationally Stable 1-C-Tributylstannyl Iminosugars

Keywords: 1-Metalated Iminosugars, Stannanes, C-C coupling, Stereospecific Reactions, Catalysis

come

Stereospecific synthesis of glycoside mimics through

Migita-Kosugi-Stille cross-coupling reactions of chemically and configurationally stable 1-C-tributylstannyl iminosugars

Introduction

Glycomimetics are carbohydrate analogs designed to interfere with biochemical pathways wherein carbohydrates play key roles and are associated with pathological disorders. [1] As potential drug candidates, analogues of glycosides and glycoconjugates are gaining increased interest because of their ability to modulate the activity of carbohydrate-processing enzymes, or enhance their affinity for carbohydraterecognizing proteins. One class of glycomimetics that is attracting currently a surge of interest is that of iminosugars (e.g., 1 and 2, Fig. 1). These compounds exhibit remarkable activities as glycosidases and glycosyltransferases inhibitors, which bestow them high therapeutic value. [2] Thus, iminosugar derivatives have emerged as new drugs for example to control glucose uptake in non-insulin-dependent diabetes patients (Miglitol ® , N-[2-hydroxy-ethyl]-1deoxynojirimycin 1a), [START_REF] Scott | [END_REF] to salvage mutant enzymes in lysosomal diseases such as Fabry disease (Galafold ® , 1-deoxygalactonojirimycin, [4] 1b) or to treat type I Gaucher and Niemann-Pick diseases by inhibition of GlcCer synthase (Zavesca ® , N-butyl-1deoxynojirimycin 1c (Fig. 1)). [5][6] However, these saccharide surrogates lack the structural and stereochemical substitution patterns that are commonly embedded within the aglycone of glycosides and, as a result, lack selectivity in their biological activities.

In that respect, 1-C-substituted iminosugars (2), which carry a C-linked substituent at the pseudo anomeric position in a well-defined configuration, i.e.

imino-C-glycosides, constitute highly relevant mimetics of glycosides or glycoconjugates, and enhance the biological and therapeutic potential of iminosugars. [2b],[2e], [7] For example, certain imino-Cnucleosides have been identified as lead anticancer, antiviral and antibacterial compounds but also for the treatment of metabolic and genetic diseases. [8] Mundesine ® (Immucillin-H) 2a, for example was approved in Japan for the treatment of relapsed/refractory peripheral T-cell lymphoma, [9] and BCX4430 (Immucillin-A) 2b has been demonstrated to protect against both Ebola and Marburg viruses in rodents and monkeys, even when administered up to 48 h after infection (Fig. 1). [10] Figure 1. Chemical structures of iminosugar derivatives and imino-C-glycosides.

The synthesis of imino-C-glycosides represents a significant challenge and various elegant stereoselective methods have been developed for the de novo preparation of 1-C-substituted iminosugars, [START_REF] Marichev | For a non-exhaustive list of valuable synthetic routes see[END_REF] or more frequently, from sugar substrates. [START_REF] Maram | For a set of original methods see[END_REF] Most of the synthetic methods from sugars are based on the addition of carbon nucleophiles to glycosylamines (3) (path a), [13] cyclic imines (4), or cyclic nitrones (5) (path b), [START_REF] Malik | Carbohydrate Chemistry. In Recent advances in the synthesis of imino sugars. An insight into the cascade addition of Grignard reagents to halonitriles/cyclization[END_REF] lactones 6, and lactames 7 (path c) (Scheme 1). [2b],[2e], [7], [15] These tactics often require long reaction sequences and give low overall yields. In addition, the stereochemistry is governed by the inherent chirality of the saccharide core, thus giving in most cases moderate diastereoselectivities.

As a result, there are only two methodologies reported so far whereby it is possible to generate either (1S)-or (1R)-imino-C-glycosides (i.e., mimics of and -glycosides) with high specificity in a tunable, and predictable manner. [11b],[12g],[16] Furthermore, there is no report describing the more straightforward disconnection that would consist in functionalizing 1deoxy iminosugars (1) at the C-1 position through a direct C(sp 3 )-H bond activation or by a metalcatalyzed reaction using a stable 1-metallated iminosugar derivative and a carbon electrophile (path d).

Indeed, prior studies on the direct functionalization of saturated cyclic amines and on the preparation of 1metallated iminoalditol derivatives 8 (e.g., M = Li, Zn) have highlighted the critical problems associated with these routes (e.g. formation of iminoglycals of type 9 through elimination of the alkoxy group at C-2 (Scheme 1), poor functional group tolerance, and difficulties in controlling the α-stereoselectivity etc.). [17], [18] Scheme 1. Synthetic routes to iminosugar C-glycosides.

To be successful, the envisioned strategy would largely depend on the choice of the metal to be installed at the C-1 position. This problem was recently addressed by Walczak and co-workers in the sugar series, by capitalizing on stereospecific crosscoupling reactions of anomeric stannanes with aromatic iodides, diaryliodonium triflates and acyl donors in the presence of a palladium catalyst. [19] However, these methods are based on the use of activated glycosyl donors as precursors and are not applicable to the synthesis of iminoglycosyl stannanes.

In our work, we have devised an efficient and stereoselective approach to iminoglycosyl stannanes whereby path d to imino-C-glycosides (Scheme 1) becomes feasible. In the course of our investigations on the reactivity of N-tert-butanesulfinyl glycosylamines (10), which are useful precursors of imino-C-glycosides by an addition-cyclisation strategy, [13] we wondered whether it would be possible to generate 1-stannylated iminoalditol derivatives by this methodology and use the resulting organometallic intermediates for the synthesis of diverse imino-Cglycosides. We describe herein that it is indeed the case. We report in this article a new method of synthesis of imino-C-glycosyl compounds by way of the highly stereoselective and tunable addition of Bu3SnLi onto (SR)-and (SS)-N-t-butanesulfinyl glycosylamines, followed by cyclisation to iminoglycosyl tributylstannanes and their stereospecific cross-coupling by Migita-Kosugi-Stille catalysis with a range of aroyl and alkanoyl chlorides, thus giving access in a fully stereocontrolled process to iminosugar-C-glycosides of either 'alpha' or 'beta' configuration.

Results and Discussion

Synthesis of Glycosylamines. At the outset of the study, a panel of N-tert-butanesulfinyl glycosylamines of interest was selected (10a-e, Scheme 2) with both furanose and pyranose structures and (R)-or (S)-Ellman's chiral auxiliaries (e.g., (SS)-10 or (SR)-10). [START_REF]Sulfinylglycosylamines 10a-e are precursors of hydrolytically stable iminosugar glycomimetics of type 25a-g with D-Galf-like, L-xylof, L-lyxof, D-Gulop-like and D-Glcp-like configuration. Compounds 25a-g are thus promising chemotherapeutics for the treatment of tuberculosis, cancers, viral infections, diabetes and cystic fibrosis[END_REF] Starting from known tri-O-benzyl pentoses (11a-e), the formation of the N-sulfinyl glycosylamines of type 10 was executed under conditions improved from the ones we previously described. [12g], [21] Scheme 2. Preparation of sulfinylglycosylamines 10a-e.

Using Ti(OEt)4 (1.5 equiv.) as the promoter and commercial (R)-(+)-and (S)-(-)-2-methyl-2propanesulfinamide ((R)-and (S)-12, 2 equiv.) in dry toluene, glycosylamines (SR)-and (SS)-10a, 10b, 10d and 10e were isolated in excellent yields (70-96%) by heating for 1.5-2.5 h at 110 °C under µW irradiation (general procedure A-1, G.P. A-1). Interestingly, for the pyranosylamine motif ((SR)-10e), a noticeable yield increase was observed (74% vs. 56%) and the reaction time was significantly shortened (2 h vs. 48 h) through the µW activation mode. The acid sensitive sugar hemiaminals (SR)-and (SS)-10c were in turn obtained in good yields (85% and 90% for (SS)-and (SR)-10c, resp.) by refluxing a mixture of ribofuranose 11c, sulfinamide (R)-or (S)-12 (2 equiv.) and Cs2CO3 (1.5 equiv.) in dichloroethane for 36-48 h (G.P. A-2).

Addition Reactions. Tributylstannyllithium was subsequently added to the glycosylamines 10a-e at -78 °C. We noted by TLC analysis that some of the lithiated N-sulfinylated intermediates (see below in Scheme 3, compounds of type 13) may require increasing the temperature for a complete reaction. We have already observed similar results in the addition of simple organolithium and magnesium reagents to a variety of glycosylamines of type 10. [12g], [21] The reactions were thus followed by TLC analysis and allowed to reach a given temperature over 1.5-6 h (general procedure B, G.P. B).

It should be noted that Bu3SnLi is a very reactive reagent and its addition to aldehydes, ketones, [22] and sulfinylimines, [23] is very rapid even at -78 °C. [START_REF] Linderman | Tri-n-butylstannyllithium[END_REF] As a result, reactions must be typically quenched at low temperatures to achieve high yields, because the tributylstannanes decompose above a certain temperature under the conditions of the reaction.

It was not the case. The corresponding stannylated aminoalditols 14a-e were obtained in good yields (71-89%, Table 1, entries 1-7, 9), with a single notable exception: for the pyranoid derivative (SS)-10e no conversion whatsoever was detected and the starting material was recovered unchanged (entry 8). Remarkably, all the other N-sulfinylglycosylamines gave upon addition a single diastereomer (dr > 98:2) as indicated by NMR analysis of the crude mixtures. Furthermore, no matched or mismatched situations could be observed, highlighting the fact that stereochemistry of the reaction appears to be solely controlled by the chiral sulfinyl auxiliary.

Most interestingly, the configuration of the newly formed stereogenic center is R when the SR chiral group is used (e.g., (SR)-10 gives (1R)-(SR)-14), whereas diastereofacial selectivity is inverted when the SS chiral group is employed (e.g., (1S)-(SS)-14 is formed from (SS)-10).

These results are in opposition with those of Chong et al., [23b] who reported the addition of Bu3SnLi to simple tert-butanesulfinylimines, the only literature precedent existing on this kind of reaction: the stereochemistry of their addition was consistent with a chelation-controlled transition state. Our results however appear to involve an open, non-chelation model such as that suggested by Plobeck et al., for the addition of conventional lithium reagents to tertbutanesulfinylimines in THF. [START_REF] Plobeck | [END_REF] In this model, it is likely that, in the lithiated species produced by excess tributylstannyllithium used to generate the reactive imine by ring opening, the lithium ion coordinates to the liberated alkoxide ( 15). The tin-lithium reagent then coordinates to the lone pair of the sulfoxide moiety and the addition proceeds through an acyclic transition state of type A, following the Felkin-Ahn rules (Scheme 3). [26] Table 1. Addition of Bu3SnLi to glycosylamines 10a-e. a) Entry Substrate Product 14 ratio (1R:1S) b) The high stereoselectivity of the addition is quite remarkable and it is important to note that the potential chelating effect of the sugar functions does not divert the stereochemical mechanism away from the sulfinyl group, thus providing high stereocontrol in a tunable mode. The lack of reactivity of a single glycosylamine, namely pyranosylamine (SS)-10e is surprising and may be rationalized. In our DFT calculations on the addition of [M]CF2P(O)(OEt)2 (M = Li, MgX) to compounds of type 10, we assumed that internal complexation of the lithium ion by either the openchain imine transient entity ( 15) and/or the various ether protecting groups would be entropically favored over solvent complexation. [12g] In the (SS)-Dxylopyranosylamine series, these interactions and possibly stereoelectronic factors might prevent opening of the metallated N-tert-butanesulfinyl-Nglycoside intermediate 13 (Scheme 3) to a reactive imine and therefore shut down the addition reaction.

14 yield (%) c) 1 (SR)-10a (1R)-(SR)-14a > 98:2 89 2 (SS)-10a (1S)-(SS)-14a > 2:98 81 3 (SR)-10b (1R)-(SR)-14b > 98:2 85 4 (SS)-10b (1S)-(SS)-14b > 2:98 87 5 (SR)-10c (1R)-(SR)-14c > 98:2 84 6 (SS)-10c (
Cyclization Reactions. As illustrated in Scheme 4, it was then possible to mesylate compound (1R)-(SR)-14a (step a) through general procedure D-1 (G.P. D-1). As a bonus, the corresponding mesylated stannylsulfinamide (1R)-(SR)-16a proved to be quite stable, surviving chromatography on silica gel and it was isolated in excellent yield (90%).

Our first attempts at cyclization using bulky bases such as t-BuOK or Et3N in THF failed. Alternatively cleavage of the sulfinyl group (HCl in MeOH) and subsequent ring closure promoted by a basic resin was also unsuccessful. [12g], [13], [27] The problem could be solved using an excess of sodium hydride (NaH, 3 equiv.) in THF at room temperature. From D-xylo (1R)-(SR)-16a, the L-arabino (1R)-(SR)-17a was obtained in good yield (90%, step b, G.P E-1), resulting from an inversion of the configuration at C-4 upon ring closure. Oxidation of the labile sulfinyl group into a robust sulfonylamide was next carried out and compound (1R)-18a was obtained in almost quantitative yield (98%, step c) through general procedure C-2 (G.P. C-2).

One of the advantages of the tert-butanesulfinyl auxiliary is that it can be easily removed by treatment with an acid to provide free amines. [28] Unlike Chong's investigations on α-sulfinylaminoorganostannanes, [23b] we were able to remove the sulfinyl group in our sugar scaffold under acidic conditions (HCl, MeOH) without any noticeable degradation. Subsequent neutralization using a basic ion-exchange resin furnished (1R)-19a in good yield (77%, step d).

Compound (1R)-19a was thereafter reacted with Boc2O in presence of DMAP in CH3CN (step e) to provide the N-Boc derivative (1R)-20a (88%). Importantly, the formation of (1R)-20a could also be effectively promoted from (1R)-(SR)-17a, carrying step d and e in a single pot, albeit with a slight decrease of the overall efficacy (55% vs. 68% overall). The N-Cbz protected iminosugar (1R)-21a was prepared similarly (see step f) and it was observed to exist as a 6:4 mixture of rotamers (77%).

the tert-butanesulfonyl (Bus) group was previously shown to be a useful protecting group, which could be used in Stille-type coupling reaction partners without notable degradation. [29] In addition, it can be removed under acidic conditions, [30] as shown in aminoacid chemistry, [31] and can be prepared from the tertbutanesulfinyl group via simple m-CPBA oxidation. [28], [30] It was thus selected as key protecting group for the rest of the study. As described in Scheme 5, oxidation of aminoalditol derivatives 14a-e was thus performed with m-CPBA (G.P. C) to give sulfonamides 22a-c (in D-xylo, D-arabino and D-ribo series and with (1R)-and (1S)-configurations) in good to excellent yields (78-98%). In the case of the (1R)-L-lyxo and D-xylo derivatives, both featuring a primary hydroxyl group, compound (1R)-22d was obtained in good yield (87%), and (1R)-22e was formed in a non-optimized 57% yield. As expected, no erosion of the stereochemistry at C-1 could be detected. In the D-xylo, D-arabino and L-lyxo series, the same mesylation procedure as the one reported in Scheme 4 was then utilized to form compounds 23a-b, (1R)-23d and (1R)-23e (G.P. D-1)).

Surprisingly, for the D-ribo structure, the tertbutyldiphenylsilyl-protected pyrrole derivative 24 was isolated as a major by-product (see the Supporting Information (SI)). However, mesylation could be readily accomplished through G.P. D-2 (DMAP (cat.), excess MsCl) in pyridine to afford (1R)-23c and (1S)-23c in 97% and 88% yield respectively. Cyclisation of mesylates 23a-23e could then be efficiently performed under the conditions of G.P.E, using NaH at room temperature or at 70 °C, and in some cases under µW irradiation (Scheme 5). Alternatively, cyclisation could also be performed under Mitsunobu conditions (e.g. (1R)-22b gave (1R)-18b in moderate yield (66%).

Gratifyingly, we found that cyclic, but also linear iminosugar stannanes 18a-e and 22a-e are chemically and configurationally stable. They can be purified by chromatography on silica gel, are stable against air and moisture, and retain anomeric configuration when exposed to light or when heated at 110 °C for several days. Furthermore, they can be stored at 0 °C under argon atmosphere for at least two years without erosion of the chirality at C-1.

Scheme 5. Synthesis of stereochemically-defined pseudo anomeric iminosugar stannanes.

Configuration Determination and J(C-Sn) as diagnostics.

As summarized below in Fig. 2, the pseudo anomeric configuration of pyrrolidinyl iminosugar 1-stannanes was established based on the analysis of 2D nOesy spectra. The 1R configuration of L-arabinitols 17-21a could be ascribed thanks to distinct nOe contacts between protons H-1 and H-4 in compounds (1R)-18a and (1R)-19a. The same relations were observed for L-arabinitol (1R)-18c. Regarding the L-xylf series, the 1S configuration of 18b could in turn, be attributed owing to the main nOe contacts between protons H-1 and H-5.

In the piperidine series, the 1R configuration and 4 C1 conformational arrangement of imino-L-lyxitol (1R)-18d and imino-D-xylitol (1R)-18e were unambiguously settled according to the value of the vicinal H,H-coupling constants between ring protons (Fig. 3). Noteworthy, both compounds exist in the conformation in which the stannyl group is axial, in spite of its bulkiness. This is consistent with the conformational properties of N-sulfonylated piperidines carrying a substituent on the alpha-carbon. For steric reasons (A1,3type strain), the substituent adopts preferentially an axial position. [32] The 13 C NMR data of the 1-stannylated iminosugars (see SI and Fig. 2 and Fig. 3), were then analyzed. Interestingly, they revealed a trend in the heteronuclear coupling constants between the 13 C-1 nucleus in the butyl chain and the anomeric 117 Sn/ 119 Sn nuclei.

As a rule, for the N-sulfonylated and N-acylated 1-C-stannylated iminoalditols with a pyrrolidine structure, in which the tin group is in a cis relationship with the group at C-2, the 1 J( 117 Sn-13 C) coupling constants are above 320 Hz (335 Hz for 1 J( 119 Sn-13 C)). In contrast, in the C-1 stannanes in which the tin group is in a 1,2-trans orientation, these coupling constants are below these cutoffs. This trend however was not observed for pyrrolidines carrying a N-sulfinyl or N-H group ((1R)-(SR)-17a and (1R)-19a).

For the six-membered piperidine series, we made observations similar to those reported for 1tributystannyl hexopyranoses: the values of 1 J( 117 Sn-13 C), when the tin group occupies the axial position in a 4 C1 pyranose conformer are smaller than 305 Hz (319 Hz for 1 J( 119 Sn-13 C)). [19e] Compounds (1R)-18d and (1R)-18e follow the rule, thus widening further this prediction tool.

At this point, we were also intrigued to see whether we could extend the tool and predict the C-1 configuration at an earlier stage of the synthesis. No such correlations emerged in compounds 14, 22 and 23, even when pushing the heteronuclear spin-spin coupling analysis through the whole n-butyl chain. Early investigations from Vasella on 1-stannylated glycosides have shown that 1 J(Sn-C) between the anomeric carbon and the tin substituent could be of interest in structural assignments. [33] Although no relationship between 1 J(Sn-13 C-1) values and the configuration at C-1 could be found in the Nsulfinylated 1-stannyl iminoalditols 14, we were delighted to identify a correlation for the series of Nsulfonylated compounds 22. As a rule, for the iminoalditols of type 22, in which the amino group is in a 1,2-syn relationship with the alkoxy group at C-2, the 1 J( 117 Sn-13 C) coupling constants are above 310 Hz (320 Hz for 1 J( 119 Sn-13 C)), whereas the C-1 stannylated iminoalditols with a 1,2anti orientation have 1 J(Sn-13 C) coupling constants below these cutoffs (see Fig. 4 and SI). Thus, it appears that the magnitude of this coupling constant is influenced by the configuration at the Sn-bearing carbon when steric effects are maximal and constitute a useful tool for configuration assignment.

Migita-Kosugi-Stille Reaction Development. We next decided to investigate the Pd-catalyzed Migita-Kosugi-Stille cross-coupling reactions of the anomeric iminosugar stannanes: this reaction might unlock the synthesis of a diversity of iminoglycoside analogs, as suggested by the stereospecific Pdmediated coupling reactions of chiral C(sp 3 ) stannanes with aryl(aroyl) or alkyl(alkanoyl) halides [19], [34] .

Since the pioneering work of Eaborn, [35] Migita and Kosugi, [36] in the 1970's and the extensive mechanistic studies by Stille, [37] and others this reaction has progressively become one of the cornerstones in Pdcatalyzed C-C bond formation. [34], [38] It is a mild process that tolerates a wide variety of functional groups, and which is frequently used in the synthesis of molecules of high complexity. Furthermore, organostannanes are relatively insensitive to moisture and oxygen, allowing for harsher reaction conditions.

In the sugar series, anomeric glycosylstannanes have been described by Sinaÿ, [39] Kessler, [40] and Vasella, [33] and their first cross-coupling reactions with benzoyl a range of investigated by [41] The D-glucopyranosyl C-glycosides of were obtained in low to moderate yields, using excess electrophile, and with complete retention of configuration at the anomeric center. Walczak et al. recently demonstrated the scope and synthetic utility of the glycosylstannanes through coupling with aromatic iodides, diaryliodonium triflates and various thioand selenoesters and generated an impressive array of mono-and oligosaccharide-1-C-glycosides featuring free or diversely protected hydroxyl groups. [19] However, no example of such reaction has been reported so far from iminosugars. Pd2(dba)3 JackiePhos 51 3 a) Typical reaction conditions: Ar atmosphere, compound (1R)-18a (50 mg, 61.5 µmol, 1 equiv.), benzoyl chloride (1.5 equiv.), MS (4Å) (100 mg), Pd2(dba)3 (5 mol %), ligand (20 mol %), CuCl (10 mol %), heating at 110 °C for 6 h in 1,4-dioxane (0.03 M). b) Isolated yields (column chromatography). c) The reaction mixture was heated for 16 h. N.D not determined.

Based on the single literature precedent by Chong et al. on the Stille coupling of stereochemically defined α-sulfonamido-organostannanes, [29] and the aforementioned informations, optimization of the conditions for the Stille coupling of our iminoglycosyl stannanes with various electrophiles was thus started (Table 2). Robust L-arabino stannane (1R)-18a was selected as model substrate and the reaction was carried out with Pd2(dba)3 (5 mol%), 1,2-bis(diphenylphosphino)ethane (dppe), copper(I)chloride (10 mol %) and benzoyl chloride (1.5 equiv.) in 1,4-dioxane for 16 h at 110 °C. A 7% yield of imino-C-glycoside 25aa was obtained (entry 1).

Variation of the ligand was next envisaged with a careful choice of the phosphine, presenting various steric and electronic properties. [42] With the chelating diphosphine 1,1'-bis(diphenylphosphino)ferrocene and the tertiary tricyclohexylphosphine (PCy3), featuring high basicity (pKa = 9.7) and large ligand cone angle (170°), the yield of the coupling product could be slightly increase to ca. 16% (entries 2 and 4). Interestingly with PCy3, we noticed considerable decomposition and formation of an elimination product (an iminoglycal, 26a). The yield of the coupling process could be increased to a decent 35% (isolated) yield by shortening the reaction time to 6 h (entry 5). Triphenylphosphine (PPh3) and BINAP are some of the most often used ligands for palladiumcatalyzed C-C bond-forming reactions. Thus, subsequent cross-coupling reactions were conducted with these phosphines, but a dramatic yield decrease or no improvement at all was noticed (entries 6 and 7).

Of note, in situ generation of tetrakis(triphenylphosphine)palladium(0) from Pd2(dba)3 and PPh3 (entry 6) vs. direct addition of Pd(PPh3)4 (entry 8) did not produce any significant change of the reaction yield. With the square planar bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2) as catalyst, the yield could be increased to a respectable 46%, although a large amount of iminoglycal 26a (38%) was formed (entry 9).

As the rate-determining step in many Stille reactions is transmetalation, ligands of lower donicity, such as P(2-furyl)3 and Ph3As can dramatically enhance coupling reaction efficiencies. [43] These two ligands were thus tested, but unsatisfactory 36% and 18% yields respectively were obtained with undetermined amounts of the iminoglycal derivative (entries 10 and 11). These disappointing results show that facilitating the Sn-Pd transmetalation step does not suppress the competitive elimination of the C-2 substituent from the transient anomeric palladium species nor facilitate reductive elimination.

The highly basic tris(2,4,6trimethoxyphenyl)phosphine (TTMPP) used to perform the convenient synthesis of amino acids from α-sulfonamidoorganostannanes was thus tested thereafter, [29] but a notable yield decrease was observed (22%, entry 12). The electron rich tri-tertbutylphosphine (P(t-Bu)3) ligand was then examined and gave 1-C-benzoyl imino-L-arabinitol 40% yield (entry 13).

JackiePhos is biaryl diphenylphosphine reported by Buchwald for the Pd-catalyzed amidations aryl halides. [44] It was recently shown by Walczak to be quite efficient for the cross-coupling reactions of anomeric stannanes. [19] It has two electron-rich methoxy and three large isopropyl groups to favor oxidative addition and possibly reductive elimination. Two electron-withdrawing 2,6bis(trifluoromethyl)phenyl groups have also been setup to counterbalance the trend towards transmetalation and reductive elimination. With this ligand, the iminoarabinitol-C-glycoside 25aa was obtained in a respectable yield of 51%, with trace amounts (3%) of the elimination product (entry 14).

To fine-tune the reaction conditions solvent, additives, palladium and copper sources, stoichiometry of reagents, and reaction temperature were then screened (for details, see the Supporting Information). Based on the literature precedent that fluoride facilitates the Stille reaction, [45] KF was selected as an additive for the optimization studies, but no beneficial effect could be observed. Given the accelerating role of Cu salts, additives such as CuCl (0.05, 0.1, 1, 2, 3 equiv.), vs. CuI, CuBr or CuCN in combination with Pd2(dba)3 as precatalyst were also studied. Of note, while CuCN and CuBr generated a yield comparable to CuCl, almost no conversion could be detected with CuI. The yield of the Stille reaction may also be improved by the use of several equivalents of the Cu(I) salt, but in this transformation, as low as 5-10 mol % of CuCl could be used without degradation of the reaction yield. It is important to mention that increasing the amount of CuCl gave rise to increased quantity of elimination product 26a.

Likewise, in polar solvents (CH3CN, NMP, DMF, DMSO, DMA) the 1-substituted iminoarabinitol derivative 25aa was not obtained, the reaction providing again mainly by-product 26a (19% in DMA and 72% in DMSO). Aromatic solvents may suppress glycal formation, [19e] but in our case, the use of PhMe resulted in a lower yield of 25aa (36%) and an increase of 26a (18%). Other inorganic additives (ZnCl2, FeCl3, LiCl) were inefficient (typically 0% yield).

It is worth noting that all these results are consistent with a mechanism by a "closed" pathway (vs. the "open" pathway that leads to stereoinversion) involving a four-membered cyclic transition state and complete retention of the configuration. This also implies probable dissociation of one phosphine ligand prior to transmetalation. In addition, although no conversion was noted when the coupling reaction was performed without CuCl or Pd2(dba)3, at this point it is difficult to determine whether copper participates with palladium in a bimetallic catalytic cycle or if it is only Pd catalysis and Cu(I) acting as a ligand scavenger. [34], [46] Overall the optimized conditions were the following: stannane (1R)-18a (1 or 2 equiv.), benzoyl chloride (1.5 or 1 equiv.), MS (4Å), Pd2(dba)3 (2.5 mol %), ligand (10 mol %), copper(I) chloride ( 10mol %), heating at 110 °C for 6 h in 1,4-dioxane (see general procedure F, G.P. F), trace amounts (6%) of the elimination product being obtained.

Migita-Kosugi-Stille Reaction Scope. The optimized iminoglycosyl-benzoyl cross-coupling conditions were thereafter tested in reactions with various iminoalditol derivatives 18a-e, (1R)-20a and (1R)-21a, (Scheme 6). Pleasingly, we found that the general set of conditions is operational for the whole series of Nsulfonylated imino-L-arabino, L-xylo and L-lyxo alditols derivatives, giving rise to the corresponding Cglycosides in moderate to good yields (62-93%) and stereospecifically. The 1S configuration of 25ca could be assigned thanks to distinct nOe contacts between protons H-1 and H-3. One notes that lower yields of the coupling products were obtained from D-xylo piperidine (1R)-18e (to give (1S)-25ea, 23% e.g., 60% brsm) and from the two L-arabino pyrrolidines (1R)-20 and 21a ((1S)-25fa and (1S)-25ga formed in 34% and 26% yield respectively), which are not protected with a tert-butanesulfonyl group. For compounds 25fa and 25ga, the low yields can be explained by the formation of some deprotected products and some degradation.

Moreover, in case of the chemically stable imino-Dxylitol derivative (1R)-25ea, 24% of iminoglycal of type 26 (e.g., 26e, see SI) was isolated along with 53% of unreacted 1-stannyl iminosugar (1R)-18e. For this particular substrate, the search for better ligands appear to be mandatory.

Importantly, N-sulfinylated compound (1R)-(SR)-17a was also tested in the Pd-catalyzed cross-coupling, but total decomposition was observed. The scope of the reaction was next further explored using anomeric stannane (1R)-18a and a range of substituted aroyl chlorides and an n-alkanoyl reagent (Scheme 7). Gratifyingly, the yield of the coupling was quite good for most products. The performance of the acylation reactions did not seem to be correlated with the electronic properties of the aroyl chloride reagents, except with a p-NO2 group. However, sterics may be more important as it is observed that the yields of the reactions with p-Cl-, m-Cl-and o-Cl-benzoyl chloride decrease in this order (72%, 64% and 45% for 25ae, 25ag and 25af resp.). Notably, with hexanoyl chloride, a significant 52% yield of 25ah was achieved. Moreover, it is worth mentioning that the synthesis of the acyl chlorides from carboxylic acids (general procedure G, G.P. G), followed by cross-coupling with the anomeric iminosugar stannanes afforded 25aa and 25ah with an efficacy similar to commercial chlorides. These results thus open opportunities to incorporate acyl groups of greater complexity, such as for instance groups derived from amino acids or peptides for the synthesis of glycosyl aminoacids and glycopeptide mimics. [47] Overall, it was demonstrated that the stereospecificity of the cross-coupling reactions is linked exclusively to the C-1 configuration of both iminoglycosyl stannanes anomers, with retention of the 'anomeric' configuration; furthermore, the problem of competitive β elimination of the oxygen-based groups at C2 was addressed and the formation of unsaturated by-products 26 minimized. No epimerization of the chiral centers was noted.

Further data from literature, in particular from the work of Behr and Defoin on the configurational and conformational analysis of five-membered N-acyliminosugars, fully support our configurational assignments: it was shown that iminofuranosyl scaffolds tend to exhibit characteristic J(2,3) and J (4,5) coupling values for cis-relationships (J > 5.0 Hz) and J < 3 Hz for trans ones. [48] As regards the stannyl iminosugar derivatives, this general trend was observed for all our pyrrolidine related to pentofuranoses (see SI): J(1,2) couplings when H-1 and H-2 are in cis-relationship were in the range of 4.3 to 6.0 Hz, whereas the coupling between H-1 and H-2 in 1,2-trans isomers is very small with H-1 appearing as a broad singlet. From the nOe analysis of L-lyxo derivative (1S)-25ca and 1 H NMR investigations on compounds of type 25, it seems likely that the rule is conserved in the aroyl and hexanoyl imino-Cglycoside series (7.1 < J < 8.7 Hz for J(1,2)-cis and broad singlet for H-1 in 1,2-trans derivatives, see SI). As a result, based on literature precedents and on our investigations, anomeric stannanes undergo highly specific cross-coupling reactions with n-alkanoyl and aroyl chlorides through a stereoretentive mechanism.

Conclusion

In conclusion, we have developed an efficient approach for the preparation of 1-C-stannylated iminosugar anomers and their utilization in Stille cross-coupling reactions. The methodology demonstrated a good substrate scope and protecting group tolerance and has the great advantage of being tunable, i.e., the pseudoanomeric configuration of the 1-C-stannyl iminosugars and of the resulting 1-Cacylated iminosugar-C-glycosides can be chosen by selecting the configuration of the sulfinyl group in the starting N-tert-butanesulfinyl glycosylamines.

The iminoglycosyl stannanes were prepared by way of the stereospecific addition of tributylstannyllithium to (SR)-or (SS)-N-tert-butanesulfinyl glycosylamines, followed by an oxidation and/or a mesylationcyclization strategy. From J(C-Sn) data, a practical tool for the assessment of the chirality at C-1 was established. As a rule, (SS)-glycosylamines [(SS)-10] gave (1S)-configured stannylated iminosugar derivatives and (SR)-glycosylamine [(SR)-10], the (1R)stereoisomer respectively. The iminosugar-derived organostannanes were then efficiently cross-coupled through Migita-Kosugi-Stille reactions with a range of variously substituted aroyl chlorides and an alkanoyl chloride, in a stereoretentive C-C bond forming process. Such compounds are desirable advanced intermediates for the synthesis of a diversity of glycoside mimics which could act as inhibitors of carbohydrate-processing enzymes. The generality of the Stille coupling was investigated on 12 1-C-stannyl anomers, exhibiting furanose-like and pyranose-like skeletons. Eighteen examples of coupling products from the various stannylated iminosaccharides were obtained in moderate to excellent yields. Under the optimized coupling conditions, the β-elimination pathway was suppressed using a bulky phosphine ligand (JackiePhos) and a high stereoselectivity of the process was attained. Experimental evidence support a mechanistic rationale according to which the anomeric stannanes undergo stereoretentive cross-coupling reaction by way of a four-membered cyclic transition state. Taken together, these methodologies represent original and powerful strategies providing iminosugar-C-glycosides through an effective and hitherto unprecedented transformation. This synthetic tool allows for late stage coupling of iminosugar moieties with various aglycones and thus provides a means of performing the synthesis of imino-C-glycosyl compounds in a diversity-oriented approach. Further extension of the coupling reaction to other electrophiles such as aryl halides or heterocyclic halides should significantly expand the scope of the process and is currently under investigations.

FULL PAPER

Stereospecific Synthesis of Glycoside Mimics

  conditions (G.P. B): (1) i-Pr2NH (3.5-4.0 eq.), n-BuLi (3.5-4.0 eq.), THF, 0 °C, 30 min; (2) Bu3SnH (3.5-4.0 eq.), -78 °C, 15 min; (3) 10a-e, THF, -78 °C; (4) The reaction mixture is allowed to reach a given temperature over 1.5-6 h. b) d.r determined on crude mixture using 1

Scheme 3 .

 3 Scheme 3. Stereochemical model for the addition of Bu3SnLi to compounds 10a-e.

Scheme 4 .

 4 Scheme 4. Synthesis of anomeric iminosugar-1tributylstannanes (1R)-(SR)-17a and (1R)-18-21a.

Figure 2 .

 2 Figure 2. List of 1-stannyl pyrrolidinyl iminosugars, main nOe contacts and 1 J(Sn-C) values.

Figure 3 .

 3 Figure 3. List of piperidinyl iminosugar 1-stannanes, main nOe contacts, coupling constants, conformations and 1 J(Sn-13 C-1) values.

Figure 4 .

 4 Figure 4. List of N-sulfonylated aminoalditol 1-stannanes 22, and 1 J(Sn-13 C-1) trends.

6 .

 6 Scope of C-glycosylation with anomeric iminoglycosyl tributylstannanes.

Scheme 7 .

 7 Scheme 7. Scope of C-glycosylation with n-alkanoyl and aroyl chlorides.

  Through Migita-Kosugi-Stille Cross-Coupling Reactions of Chemically and Configurationally Stable 1-C-Tributylstannyl Iminosugars Adv. Synth. Catal. Year, Volume, Page -Page Sizhe Li, a Justyna Jaszczyk, a Xavier Pannecoucke, b Thomas Poisson, b,c Olivier R. Martin, a and Cyril Nicolas a *

Table 2 .

 2 Stille-Migita reaction optimization. a)

	Entry Catalyst	Ligand	(1S)-25aa	26a (%) b)
				(%) b)
	1	Pd2(dba)3	DPPE	7 c)	N.D.
	2	Pd2(dba)3	DPPF	16 c)	N.D.
	3	Pd2(dba)3	P(o-tol) 3	7 c)	N.D.
	4	Pd2(dba)3	P(Cy) 3	15 c)	N.D.
	5	Pd2(dba)3	P(Cy) 3	35	N.D.
	6	Pd2(dba)3	PPh 3	33	N.D.
	7	Pd2(dba)3	R-(+)-BINAP	0	N.D.
	8	Pd(PPh3)4	-	31	N.D.
	9	Pd(PPh3)2Cl2	-	46	38
	10	Pd2(dba)3	P(Fu) 3	36	N.D.
	11	Pd2(dba)3	AsPh 3	18	N.D.
	12	Pd2(dba)3	TTMPP	22	N.D.
	13	Pd2(dba)3	P(t-Bu) 3	40	N.D.
	14

Acknowledgements

The authors are grateful for financial support from the Centre National de la Recherche Scientifique (CNRS) and Labex SynOrg (ANR-11-LABX-0029).


Thomas Fix
email: tfix@unistra.fr

Romain Vollondat

Arechkik Ameur

Stéphane Roques

Jean-Luc Rehspringer

Céline Chevalier

Dominique Muller

Abdelilah Slaoui

Silicon Clathrate Films for Photovoltaic Applications

Silicon clathrates are allotropes of silicon that show great promises for optoelectronics and batteries.

However silicon clathrates in the form of films are relatively new and devices based on this material still have to be engineered. In this work we present a protocol for silicon clathrates film fabrication that does not necessitate a glove box. We show that dense films can be obtained with a pressure annealing treatment and that the films can be measured by atomic force microscopy. Ion implantation of P, B and As is performed on the clathrate films. Early photovoltaic devices are presented, with a maximum short circuit current density of 0.11 mA/cm². 10 µm

Introduction

Diamond silicon is the second most abundant element on earth. Silicon (Si) is the most used element in semiconductor technology. The main forms of Si encountered in research and industry are monocrystalline, multi-crystalline and amorphous Si. An allotrope of Si called Si clathrate was discovered in 1965 [START_REF] Kasper | Clathrate Structures of Silicon[END_REF] . It is composed of a framework of hollow spheres of various sizes, similar to carbon fullerenes further discovered in 1985, with the ability to host guest atoms. Although practical applications of Si clathrates were not developed, this topic has regained interest with the possibility to fabricate Si clathrates in the form of films [START_REF] Narita | Preparation of NaSi Thin Films for the Guest Free Si Clathrate Thin Films by Heat Resistance Apparatus Using NaSi Target Materials[END_REF][START_REF] Ohashi | Thin-film Formation of Si Clathrates on Si Wafers[END_REF][START_REF] Krishna | Efficient Route to Phase Selective Synthesis of Type II Silicon Clathrates with Low Sodium Occupancy[END_REF] . There are two main types of applications for these materials: optoelectronics [START_REF] Amsler | Low-density Silicon Allotropes for Photovoltaic Applications[END_REF] and batteries [START_REF] Krishna | Inorganic and Methane Clathrates: Versatility of Guest-host Compounds for Energy Harvesting[END_REF] . In the first case, the NaxSi136 type II binary clathrate structure presents semiconducting properties and is able to reach a 1.9 eV quasi-direct bandgap [START_REF] Gryko | Low-density Framework Form of Crystalline Silicon with a Wide Optical Band Gap[END_REF] . Furthermore the material presents the advantage of a direct bandgap as opposed to conventional diamond silicon [START_REF] Kume | Group IV Clathrates for Photovoltaic Applications[END_REF] . In the second case, the large size of spheres composing the Si clathrates enables a high storage capacity within the cage that can be used for lithium or sodium ion batteries [START_REF] Chan | First Principles and Experimental Studies of Empty Si46 as Anode Materials for Li-ion Batteries[END_REF][START_REF] Chan | Synthesis and Characterization of Empty Silicon Clathrates for Anode Applications in Li-ion Batteries[END_REF][START_REF] Li | Electrochemical Properties and Challenges of Type II Silicon Clathrate Anode in Sodium Ion Batteries[END_REF][START_REF] Warrier | Silicon Clathrates for Lithium Ion Batteries: A Perspective[END_REF] . Thermoelectric properties of clathrates also seem favourable [START_REF] Chae | Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure[END_REF][START_REF] Li | A Study of the Vibrational and Thermoelectric Properties of Silicon Type I and II Clathrates[END_REF] . Coming back to optoelectronics, Si clathrate films (SCF) have been proposed to be used in solar cells [START_REF] Amsler | Low-density Silicon Allotropes for Photovoltaic Applications[END_REF][START_REF] Kume | Group IV Clathrates for Photovoltaic Applications[END_REF][START_REF] Botti | Low-energy Silicon Allotropes with Strong Absorption in the Visible for Photovoltaic Applications[END_REF][START_REF] Martinez | Synthesis of Group IV Clathrates for Photovoltaics[END_REF][START_REF] Wu | Silicon Clathrates for Photovoltaics Predicted by a Two-step Crystal Structure Search[END_REF][START_REF] Krishna | IV Clathrates: Synthesis, Optoelectronic Properties, and Photovoltaic Applications[END_REF][START_REF] Kume | Thin Film of Guest-free Type-II Silicon Clathrate on Si(111) Wafer[END_REF] . Their main advantage compared to diamond Si is that diamond Si has an indirect bandgap of 1.1 eV and a direct bandgap of 3.4 eV which makes it a poor solar spectrum absorber. Thus the diamond Si absorber has to be thick (100 µm) and at the same time very pure to provide a large minority carrier diffusion length [START_REF] Botti | Low-energy Silicon Allotropes with Strong Absorption in the Visible for Photovoltaic Applications[END_REF] . Many efforts have been undertaken to improve the optical properties of diamond Si, such as superlattices [START_REF] Oh | Dipole-allowed Direct Band Gap Silicon Superlattices[END_REF] , nanopatterning [START_REF] Cloutier | Optical Gain and Stimulated Emission in Periodic Nanopatterned Crystalline Silicon[END_REF] , and plasmonics [START_REF] Lecler | Plasmonic Effect of Ag Nanoparticles in a SiON Antireflective Coating: Engineering Rules and Physical Barrier[END_REF][START_REF] Bläsi | Study of Plasmonic Nanoparticle Arrays for Photon Management in Solar Cells[END_REF] .

Recently, an attempt was made to fabricate solar cells based on NaxSi136 8 . The short-circuit current, open-circuit voltage and conversion efficiency were low, 9.2 nA, 0.036 V and 10 -5 % respectively. In this work, we describe our fabrication process to obtain type I and type II SCF and we describe a substantial improvement of the photovoltaic (PV) properties using ion implantation.

Methods

The method to fabricate SCF is similar to the ones reported in the literature, involving two annealing processes. The first annealing is performed as follows: a Na slice of mass around 1.2 g is cut from sodium sticks (purity 99%, Alfa Aesar) and placed into cyclohexane (99+%) to prevent oxidation. The Na material and related compounds should be handled with safety precautions. The Na slice is then put at the bottom of an Inconel crucible. A c-Si (001) substrate of thickness 0.5 mm and size 54 x 37 mm² is cleaned with HF (HF:H2O (1:10) for 2 min), rinsed with deionised water, placed on top of the crucible. The c-Si substrate is p-type with a resistivity of 1-5 Ω•cm. The substrate is covering totally the crucible and is about 1 cm above the Na slice. The crucible and sample are then inserted into a stainless-steel tube (diameter 44.5 mm, length 1 m, Inox SST304) which is itself placed into a tubular furnace. Some Ta in the form of wire is also inserted in the tube with the aim to absorb any residual oxygen in the tube. The stainless-steel tube is flushed with Ar (99.99990%) for 15 min, a pressure of Ar of 1.6•10 5 Pa is applied by closing the tube with valves on each side. The tube is then heated with a ramp of 5°C/min to a temperature of 480-650°C and maintained at this temperature for 19h. The tube is then naturally cooled down to room temperature. The tube is flushed with Ar and the substrate is taken out. The sample is then quickly transferred to a quartz tube which is pumped down to a dynamic pressure of 10 -5 Pa. A furnace is placed around the quartz tube which is heated to 400°C for 3h under dynamic vacuum conditions and naturally cooled down to room temperature. Finally, the samples are washed with deionised water to remove remaining Na on the surface.

In order to improve the quality of the films, some SCF samples were press annealed in air at a temperature of 200°C and 250°C for 30 min with a force of 2 kN. The thermal treatment under pressure system is a hydraulic system (Rondol) with two steel plates with integrated and regulated heating up to 300°C. The applied force of 2 kN is the one applied by the system to the entire plates (15x15 cm²) which corresponds to a pressure of 10 6 Pa for the size of sample (about 20 cm²). X-ray diffraction (XRD) was performed with a Bruker D8 diffractometer in Bragg-Brentano configuration with a Cu Kα wavelength of 0.15418 nm. Scanning electron microscopy (SEM) was performed with a JEOL 6700F equipped with a field-emission gun (FEG), operating at 3 kV in the SEI mode instrument.

The surface of SCF was investigated using atomic force microscopy (AFM) with a NT-MDT Smena B AFM in tapping mode.

Concerning optical measurements, Raman spectroscopy was performed at room temperature with a JOBIN YVON LabRam ARamis with a 532 nm excitation. Spectroscopic ellipsometry was performed with an HORIBA Uvisel Lt M200 FGMS (210-880 nm). Photoluminescence (PL) was carried out between 24K and room temperature using an excitation at 355 nm with a frequency-tripled neodymium-doped yttrium aluminum garnet (Nd-YAG) laser. The laser pulse duration, frequency and power were 10 ns, 20 kHz and up to 100 mW (15.6 W•cm -2 ), respectively. The PL signal was collected by means of a concave mirror into an optical fiber and analyzed by a multichannel CCD. The PL signal was integrated over time for a duration of 800 ms.

The ion implantation of B, P and As in the as-grown SCF was performed at ICube with an EATON 200 MC.

The dose was 5x10 [START_REF] Botti | Low-energy Silicon Allotropes with Strong Absorption in the Visible for Photovoltaic Applications[END_REF] at/cm 2 and the energy was 30 keV. The samples were then annealed in a rapid thermal annealing (RTA) system at 950°C for 30 s for the activation of dopants. More specifically the samples were RTA annealed under Ar atmosphere (30 s purge), heated from about 20°C to 950°C in 6 s, kept at 950°C for 30 s and cooled down with water cooling (950°C to 500°C reached in 12 s, total cooling time from 950°C to 35°C in 180 s). I-V characteristics were performed at room temperature using an Oriel Solar Simulator in AM1.5G conditions and a Keithley 2461 SourceMeter. Unless stated otherwise, the contacts were made of thick Ag paste in a corner of the sample for the top contact and over the whole back surface for the bottom contact.

Results and Discussion

Figures 1a) andb) show the experimental procedure to obtain SCF on c-Si (001) as described in the experimental section. Before having switched to this procedure involving bulk Na as a precursor, two alternative methods were tested. The first one involved deposition of NaF on c-Si and subsequent annealing, and the second ion Na implantation in c-Si. For both methods the amount of Na incorporated in Si was insufficient to form SCF. Therefore in this work only the fabrication of SCF with Na source is described. It should be noted that in contrast to previous work [START_REF] Ohashi | Thin-film Formation of Si Clathrates on Si Wafers[END_REF][START_REF] Krishna | Efficient Route to Phase Selective Synthesis of Type II Silicon Clathrates with Low Sodium Occupancy[END_REF][START_REF] Kume | Group IV Clathrates for Photovoltaic Applications[END_REF] , no glove box is necessary in the protocol and that the design of the experiment is quite simple which should enable more scientists to elaborate SCF in the future. Using this method, a large area of SCF of size about 4.5cm x 1.5cm can be produced as shown in Figure 1c). It should be noted that initial experiments were producing SCF that were exfoliating from the substrate. This issue was solved by naturally cooling down the sample after the second annealing and maintaining the furnace around the quartz tube so that the cooling down is slow and takes about 10h to reach room temperature. Further confirmation of the presence of silicon clathrates is given by Raman spectroscopy (see Supplementary Information (SI) Figure S2) where several peaks are identified for type II clathrates [START_REF] Guyot | Raman Scattering of Silicon Clathrates[END_REF] and two peaks for type I clathrates [START_REF] Fang | Raman Scattering from Vibrational Modes in Si46 Clathrates[END_REF] . Figure 3a) shows the SEM image in cross-section of a SCF sample. It shows a very porous morphology, large variations in the surface (see SI Figure S1) and the presence of cracks on the surface originating from the decomposition of the Zintl phase. Such a surface and morphology is detrimental for the production of devices. Therefore we performed press annealing on SCF films. Figure 3b) shows a SEM cross-section of a SCF sample press annealed at 250°C. The porous structure observed in the as-grown samples has disappeared and a compact film is observed. Thanks to the improvement of the surface properties, it was possible to perform AFM on such samples. Figure 4 shows the topography observed by AFM of the surface of SCF on c-Si after press annealing at 250°C. This is the first time that SCF roughness is low enough to be able to be measured by AFM. Grains of size about 200 nm can be observed and the root-mean-square roughness (RMS) in a clean 5x5 µm² and 1x1 µm² area is about 100 nm and 10 nm respectively. While such values are still very high compared to film roughness prepared by physical methods such as pulsed laser deposition (PLD) or magnetron sputtering, it represents a first step into the integration of SCF into film technologies. As we will describe in the section on PV properties, contacts could be deposited on the SCF without creating short-circuits which means that the films are viable for further processing. The pressure annealing not only resulted in a morphology change, but also in a recrystallization of SCF as the annealing temperature was increased; the recrystallization led to more intense and less numerous XRD reflections (see SI Figure S3). The position of the peaks does not vary after pressure annealing but only their intensity, which indicates that the same phase is preserved but the grains are reorganised and recrystallised to allow a denser film. However, after press annealing at 250 °C no remaining type II silicon clathrate peaks are visible. and after washing the SCF with deionised water respectively. Further repetition of the second annealing in vacuum tends to reduce the amount of Na. Some solutions using a cold plate [START_REF] Krishna | Efficient Route to Phase Selective Synthesis of Type II Silicon Clathrates with Low Sodium Occupancy[END_REF] or iodine treatment [START_REF] Kume | Group IV Clathrates for Photovoltaic Applications[END_REF][START_REF] Ammar | On the Clathrate Form of Elemental Silicon, Si136: Preparation and Characterisation of NaxSi136 (x -> 0)[END_REF] have been proposed to effectively reduce the amount of Na to negligible levels and this will represent a further improvement of the current work.

We now consider the optical properties of the SCF. Spectroscopic ellipsometry was used to determine the refractive index, extinction and absorption coefficients (see SI Figure S4). At 600 nm, the absorption coefficient is comparable to that of a-Si. It is about 2x10 [START_REF] Krishna | Efficient Route to Phase Selective Synthesis of Type II Silicon Clathrates with Low Sodium Occupancy[END_REF] cm -1 which is very close to previous experiments by photothermal deflection spectroscopy [START_REF] Martinez | Synthesis of Group IV Clathrates for Photovoltaics[END_REF] . Noisy signal is obtained due to the low reflective nature of the SCF and rough surface. It is interesting to notice that the refractive index is about 1.5, much lower than for d-Si in the visible range. The Tauc-plot (SI Figure S4c) indicates a direct bandgap in the 1.7-1.9 eV range which is comparable to previous values [START_REF] Gryko | Low-density Framework Form of Crystalline Silicon with a Wide Optical Band Gap[END_REF] . The PL properties were also measured (SI Figure S5). A very wide PL peak of centre around 600 nm and full width at half maximum about 300 nm is observed, but no sign of the bandgap of SCF could be observed, even by lowering down the temperature to 24 K. The fact that the intensity of the PL decreases at low temperature can be a sign that the PL observed is mainly due to defects. It was proposed in Ref. 18 that the broadness of the PL is due to defects occurring with Na sublimation during the annealing under dynamic vacuum.

Concerning the electrical properties, because our SCF is fabricated on p-Si, no Hall effect measurements are possible. However in Ref. [START_REF] Kume | Group IV Clathrates for Photovoltaic Applications[END_REF] it was reported that the SCF was of n-type nature. Thus, early PV devices were fabricated in the hope of having a p-n junction (p-Si substrate -n-SCF). In contrast to previous work in the present case a hot-point probe method indicates for SCF neither a clear n-or p-type behaviour, and therefore it is presented in the following as "i" for intrinsic. This discrepancy can be explained by several reasons such as the fact that here we have a mixture of type I and type II Si clathrates and that the Na content is still high compared to the literature. It should be noted that "i" is only used as a guideline and that the non-implanted SCF is conductive (10 Ω•cm resistivity) and thus not a real intrinsic material. Still it shares the property common to intrinsic semiconductors that neither a clear n-type or ptype behaviour is observed. This was also confirmed by Hall effect on SCF produced from real intrinsic Si wafers.

The size of the cell is about 0.5 cm². Figure 5a) shows the IV in dark and under 1 sun of SCF on p-Si contacted with Ag. It shows a very small short circuit current density (JSC) of 9.2•10 -4 mA/cm² and a very small open circuit voltage (VOC) of 41 mV. It can be observed in the inset that the IV curve is slightly asymmetric but does not show a nice diode rectification behaviour. We expect here an i-n junction and therefore PV properties are poor. To improve this, the SCF films were doped at the surface by ion implantation, using a low energy to minimise amorphization by the ion beam. It should be noted that little is known about doping in silicon clathrates and therefore we performed pioneering work using B, P and As as dopants. At 30 keV, we calculated with the SRIM software that the projected range Rp of the implantation is very small compared to the thickness of the SCF (Rp of 132 nm for B, 52 nm for P, 32 nm for As, compared to SCF thicknesses higher than 50 µm). Rapid thermal annealing was performed after implantation with the aim to activate the dopants, while limiting exodiffusion. Therefore, doping affects only the top part of the SCF. It was checked by Raman that the implantation and RTA do not alter the clathrate phase (SI Figure S6). Figure 5b) shows the device implanted with B. B is traditionally a p-type dopant in diamond silicon. In this case, the PV properties have disappeared and the IV curve is quite symmetrical. This is not surprising if we consider that we have formed a p-i-p structure. Next, implantation of P was performed (Figure 5c). P is a n-type dopant in diamond Si. A small JSC and VOC are observed as in the undoped case. However it should be observed that the short-circuit current is of opposite sign as that of the undoped case. We can consider that this is due to the formation of a n-i-p structure. Using the same conditions and dose, As implantation was performed (also n-type dopant in diamond Si, therefore also generating a n-i-p structure). Figure 5d) shows the IV curves after implantation by As. It can be seen that the short-circuit current has the same sign as that for P but the JSC and VOC are greatly improved, with 0.11 mA/cm² and 91 mV respectively. The JSC obtained is very promising and much higher than a previous attempt using undoped SCF [START_REF] Kume | Group IV Clathrates for Photovoltaic Applications[END_REF] . The resulting efficiency is still low, about 2.5x10 -3 %, but more than two orders of magnitude higher than the only previous report of 10 -5 % 8 . To further understand why As implantation gave greatly improved results compared to P implantation, it can be reminded that As in diamond Si is well known to provide better incorporation in the lattice than P. The fill factor in all cases is low, about 25%. In order to check if this low value is due to series resistances, fingers of Ti (50 nm)/Pd (50 nm)/Ag (1 µm) were deposited on SCF implanted by As and no significant improvement in the fill factor or JSC and VOC were observed. Therefore the low fill factor is probably mainly due to the poor rectifying properties of the devices. The short-circuit current decreased slightly compared to the previous case due to the reduction of active area by the fingers. In the present work we do not demonstrate directly that the electron-hole pairs are generated in the SCF and not in the p-Si base. The short-circuit currents were too small to measure external quantum efficiency (EQE). Further work will be to improve the JSC and focus on the carrier generation.

Conclusions

To conclude, this work presents substantial improvement in the field of SCF. The number of laboratories capable of producing them is very limited, which was probably hindered by the fact that a dedicated glove box was necessary. With the protocol described here, we show that no glove box is needed and that most laboratories have the necessary tools to produce their own SCF. Progress was also obtained in the morphology of the films, thanks to pressure annealing. This allowed to obtain the first AFM images on SCF. Finally, early solar cells were produced and showed that implantation has a great impact on the PV properties and thus the SCF semiconducting behaviour. Further work will be to systematically investigate the control of doping in SCF which is the bottleneck for the improvement of PV properties with this material. To do so, SCF should be produced from Si films on insulating substrates rather than on p-Si crystalline substrates in order to allow advanced electrical characterisations.

Supporting Information

SEM images of as-grown SCF; Raman spectra of as-grown SCF; XRD spectra before and after pressure annealing; optical properties of SCF; PL spectra of as-grown SCF; Raman spectra before and after implantation and RTA.

Figure 1 :

 1 Figure 1: a), b) sketches of the setup for the first and second annealing respectively; c) picture showing

Figure 2 :

 2 Figure 2: a) XRD spectrum of SCF (first annealing at 550°C) and the JCPDS reference for type I (Na8Si46)

Figure 3 :

 3 Figure 3: cross-sectional SEM image showing SCF on c-Si a) as fabricated b) after press annealing at

Figure 4 :

 4 Figure 4: AFM topography images of a silicon clathrate film on c-Si after press annealing at 250°C.

Figure 5 :

 5 Figure 5: Current density vs. voltage characteristics in the dark and under 1 sun for a) SCF as fabricated;

Acknowledgments

We thank Y. Mohamed Salah, G. Schmerber and J. Bartringer for technical assistance, C. Leuvrey and J.

Faerber for SEM observations, and Y. Le Gall for ion implantation. A. A. acknowledges partial support from the European Union (ERASMUS+ "MEDSOL"). This project has received financial support from the CNRS through the MITI interdisciplinary programs.


Non network-former cations in oxide glasses spotted by Raman scattering

Bernard Hehlen, * a and Daniel R. Neuville, b The depolarized Raman spectra of about 30 binary and ternary aluminosilicate glasses have been collected. Two spectral responses involving the cations are observed below 400 cm -1 . One of the two bands arises solely from cations close to non-bridging oxygens. This observation holds for all the glasses investigated providing thereby a simple test for qualifying the polymerization state of oxide glasses. The second feature involves all cations whatever their role in the glass and is found to be twofold: one contribution arises form cations charge compensating AlO - 4 tetrahedra and the other one by network modifier cations. These results confirm the net vibrational contrast of cations depending on their structural surrounding. Finally, while alkali atoms vibrate independently from the rest of the network, alkali-earth cations close to non-bridging oxygens drag the latter in their motion.

I. Introduction

Raman scattering is very sensitive to structural modifications in materials and provides in principle a powerful tool for probing the atomic structures. In disordered solids however the vibrational responses are usually broad, dissymmetric, and sometimes overlapping. In addition, the structural disorder often prevents for a selection rule analysis similar to that developed for crystals and molecules. Glasses suffer from these spectroscopic limitations which considerably limit the spectral analyses. Despite, there exists spectral indicators in Raman which provide valuable information on local and medium range structure, some being hardly accessible otherwise. In simple oxide glasses, these are for example the number of threefold and fourfold Si-O-Si rings [1][START_REF] Galeener | Planar rings in glasses Solid State[END_REF][START_REF] Pasquarello | Identification of Raman defect lines as signatures of ring structures in vitreous silica[END_REF][START_REF] Hehlen | Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra[END_REF] , the distribution of SiO (4-n)-4 tetrahedra (so-called Q n species) [START_REF] Mcmillan | Structural studies of silicate glasses and meltsapplications and limitations of Raman spectroscopy[END_REF][START_REF] Mysen | Structure and properties of alkali silicate melts at magmatic temperatures[END_REF][START_REF] Mysen | Experimental, in situ, high-temperature studies of properties and structure of silicate melts relevant to magmatic processes[END_REF] , the Si-O-Si angle distribution in silicates 1,[START_REF] Hehlen | Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra[END_REF][START_REF] Hehlen | Bimodal distribution of Si-O-Si angles in sodo-silicate glasses[END_REF] , the amount of BO 3 triangles and BO 4 tetrahedra in botates [START_REF] Angeli | Effect of temperature and thermal history on borosilicate glass structure[END_REF] , among others. Very often though, relating the Raman data to structural numbers deserves calibration with a third party experiment such as NMR, X-Ray diffraction, or numerical simulations.

Vibrations involving the cations are located at low frequency, below 400 cm -1 typically and by now have been studied mostly by infrared spetroscopies [START_REF] Merzbacher | Structure of Na in aluminosilicate glasses: A far-infrared reflectances spectroscopics study[END_REF][START_REF] Kamitsos | Infrared reflectance investigation of alkali diborate glasses[END_REF][START_REF] Kamitsos | Alkali sites in glass[END_REF][START_REF] Sousa Meneses | Optical and structural properties of calcium silicate glasses[END_REF][START_REF] Sousa Meneses | Investigation of medium range order in silicate glasses by infrared spectroscopy[END_REF] . Interestingly, there exists very few reports on Raman scattering, althought cations play a crucial role in the glass, for example when considering the melting temperature, the diffusion processes, the mechanical properties... From a structural point of vu, the cations (M) depolymerize the Si-O-Si connected network by creating ionic bonds with alkali and alkaline-earth units, in which the oxygen ions are called non-bridging oxygens (NBO). In aluminosilicates, when Al is in fourfold coordination it acts as a network former by forming AlO - 4 tetrahedra covalently connected to the silica network. The cations can therefore be either network modifiers as in the previous case, or charge compensators when they compensate the negative charge carried by the AlO - 4 tetrahedra. The balance between these two distinct functions varies as a function of the M/Al ratio and controls many of the physical properties of the glass, such as the viscosity.

In a previous report we show that the depolarized Raman spectra of alumino-silicate glasses exhibit two cation bands: a low frequency one around 150 cm -1 hereafter called ω 1 and a high frequency one around 330 cm -1 hereafter referred to as ω 2 . [START_REF] Hehlen | Raman response of network modifier cations in alumino-silicate glasses[END_REF] In the two glass families investigated, ω 2 involved only cations at modifier place, while all cations where supposed to contribute to ω 1 . The atomic displacements underlying these modes are not yet identified. However, a visual inspection of the vibrations calculated using first principle molecular dynamic simulations suggests that the latter corresponds to a dangling of the Na atom around the oxygen atom to which it is attached [START_REF] Kilymis | Vibrational properties of sodosilicate glasses from first-principle calculations[END_REF] .

The aim of the present work is to investigate a large set of alkali and alkali earth silicate glasses in order to get more insights into the vibrational motions underlying the cations bands, enabeling conclusions which apply over the whole aluminosilicate glass family. To reach this objective, we investigated more than 30 glass compositions, the most prominant series being sets of alcalineearth aluminosillicates with constant silica content, 50SiO 2 -zMO-yAl 2 O 3 and M = Mg, Ca, Sr, Ba, with y varying between 0 to 40 in mole %. The ratio R =MO/Al 2 O 3 ranges from 0.25 to infinity and therefore crosses the tectosilicate join at R = 1 (Table 1). The glasses contain both modifier and charge compensator cations for compositions in the paralcaline domain (R > 1), and only charge compensator cations for compositions in the peraluminate region (R < 1), in a simple model assuming all aluminum atoms fourfold coordinated. Sodo-silicates and sodo-aluminosilicates have also been studied to complement these series with alkali-based glasses. The compositions are summarized in Table 1.

Spectroscopic details and sample preparation are shortly described in section II. The spectroscopy in the binary glasses is presented in section III while ternary systems are discussed in detail in section IV. A special attention has been given to the low frequency cation mode which turned to be twofold in the aluminosilates glasses.

II. Experimental conditions and sample preparation

The Raman spectra have been recorded using a single pass HR630 Jobin-Yvon diffractometer. The incident light was delivered by a solid state laser working at 532 nm with a linearly polarized beam focused into the sample with a × 100 microscope objective. The polarization of the light scattered in the backscattering geometry was analysed by a half plate wave followed by a broad band Glan-Thomson polarizer while the strong elastic line was rejected using two Bragg filters 17 . To ensure a good reproducibility of the scattered intensities, the irradiated surface of the samples were all flats and polished to optical quality. Finally all spectra were recorded after each other keeping the same experimental conditions.

The samples were obtained by melting mixtures of Na 2 CO 3 , CaCO 3 , SrCO 3 , BaCO 3 , MgO, Al 2 O 3 and SiO 2 (Rectapur from Merck). The mixing was ground for 1 hour under alcohol in an agate mortar, heated slowly to decompose the carbonates, and then heated above the melting point following [START_REF] Neuville | Viscosity, structure and mixing in (Ca, Na) silicate melts[END_REF] . The melts were maintained for a few hours at high temperature (1900K for aluminosilicate melt and 1400 K for silicate melt) in air. The sample was quenched in a few seconds from high temperature by dipping the bottom of the platinum crucible into pure water. Peraluminous glasses were obtained by melting a ceramic firstly made at high temperature following previous paper [START_REF] Neuville | Al speciation in calcium aluminosilicate glasses: A NMR and Raman spectrocospie[END_REF][START_REF] Neuville | Structure of Mg and Mg/Ca aluminosilicate glasses: 27 Al NMR and Raman spectroscopy investigations[END_REF][START_REF] Novikov | Al and Sr environment in tectosili-8 | 1-9 cate glasses and melts: viscosity, Raman and NMR investigation[END_REF] respectively for Ca-, Mg-, and Sr-aluminosilicate glasses). The heating procedure was repeated until no crystallization could be detected by optical microscope and X-ray diffractometry. All materials were found to be chemically homogeneous glassy phases before and after viscosity measurements. Glass sample were analyzed using an electron microprobe analyses, Cameca SX100.

III. Binary glasses

Sodo-silicates: fitting strategy

For sake of data homogeneity, the Raman spectra of the sodosilicates investigated in [START_REF] Hehlen | Raman response of network modifier cations in alumino-silicate glasses[END_REF] were recorded again using the same experimental conditions as for all the other glasses studied in this work. The depolarized Raman spectra shown in Figure 1 provides examples for describing the fitting procedure. Extracting information on the cation modes deserves fitting simultaneously the boson peak (BP) and its decay at high frequency. The steep increase up to its maximum arises from quasi-local optic vibrations competing with a continuum of acoustic-like excitations. Above the maximum, incoherent scattering from optic modes possibly comes into play giving rise to a sample dependent response. [START_REF] Mizuno | Continuum limit of the vibrational properties of amorphous solids[END_REF][START_REF] Beltukov | Boson peak and Ioffe-Regel criterion in amorphous silicon-like materials: effect of bond directionality[END_REF] Whatsoever the origin, there exists no analytical way to describe this complex spectral shape and one therefore has to find acceptable compromises. In our former paper [START_REF] Hehlen | Raman response of network modifier cations in alumino-silicate glasses[END_REF] this signal was modeled by an exponential decay in the frequency region surrounding the high frequency cation mode ω 2m around 330 cm -1 . We now want to analyse ω 1 at low frequency together with ω 2 . This requires fitting a larger spectral range and hence the boson peak as a whole. The model chosen for fitting the boson peak is composed by a lognormal function accounting for its low frequency part, an exponential decay taking over continuously at frequencies above its maximum (above ∼150 cm -1 typically), and a cutoff with a gaussian decay shape around 450-500 cm -1 . This "modified lognormal" function is clearly a rough approximation, but it has the merit of being relatively stable during the fitting procedure, with fitting parameters evolving smoothly and monotoneously from one spectrum to the other, reproducing thereby the evolution of the boson peak. The cation modes ω 1 and ω 2 on top of this signal have been modeled by gaussian functions characterized by their frequency, width, and area. The numerous fitting tests we performed reveal that the output parameters of ω 2 are quite robusts.

The reason is that it stands on a relatively flat background in all glasses. The steep and curved slope on which stands ω 1 leads to larger uncertainties, sometimes limiting the spectral analysis.

The superposition of the model (solid line) with the experimental data in sodosilicates (Fig. 1) highlights the good quality of the fits, at least for that glass series. Despite, in pure silica the modified lognormal function is unable to reproduce the bump around 120 cm -1 24 . For some glasses this peculiar shape of the BP is an additional drawback limiting the fitting quality. Therefore, we will only present below the trends of the fitting parameters which are not (or weakly) sensitive to the fitting quality of the boson peak "background" signal. The only exception is the area of ω 1 , subject to large uncertainty, but whose behavior turns out to be useful for the discussion. It is finally worth noticing here that the IR response of ω 1 is most likely bimodal [START_REF] Kamitsos | Infrared reflectance investigation of alkali diborate glasses[END_REF][START_REF] Kamitsos | Alkali sites in glass[END_REF] but still, we fitted its Raman signature with a single gaussian according to the fact that the proximity of the strong boson peak prevents from a more detailed analysis.

Alkali earth silicates

ω 1 and ω 2 are also present in alkali-earth silicates as shown in the depolarized Raman spectra of the 50SiO 2 :50MO glasses with M=Mg, Ca, Sr, and Ba (Figure 2). The responses are intense and can also be observed in experiments performed without polarization analysis. In the latter situation however, the Si-O-Si bending motions (R-band) together with the signal arising from the boson peak drastically confuse the spectral analysis. The arrow in Figure 2a indicates the position of ω 2 in MgSiO 3 extrapolated from the three other glasses. It shows that ω 2 is absent (or very weak) in that glass while very strong in the three other compounds. This observation provides an additional evidence that Mg-based silicates behave in a peculiar way as compared to other silicates. For example, they exhibit a highly disordered local structure with a coordination number of ∼ 4 to 4.5 [START_REF] Neuville | Structure of Mg and Mg/Ca aluminosilicate glasses: 27 Al NMR and Raman spectroscopy investigations[END_REF][START_REF] Sen | Short-range structure of invert glasses along the pseudo-binary join MgSiO 3 -Mg 2 SiO 4 : results from 29 Si and 25 Mg MAS NMR spectroscopy[END_REF][START_REF] Cormier | Mg coordination in a MgSiO 3 glass using neutron diffraction coupled with isotopic substitution[END_REF] significantly lower than for the other alkaline earth cations, between 6 and 8. These effect correlate with the value of the field strength which is the highest for magnesium, bringing the authors to sometimes position this cation as intermediate between glass modifier and glass former. These structural specificities are also captured in the Raman response of the Q n species shown in Figure 2b. The Q 2 and Q 1 bands are rather well defined except in the Mg-based glass where the Q n feature transforms into a single and very broad structure. This behavior likely results from a pronounced structural disorder at the Si-NBO sites giving rise to ill-defined Si-NBO stretching motions. Finally, the strong disorder close to the Mg cation also explains the very broad response of ω 1 in MgSiO 3 (Fig. 2a) as compared to other MSiO 3 glasses. . ω 2 falls in the deep between the broad feature ω IR assigned to cation motions [START_REF] Kamitsos | Alkali sites in glass[END_REF] and the transverse optic mode TO4 around 480 cm -1 corresponding to the rocking of the Si-O-Si bonds [START_REF] Kirk | Quantitative analysis of the effect of disorderinduced mode coupling on infrared absorption in silica[END_REF] . ω 1 has a frequency similar to that of ω IR suggesting that both responses correspond to the same type of vibrations. For the reasons mentioned in the preceding section (lack of accuracy owing to the strong boson peak nearby), we prefered to fit ω IR with one single gaussian although the IR data clearly show a structured spectral shape du to different cation environments motivating fitting procedures using two gaussians [START_REF] Kamitsos | Alkali sites in glass[END_REF] .

For a harmonic oscillator involving n atoms, like e.g. a molecule, the eigenfrequencies are given by ω 0 = k c /µ where k c is the caracteristic spring constant and µ the reduce mass of the eigenmode under consideration. The frequency of the two IR components building the infrared response ω IR is reproduced in Figure 4. It exhibits a linear behavior when plotted as a function of 1/m M , where m M is the mass of the cation [START_REF] Kamitsos | Alkali sites in glass[END_REF] . Since µ = m M , this lead the authors to conclude that the modes building ω IR involve solely the cations, i.e. in a motion almost independent from their anionic surrounding. Lets now consider the case of alkaline-earth silicates (Figure 4b). The frequency ω 1 now deviates from the linear behavior and the effect becomes more pronounced for lighter cations. This different behavior between alkali and alkali earth ions goes along with the increase of the atomic field strengths which are higher for the former. Indeed, when the electrostatic interaction increases, the cation become more and more bonded to the oxygens nearby and drag them along in its motion. The mass of the vibrating structure increases and hence m M should be replaced by µ.

When considering m M instead, the curve ω = f ( 1/m M ) artificially bends down, exactly as ω 1 does in Figure 4b. The increasing network connectivity for light atoms goes along with a decrease of the cation diffusivity D, revealing in addition the close link between mode frequency, field strength, and diffusivity. This analysis captures the overall motions of the cations : two main vibrations in their cage, ω 1 and ω 2 , and the relaxations at high temperature, i.e diffusion processes characterized by the diffusion coefficient D.

IV. Aluminosilicates

The depolarized Raman spectra of the ternary aluminosilicates series xSiO 2 :yAl 2 O 3 :zMO with M=Ba, Sr, Mg, are shown in Figure 5. The spectra of calcium glasses are displayed in Figure 4b of Ref. [START_REF] Hehlen | Raman response of network modifier cations in alumino-silicate glasses[END_REF] . Surprisingly, the appearance of an additional structural surrounding for the cations in the ternary systems does not translate into additional Raman bands. However, a direct inspection shows that ω 1 is present down to the lowest cation concentration suggesting that the band now gather the motions of both network modifier and charge compensator cations. One also observes that ω 2 vanishes around z = 25, that is at the joint R = 1 (Ba and Sr glasses) confirming previous anticipations that this band arises solely from cations close to non-bridging oxygens. Similarly to the binary MgO:SiO 2 system, ω 2 is still absent (or very weak) in all of the MgO-aluminosilicates investigated. For this set of glasses the silica content is close but not always equal to 50% and the spectra are displayed by decreasing concentration of MgO to highlight the concomitant intensity decrease of ω 1 .

Low-frequency cation mode ω 1

Figure 6a gather the fitted integrated intensities of ω 1 in the aluminosilicates and sodosilicates series. In calcium glasses ω 1 is embedded under the strong ω 2 band and the fitting parameters could not been exploited (see Fig. 2a). For the other glasses, the relatively large dispersion of the points arises from the fitting limitations mentioned in section III. Despite, when scaling all the intensities on the same value at z = 25 (R = 1) one clearly observes a change in the slope between peraluminate and paralcaline domains. The effect is more pronounced for heavy cations and likely accounts for a change in the Raman efficiency when network modifiers (R > 1) or charge compensators (R < 1) are involved. This and the step increase of the frequency of ω 1 around R = 1 shown in Figure 6b suggests a doublet structure of the Raman response : when all cations are charge compensators ([z] < 25, neglecting Al V or Al V I structures) the frequency ω 1 (≡ ω 1c ) is fairly constant and has a different value than ω 1 (≡ ω 1m ) for the binary glasses for which all cations are network modifiers ([z] =50). In between, the glasses have both network modifier and charge compensator cations, and the Raman response likely becomes twofold, i.e. ω 1 = ω 1c + ω 1m . Unfortunately, the spectroscopy is unable to separate the two contributions, leading to an apparent up-shift (or down-shift in Mg-based glasses) of the frequency when cation oxide is replaced by aluminum oxide. This is however well correlated with the [START_REF] Beltukov | Boson peak and Ioffe-Regel criterion in amorphous silicon-like materials: effect of bond directionality[END_REF] Na NMR observed between sodium silicate and aluminosilicate glasses [START_REF] Le Losq | The role of Al 3+ on rheology and structural changes in sodium silicate and aluminosilicate glasses and melts[END_REF] where a NMR chemical shift is observed between charge compensator and network modifier cations, and also with XANES results at the Ca K-edge of CaO-SiO 2 and 25CaO-25Al 2 O 3 -50SiO 2 glasses, where a shift in the XANES pre-edge is observed as a function of the cation site [START_REF] Cicconi | Ca Neighbors from XANES spectroscopy: a tool to investigate structure, redox and nucleation processes in silicate glasses, melts and crystals[END_REF] . In an attempt to confirm the above assumption we gather in One observes two distinct behaviors. For a predominance of network modifier cations in the glass (peralkaline domain), cation motion couples with the oxygen atoms nearby leading to the observed bending of the frequency dependence of ω 1 , as explained in the preceding section. The situation is very different for glasses in the peraluminate domain. In that case, there is only one possible site for the cations, i.e. charge compensators, and the three data sets merge in a single straight line meaning that the motion of those cations is now decoupled from the rest of the network. Indeed, the negative charge carried out by the AlO - 4 to which the cation is attached is delocalized leading to a loose directional bonding. Keeping in mind that ω 1 likely results from a dangling motion of the cations, this and the large mass difference between cations and tetrahedra can produce the observed "vibrational decoupling effect". However, although directionally loose, the bonding of the cation to its AlO - 4 tetrahedra should remain strong along the radial axis, as indicated by the weaker value of the diffusion coefficient of charge-compensator cations as compared to network modifiers ones. This result is in line with the IR data of alkalineearth borate glasses [START_REF] Yiannopoulos | Structure and properties of alkaline earth borate glasses[END_REF] where the authors also found a linear behavior in a plot of ω IR versus 1/m M . For the studied concentration (0.45MO:0.55B 2 O 3 ) most of the cations compensate BO - 4 tetrahedra [START_REF] Dell | 11 B NMR studies and structural modeling of Na 2 0-B 2 O 3 -SiO 2 glasses of high soda content[END_REF] which corresponds to a situation similar to the peraluminate domain in alumino silicates. Finally, Table 2 summarizes the values for these three sets of frequencies, i.e. ω 1c , ω 1m , and 6c and 8a, respectively. The latter compiles the data obtained in alkaline-earth aluminosilicates, sodo aluminosilicates, and sodo-silicates glasses. These families define three sets of data highlighted by blue (triangles), pink (filled circles), and yellow (empty circles) background colors. Within the alkalineearth group (triangles), the intensity of the three glass family has been assigned a multiplication factor and as a consequence, all curves superpose. As expected, the intensities decrease to zero close to the joint R = 1 in all of the ternary glasses. Considering only foufoldcoordinated [Al] IV atoms in the glasses, this value is reached for z = 12.5 in the sodium aluminosilicate, and z = 25 in the alkali earth glasses (arrows in Fig. 8a). By multiplying all the sets by a constant accounting for the cation scattering efficiency, all the curves overlapp into a single one, at least when plotted as a function of [z] mod , the molar fraction of modifier cations (Fig. 8b). Within our structural approximation, each couple of AlO -

1-9 | 5

Conclusion

The Raman spectra of alkaline and alkaline-earth aluminosilicates exhibit two spectral responses involving cation motions. The first one at low frequency, typically around 150 cm -1 (ω 1 ), is likely twofold though not resolved by our experiment. One component (ω 1c ) arises from cations compensating the negative charge carried by the AlO 4 tetrahedra. Those cations vibrate independently to the heavy structure to whom they are attached. This decoupling is further amplified by the loose and delocalised electrostatic bonding. The second component (ω 1m ) involves cations at modifier's place, that is attached to non-bridging oxygens. In alkaline-earth alumino silicate glasses the cations drag the NBOs nearby in a coupled motion, a situation different than for alkali cations where the vibration is defined solely by the motion of the cation. A likely explanation is the difference of strength fields which are significantly higher for alkali earth cations. The second cation band appears around 330 cm -1 (ω 2 ). In aluminosilicates the analysis shows that whatever the cation type, only those at modifier's place contribute to that vibrational response. Overall, these results point-out a clear vibrational contrast between the cations attached to NBOs and those close to AlO 4 tetrahedra. The presence or not of ω 2 in a Raman spectra also provides a simple test for qualifying the polymerization state of a glass. Since responses at similar frequencies appear in the Raman spectra of borate and borosilicate glasses these conclusions likely hold also in these systems.

Fig. 1

 1 Fig.1Depolarized Raman spectra (I V H ) of selected sodosilicate glasses and pure silica in the region of the cations modes ω 1 and ω 2 . The solid lines result from a fit using a modified lognormal function for the boson peak (see text) and gaussians for the cation modes ω 1 and ω 2 (dashed lines).

Fig. 2 a

 2 Fig. 2 a) Depolarized Raman spectra (I V H ) of MSiO 3 glasses (50MO:50SiO 2 ) with M=Mg, Ca, Sr, Ba, and their fits. The arrow indicates the position of ω 2 in MgSiO 3 extrapolated from the three other glasses. b) Polarized Raman spectra (I VV ): zoom on the Q n bands. For sake of visibility, the spectra have been translated vertically.

Fig. 3

 3 Fig. 3 Comparison between the dielectric function ε 2 measured in IR (circles) and the response of the two cations bands ω 1 and ω 2 extracted from the Raman spectra (lines, see text for details) in a) CaO:SiO 2 13 and b) Na 2 O:2SiO 2 10 .

Figure 3 13 (

 313 Figure 3 compares the Raman signature of the cation modes ω 1

Fig. 4

 4 Fig. 4 Frequency of ω 1 in Raman and ω IR in Infrared plotted as function of 1/m M . The size of the symbols refers to the atomic radii of the cations a) Alkaline-silicates: The circle is our work, the filled squares and filled losanges correspond to the two-component description of the IR response ω IR 12 , and the star is reproduced from 10 . b) Alkaline-earth silicates (our work). The numbers in parentheses are the field strengths 28 and those in braquets are log 10 (D) where D are the diffusion coefficients compiled from ? .

Fig. 5

 5 Fig. 5 Depolarized Raman spectra (I V H ) of aluminosilictes, xSiO 2 :yAl 2 O 3 :zMO. a) M=Ba, with x=50% and z in the legend, b) M=Sr, with x=50% and z in the legend, c) M=Mg, with the legend in the format x.z. Examples of fitted boson peaks are shown to give an idea of the spectral shapes underneath ω 1 and ω 2 (dashed lines). The spectra in Mg-based glasses have been translated-up for sake of clarity. The baselines are given by the horizontal lines on the right. The arrows indicate the expected positon of ω 2 in MAS.

Fig. 6 a

 6 Fig. 6 a) Integrated intensity of ω 1 as a function of the cation content [z]. The data have been scaled to coincide at concentration z = 25 (R = 1). b) and c) Frequency of the cations modes ω 1 and ω 2 as a function of [z], respectively.

Fig. 7

 7 Fig. 7 Frequency of ω 1 as a function of 1/m M in the aluminosilicate glasses. Red filled circles: binary glasses MOSiO 3 ([z]=0); green diamonds: paralkaline glasses ([z]=12%); blue squares: peraluminate glasses, ([z]=25%, 20% and 10%, from dark blue to light blue). In the latter the frequency is similar for the three [z] values and the points superpose. Lines are guide to the eye.

Figure 7

 7 Figure 7 the curves ω 1 (M) as a function of 1/m M for cations concentrations [z]=10, 20, 25, 38, and 50mol%. The filled circles correspond to the binary silicates, [z]=50 mol%, the empty losanges correspond to aluminosilicates with [z]=38 mol% (peralkaline domain), and the squares to aluminosilicates with [z]= 25, 20 and 10mol% (peraluminate domain).One observes two distinct behaviors. For a predominance of network modifier cations in the glass (peralkaline domain), cation motion couples with the oxygen atoms nearby leading to the observed bending of the frequency dependence of ω 1 , as explained in the preceding section. The situation is very different for glasses in the peraluminate domain. In that case, there is only one possible site for the cations, i.e. charge compensators, and the three data sets merge in a single straight line meaning that the motion of those cations is now decoupled from the rest of the network. Indeed, the negative charge carried out by the AlO - 4 to which the cation is attached is delocalized leading to a loose directional bonding. Keeping in mind that ω 1 likely results from a dangling motion of the cations, this and the large mass difference between cations and tetrahedra can produce the observed "vibrational decoupling effect". However, although directionally loose, the bonding of the cation to its AlO - 4 tetrahedra should remain strong along the radial axis, as indicated by the weaker value of the diffusion coefficient of charge-compensator cations as compared to network modifiers ones. This result is in line with the IR data of alkalineearth borate glasses[START_REF] Yiannopoulos | Structure and properties of alkaline earth borate glasses[END_REF] where the authors also found a linear behavior in a plot of ω IR versus 1/m M . For the studied concentration (0.45MO:0.55B 2 O 3 ) most of the cations compensate BO -

Fig. 8

 8 Fig. 8 Integrated intensity of ω 2 a) as a function of the total cation content [z] and b) as a function of the modifier cation content [z] mod . The arrows indicate the join R = 1.

Table 1

 1 List of glass samples and location in the ternary phase diagram.

	Alkaline earth	M=Mg	M=Ca	M=Sr	M=Ba
	aluminosilicates	x-y 50-00	x-y 50-00		x-y 50-00	x-y 50-00		SiO 2
			50-06
		52-12	50-12		50-12	50-12
		48-13					x=50
	xSiO 2 :yAl 2 O 3 :zMO	49-16
		50-25	50-18 50-25		50-25	50-25	MO	(R=1)	Al 2 O 3
		50-30	50-30		50-30	50-30
		50-40	50-40		50-40	50-40
	Alkaline aluminosilicates	M=Na x-y		SiO 2
		75-02
		75-05	x=75
	xSiO 2 :yAl 2 O 3 :zM 2 O	75-09
		75-12
		75-16
	Sodo-Silicates	x-z	Na 2 O	SiO 2	Al 2 O 3
		100-0
		87.5-12.5
	xSiO 2 :zNa 2 O	80-20
		75-25	x=50
		67-33
		60-40	Na 2 O		Al 2 O 3

Table 2

 2 Frequency of the two cation modes ω 1 and ω 2 in alkaline-earth aluminosilicates (in cm -1 ): contribution of charge compensator cations ω 1c and network modifier cations ω 1m to the low frequency response ω 1 , and high frequency response ω 2 originating solely from modifier cations and taken at [z]=50. Let's now consider the high frequency mode ω 2 . Its frequency and integrated intensity are presented as a function of the cation content in Fig.

	Cation (M)	ω 1		ω 2
		ω 1c	ω 1m
	Mg	316 270	-
	Ca	-	265 352
	Sr	155 230 334
	Ba	135 193 310
	4.2 High frequency cation mode ω 2

| 1-9

tetrahedra is charge compensated by one alkali earth atom, and the latter reads [z] mod = (zy)/atm, where atm is the number of atoms per mole. When [z] mod becomes low, i.e. when the concentration of alumina increases up to the peraluminate region, the intensity of ω 2 reaches zero before the joint R = 1 ([z] mod = 0) in the alkaline-earth glasses, a behavior also visible in Figure8a. This observation goes along with an increase of [Al] V and [Al] V I environments. Compensating these structures requires additional cations which hence are lost as network modifiers, leading to a faster decrease of the intensity of the mode as compared to the linear regime extrapolated from high [z] mod values (dashed line in Figure8b). However, the effect is rather weak and the experimental errorbars prevent for a comparative analysis between Ca, Sr, and Ba.