Jennifer Bidal

Christine Cézard

Benjamin Bouvier

Caroline Hadad

Albert Nguyen Van Nhien

Matthieu Becuwe

Fundamental insight into the

come

Introduction

The field of electrochemical energy storage (especially metal-ion batteries) is currently undergoing rapid change due to the massive electrification of our daily life and the drastic decrease of the use of fossil fuels for our mobility. As a result, the demand for increasingly efficient and safer electrical energy storage systems is exponentially growing. To meet this demand, new technologies beyond classical batteries need to be developed to limit price fluctuation and avoid raw material restrictions. [START_REF] Iglesias-Émbil | Raw material use in a battery electric car -a thermodynamic rarity assessment[END_REF] While new chemistries involving diverse ions (Na + , K + , Ca 2+ , Mg 2+ ,…and even anions), [START_REF] Delmas | Sodium and Sodium-Ion Batteries: 50 Years of Research[END_REF][START_REF] Dominko | Magnesium batteries: Current picture and missing pieces of the puzzle[END_REF][START_REF] Dong | A novel coronene//Na2Ti3O7 dual-ion battery[END_REF] sulfur [START_REF] Cohn | A novel non-aqueous aluminum sulfur battery[END_REF][START_REF] Wei | A stable room-temperature sodium-sulfur battery[END_REF] or organic-based compounds [START_REF] Poizot | Opportunities and Challenges for Organic Electrodes in Electrochemical Energy Storage[END_REF][START_REF] Lu | Prospects of organic electrode materials for practical lithium batteries[END_REF] are currently under intense scrutiny, the next generation of batteries is likely to be all-solid since the use of a solid electrolyte can significantly increase the energy density (through bipolar stacking and use of metal anode) but also make the batteries safer. [START_REF] Famprikis | Fundamentals of inorganic solid-state electrolytes for batteries[END_REF][START_REF] Janek | A solid future for battery development[END_REF] The switch to such a technology implies many changes and above all new challenges to overcome such as interfaces management, volume variation control during use and recycling of these batteries at the end of their life. In this context, all attention is focused on the electrolyte, which can be of different chemical natures like inorganic, organic (molecular or polymer) or even hybrid. Each family has its specific advantages, such as the high conductivity of inorganic compounds or the excellent flexibility and processing of polymers, but to date, the perfect electrolyte, combining all the required criteria like good conductivity, broad electrochemical stability, sufficient transport, relatively low cost, etc., has not yet been found. In addition, there are increasing constraints on the availability of raw materials and the recycling of electrolytes, which are more complex for all-solid systems, especially those obtained by sintering. Most probably, as it is currently the case for vehicles, the trend is to progressively evolve from pure inorganic-based electrolyte to hybrid material with different proportions of earth-abundant elements. In this sense, different approaches can be used to design a hybrid electrolyte, such as tethered organic charge carrier, [START_REF] Korf | Piperidinium tethered nanoparticle-hybrid electrolyte for lithium metal batteries[END_REF][START_REF] Delacroix | SiO2/Ionic Liquid Hybrid Nanoparticles for Solid-State Lithium Ion Conduction[END_REF][START_REF] Lu | Ionic Liquid-Nanoparticle Hybrid Electrolytes and their Application in Secondary Lithium-Metal Batteries[END_REF][START_REF] Bidal | Van Hybrid Electrolytes Based on Optimized Ionic Liquid Quantity Tethered on ZrO2 Nanoparticles for Solid-State Lithium-Ion Conduction[END_REF] ionic liquid confinement, [START_REF] Chen | Silica gel solid nanocomposite electrolytes with interfacial conductivity promotion exceeding the bulk Li-ion conductivity of the ionic liquid electrolyte filler[END_REF][START_REF] Guyomard-Lack | Enhancement of lithium transport by controlling the mesoporosity of silica monoliths filled by ionic liquids[END_REF] metal-organic framework [START_REF] Wang | Boosting interfacial Li+ transport with a MOF-based ionic conductor for solid-state batteries[END_REF][START_REF] Wang | A Metal-Organic-Framework-Based Electrolyte with Nanowetted Interfaces for High-Energy-Density Solid-State Lithium Battery[END_REF] or simply by introducing an inorganic charge, used as filler, into a polymer (CPE, Composite Polymer Electrolyte). [START_REF] Brandell | Molecular dynamics simulation of the LiPF6•PEO 6 structure[END_REF][START_REF] Das | Influence of oxide particle network morphology on ion solvation and transport in "soggy Sand" Electrolytes[END_REF][START_REF] Groce | Nanocomposite polymer electrolytes for lithium batteries[END_REF] The latter represents the most widely used type of hybrid electrolyte.

The main fillers used are ceramic particles of nanometric oxides such as Al 2 O 3 , SiO 2 , ZrO 2 , and TiO 2 . These fillers do not participate directly in ionic transport but act as "solid plasticizers". They reduce the crystallinity and glass transition temperature of the polymer [START_REF] Zhang | Recent advances in solid polymer electrolytes for lithium batteries[END_REF] and could kinetically inhibit the crystallization of the PEO matrix through Lewis acid/base interactions between the surface groups of the ceramic fillers and the polymer segments as well as the lithium salt anion. Since the interface between the inert fillers and the polymer matrix plays a role in improving Li + ion conduction, particle size and specific surface area are, therefore, important factors for this improvement. [START_REF] Przyluski | Effective medium theory in studies of conductivity of composite polymeric electrolytes[END_REF][START_REF] Wieczorek | Composite polyether based solid electrolytes. The Lewis acid-base approach[END_REF] In addition to the size of the "inert" ceramic particles, the concentration of the nanoparticles is considered as a second key factor impacting the ionic conductivity of composite electrolyte and can be explained by the cooperation of several factors [START_REF] Przyluski | Effective medium theory in studies of conductivity of composite polymeric electrolytes[END_REF][START_REF] Rajendran | Effect of ceramic oxide on PVC-PMMA hybrid polymer electrolytes[END_REF]: (i) the decrease of the glass transition temperature with the addition of fine ceramic particles; (ii) the formation of crystallites from the ceramic particles, acting as nucleation centers; (iii) the formation of amorphous phases in the polymer electrolyte, assisted by the inorganic fillers; and (iv) the formation of a new kinetic pathway through the interphase of the polymer matrix and the inert ceramic fillers. [START_REF] Zhang | Recent advances in solid polymer electrolytes for lithium batteries[END_REF] Lastly, the surface chemistry of inorganic fillers is also a factor impacting the performance of CPEs. For instance, it has been shown that the more acidic surface of TiO 2 particles allows CPEs to acquire a higher ion transfer number than in the case of Al 2 O 3 . [START_REF] Groce | Nanocomposite polymer electrolytes for lithium batteries[END_REF] SiO 2 is also known to further stabilize the charge-polymer interface due to the hydroxyl groups, responsible for many chemical and physical interactions. [START_REF] Lin | High Ionic Conductivity of Composite Solid Polymer Electrolyte via in Situ Synthesis of Monodispersed SiO2 Nanospheres in Poly(ethylene oxide)[END_REF] In hybrid polymer electrolytes (lithium salt, polymer and inorganic fillers), it is always difficult to estimate which chemical species and which parameters affect and control ion mobility and also how each component interacts together and concomitantly. Thus, a complete understanding of such a system is quite difficult and requires a thorough understanding of the bilateral interactions before the complete ion mobility scheme can be modeled. The present study is the first in a series dedicated to this objective. To the best of our knowledge, lithium ion conduction in lithium salt-filler material has not yet been studied without any other component. To investigate this fundamental question, we propose here a dual experimental-theoretical study focusing on the fillers-salt couple and more specifically on how the surface chemistry of the filler affects the lithium mobility. To this purpose, silica nanoparticles (synthesized using the modified Stöber method) in association with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) have been selected for this study. In order to see clearly the influence of the surface-lithium salt interaction on the conduction properties, the quantity of LiTFSI was intentionally chosen as low (resulting in deliberately low conductivity values) but is perfectly justified to avoid any masking effect of the lithium salt on the conductivity. Different modifications of the SiO 2 surface were carried out, like suppression of hydroxyl groups, surface lithiation and even surface-capping with an organosilane derivative to increase lipophilic interaction.

Results and discussion

Characterization of the synthesized silica NPs (OH-SiO 2 )

In accordance with the literature, [START_REF] Dugas | Surface hydroxylation and silane grafting on fumed and thermal silica[END_REF] infrared spectroscopy of fully hydroxylated silica nanoparticles displays a large and intense band at 3670 cm -1 corresponding to free Si-OH groups (Figure 1a). The size distribution for diameters measured on a sample of 300 nanoparticles was between 40 and 45 nm (mean: 44 nm, standard deviation: 4). The silica nanoparticles were found to be spherical and their size was homogeneously distributed in the sample (Figure 1b). Subsequently, nitrogen adsorption and desorption isotherms (Figure 1c) were performed to evaluate the specific surface of the nanoparticles using the equation determined by Brunauer, Emmett and Teller (BET method). [START_REF] Brunauer | Adsorption of gases in Multimolecular layers[END_REF] The BET specific surface measured for NPs of size 40-45 nm in diameter was 64 m²/g. According to the literature, the shape obtained corresponds to type IV to which a hysteresis (type H1) is added. [START_REF] Sing | Adsorption methods for the characterization of porous materials[END_REF] The shape of these isotherms, characteristic of the texture of a material, is usually found in mesoporous solids.

Surface modifications of hydroxylated SiO 2 NP (OH-SiO 2 )

Several surface modifications were performed on OH-SiO 2 NPs such as lithiation (Li-SiO 2 ), calcination to give hydroxyl-free silica (SiO 2 ), and surface capping with a hydrophobic molecule (MS-SiO 2 ). These modifications were performed in order to probe the impact of the surface state on the ionic conductivity within these samples (Figure 2). The modified nanoparticles were first analyzed by FTIR spectroscopy in diffuse reflectance mode to only observe the surface evolution (Figure 3). For the unmodified (OH-SiO Specifically, the removal of labile protons (which can impact the lithium conduction mechanism) was achieved by coating the NPs surface with HMDS reagent under inert conditions (Figure 3, red curve). In this case, an intense and shifted absorption band at 2963 cm -1 attributed to the alkyl groups of the trimethylsilyl moiety (TMS) is observed. The disappearance of the absorption band at 3740 cm -1 , corresponding to the stretching vibrations of the isolated Si-OH groups, testifies in favour of the existence of a covalent bond linking the TMS group to the surface OH groups through a Si-O-Si bond. In addition, surface lithiation was performed to study both the influence of lithium addition on the ionic conductivity as well as the role of labile protons (Figure 3, blue curve). The decrease of the Si-OH absorption band of the isolated silanols at 3740 cm -1 attests to an at least partial lithiation of the surface silanols. Finally, the FTIR spectrum analysis of calcined silica NPs (Figure 3, green curve) reveals the increase in intensity of the absorption band of the isolated Si-OH groups at 3740 cm -1 . In accordance with the literature, these groups were formed by the dehydroxylation of silanols involved in hydrogen bonds (with water molecules or vicinal silanol groups). [START_REF] Dugas | Surface hydroxylation and silane grafting on fumed and thermal silica[END_REF] Conversely, after rehydroxylation (black curve, OH-SiO 2 ) the absorption of the isolated Si-OHs loses intensity to the detriment of silanols linked with hydrogen bonds, whose absorption band gains intensity at 3670 cm -1 . Finally, the absorption band located at 970 cm -1 , identified as the δSi-OH deformation vibration, is absent from the spectrum of calcined NPs. This information attests to the presence of siloxane bridges. Indeed, the literature testifies to the formation of these bridges from 400 °C, by the thermal dehydration reaction during which two neighbouring silanols condense into a siloxane bridge. [START_REF] Dugas | Surface hydroxylation and silane grafting on fumed and thermal silica[END_REF] At 500 °C, we thus observe at the surface of the nanoparticles a decrease in the Si-OH groups linked by hydrogen bonds, in parallel with an increase in the isolated silanol groups as well as an increase in siloxane bridges.

Ionic conductivity measurements of unmodified and modified silica NPs

To prepare ionic conductivity measurements by Electrochemical Impedance Spectroscopy (EIS), 13.8 wt% of LiTFSI was mixed with an aqueous suspension containing OH-SiO 2 or Li-SiO 2 , in order to add mobile lithium ions into the material. On the other hand, MS-SiO 2 was suspended in acetonitrile due to its hydrophobic surface; the calcined SiO 2 sample was similarly handled to prevent the rehydratation of its calcined surface. This relatively low proportion of LiTFSI was chosen intentionally to allow the measurement of the ionic conductivities of the materials (above the detection limit of the measuring devices) while not making the conductivity of LiTFSI interfere with the conductivity values of the studied samples. This fact is important since the goal of this study is to clearly observe and understand the contribution of the surface on the conduction properties.

All samples were transformed into powder, then dried under vacuum at 110 °C overnight before being transferred to a glove box under argon atmosphere. The solid electrolyte obtained did not degrade under air; however, it becomes more hygroscopic as an increasing amount of LiTFSI is added. This is why a particular attention was paid to avoid any contact with water and moisture during the transfer, at the risk of distorting the conductivity results

with the presence of water traces. [START_REF] Bidal | Van Hybrid Electrolytes Based on Optimized Ionic Liquid Quantity Tethered on ZrO2 Nanoparticles for Solid-State Lithium-Ion Conduction[END_REF] The ionic conductivity of SiO 2 /13.8 wt% LiTFSI mixtures of unmodified and modified SiO 2

NPs were then studied by EIS at different temperatures (Figure 4). Initially, the EIS results

were plotted in a complex plane in the same way as the OH-SiO 2 /13.8 wt% LiTFSI sample below (Figure 4a). All samples have the same appearance: at high frequencies, a semicircle can be seen on each Nyquist plot. At lower frequencies, there is a straight line due to a capacitive effect characteristic of ion blocking electrodes. The line is not completely vertical due to the surface roughness and porosity of the pellets. An appropriate equivalent circuit (Figure 4a, inset) was used to extract the values corresponding to the resistance of the R1 electrolytes. The ionic conductivity is then calculated for each temperature from the expressions proposed by Ingram et al. [START_REF] Ingram | Transport and defect mechanisms in cuprous delafossites. 2. CuScO 2 and CuYO2[END_REF] The temperature dependence of the ionic conductivity for X-SiO 2 /13.8 wt% LiTFSI is shown in Figure 4b. The conductivity of each sample was calculated from 20 °C to 100 °C. No decomposition or phase transition was observed in this temperature range, as shown by the linear increase in conductivity with temperature for all samples. Depending on the surface treatment, different behaviours have been observed. The ionic conductivity of OH-SiO 2 is very low, below 60 °C, therefore it could only be measured from 60 °C to 100 °C (8.0 x 10 -10 and 4.5 x 10 -8 S.cm -1 respectively). After calcination at 500 °C, the ionic conductivity of SiO 2 could only be recorded from 80 °C to 100 °C, respectively 1.5 x 10 -9 to 1.0 x 10 -8 S.cm -1 . The decrease in conductivity can be explained by the increase in the number of siloxane bridges at the expense of isolated Si-OH groups and silanols engaged in hydrogen bonds. Contrary to Si-O-Si bridges, free surface silanols seem to promote conductivity through interactions and complexations of the lithium cation with oxygen atoms.

Thus, the -OH groups located on the silica surface are involved in ionic conduction.

The coating of the nanoparticle surface with HMDS (MS-SiO 2 ), was performed to evaluate the role of the labile proton in the conductivity. In the case of MS-SiO 2 , the conductivity is relatively stable between 20 °C and 100 °C which is quite surprising. These measurements suggest an absence of interaction between the surface and the whole lithium salt, thus inducing a low conductivity (no dissociation) and independent of temperature (no interaction with the surface) even if the ionic conductivities of OH-SiO 2 and MS-SiO 2 are comparable at 80 °C. Therefore, although labile protons have been removed, ion conduction is still allowed, demonstrating that labile protons are not involved in the conductivity. On the other hand, the ionic conductivity within Li-SiO 2 at 60 °C is one decade higher than that of OH-SiO 2 ; this increase is directly attributed to the presence of additional lithium cations on the surface of the Li-SiO 2 material and borne by silanolate groups. Nevertheless, the ionic conductivities of the lithiated and hydroxylated NPs can be measured only at 60 °C and above. This confirms that the trimethylsilane fragment and TFSI interaction releases the lithium cation and thus improves the conduction at lower temperatures. Surface modifications of silica nanoparticles have shown that oxygen atoms participate in conduction while labile protons are not responsible.

Understanding evolution of the conductivity using molecular dynamics simulations

200 ns of molecular dynamics simulations were performed for each of the silica NPs-LiTFSI systems featuring OH-SiO 2 , OH-SiO 2 /Li-SiO 2 (9%), Li-SiO 2 , MS-SiO 2 or SiO 2 surfaces at various temperatures (250 K, 300 K, 350 K, 400 K and 450 K). We then characterized the effects of the distinct silica surface states using the radial distribution functions (RDF) of ions relative to the surface and the ionic number density profiles perpendicular to the surface.

Structuration of LiTFSI around the silica surfaces.

Figure 5 presents the RDF of the Li + and TFSI -ions relative to the silica surfaces at 350 K. It is clear that the surfaces possessing silanol oxygens do not behave the same in presence of LiTFSI than those that do not. For the surfaces bearing silanols, the first peak, sharp and well defined, is located at a distance of 0.2 nm of the silanol oxygens for Li + and 0.3 nm for TFSI -; for the latter, the second peak around 0.55 nm is also remarkably apparent.

This indicates a medium-range structuration of the ionic atmosphere due to strong interactions with the surface. On the other hand, the HDMS and calcinated surface feature monotonous RDFs for both ions, devoid of any peaks, which is indicative of high ionic mobility caused by very weak ion-surface interactions. We now examine the number density profiles for both ions along the axis perpendicular to the silica surface (Figure 6). The density is negligible inside the silica slab (where the ingress of ions is almost never observed) and has a relatively constant value far away from the surface which corresponds to the density of bulk LiTFSI. Near to the surface, interactions with the silica may result in excess of ionic densities which manifest as peaks.

Because amorphous silica surfaces are slightly irregular, these peaks can be split into several irregular sub-peaks; however, the resolution still allows to see a distinct alternance of cationic and anionic layers. These observations are consistent with published results obtained for other surfaces and/or ionic liquids. [START_REF] Aliaga | Surface chemistry of room-temperature ionic liquids[END_REF][START_REF] Federici Canova | Structural stability and polarisation of ionic liquid films on silica surfaces[END_REF][START_REF] Pal | Effects of Silica Surfaces on the Structure and Dynamics of Room-Temperature Ionic Liquids: A Molecular Dynamics Simulation Study[END_REF] Again, it is apparent that the surfaces bearing silanol oxygens interact differently with ions than those without. As expected, Li-SiO 2 shows the highest concentration of Li + ions on its surface, with the TFSI -ions relegated to the second layer. Li + also markedly interacts with the OH-SiO 2 surface, although the first Li + layer is a bit more distant from the surface. On the contrary, for MS-SiO 2 and calcinated surfaces the first peak is much further away, behind the first layer of anions which become the closest ions to the surface. While this effect could be ascribed purely to steric hindrance in MS-SiO 2 (due to the bulky -(CH 3 ) 3 groups), its occurrence for the calcinated surface indicates other causes.

Comparatively, the layering behaviors of TFSI -anions around all silica slabs are quite similar (considering the fact that the MS-SiO 2 surface is broader), even though these anions represent the first layer for calcinated and MS-SiO 2 systems and the second layer for the rest. Anta et al. argue that the number of TFSI -ions coordinated to Li + governs the conducting properties of the electrolyte. [START_REF] Vicent-Luna | Molecular Dynamics Analysis of Charge Transport in Ionic-Liquid Electrolytes Containing Added Salt with Mono, Di, and Trivalent Metal Cations[END_REF] In surfaces with silanol oxygens (whether protonated or not), the interfacial Li + ions interact with the silanol functions and are thus less bound to their TFSI -counterions than in pure LiTFSI, as indicated by the RDFs between Li + and TFSI -presented on Figure 7. While the RDF in bulk LiTFSI (i.e., far from the surface) is not affected by the surface type, differences near the surface are profound: for MS-SiO 2 and calcinated silicas, LiTFSI g(r) feature marked a first and a second peaks comparable to those found in the bulk, showing that the Li + /TFSI -ion pair is not dissociated by the presence of the surface groups; conversely, silanolated silica surfaces compete with TFSI -for interacting with Li + and thus at least partially dissociate the Li + /TFSI -ion pairs, which translates on the RDF plots as weaker peaks with no resemblance to the bulk.

Conductivity and its origins.

Figures S4a-e show the RDFs for the extreme temperatures of our simulations, i.e. 250

K and 450 K. The locations of the different peaks are not modified: they only differ by their heights and widths from their counterparts at 350 K. Indeed, the thermal energy injected into the system by the rise of temperatures is sufficient to lower or break the interactions of the ions with the surface: the ions thus become more mobile, which tends to increase the conductivity. As shown by the 9%-lithiated surface, this effect appears proportional to the surface's degree of lithiation. The number density profiles at 250 K and 450 K (Figures S5ay) paint a similar picture: at higher temperatures, the layering effect is less pronounced (especially for TFSI -). Sets of radial distribution functions and number density profiles for all systems at extreme temperatures (250 and 450 K) can be found in Supplementary

Information. From the molecular dynamics simulations at different temperatures, we compute the theoretical conductivities of our systems using the Einstein-Helfand model [START_REF] Eugene | C4 journal of experimental and theoretical physics established by K L. Nichols in 1S93 Transport Coefficients from Dissipation in a Canonical Ensemble[END_REF] (Figure 8)

and compare them to their experimental counterparts on Figure 4b. Theory and experiment agree that the ionic conductivity for the MS-SiO 2 surface is only slightly affected by temperature changes. As previously stated, the RDF and number density profiles clearly indicate that there is no strong interaction between the silica surface and the salt in this system. Raising the temperature thus only increases the thermal motion of the electrolyte ions, only causing a small elevation of the total conductivity. Ionic conductivities measured for the MS-SiO 2 /LiTFSI system also show almost no temperature dependence, just like in our simulations; considering how little the surface affects the Li + /TFSI -ion pairs (see above), we expect these conductivities to be close to those of bulk LiTFSI. Conversely, for all other systems, there is a marked correlation between conductivity and temperature, observed both in experimental measures and molecular dynamics results. The substantial augmentation of conductivity observed upon temperature elevation is not only linked to the increase of thermal motion of Li + /TFSI -, but is a consequence of the detachment of Li + cations from the surface via the thermal weakening of electrostatic interactions with the silanol oxygens. The resulting increase in the mobility of these cations, clearly indicated by the broadening and the shortening of the first peak of the Li-O silanol radial distribution function (Figure 5) upon temperature increase, contributes positively to conductivity via the facilitated diffusion of charges. In our model, conductivity thus appears as a tradeoff mediated by the relative strengths of the Li + /O silanol and Li + /TFSI -interactions. Once dissociated from its bulky TFSI -counterion by interaction with the silanols, Li + ions can diffuse efficiently due to their small radii, enhancing conductivity; however, at lower temperatures, they become "trapped" by the silanols and conductivity falls. Conversely, in the absence of strong interactions with the surface, the Li + /TFSI -ion pair endures regardless of temperature, but its large radius strongly limits its diffusive capabilities. The good match between experimental and theoretical results proves the dominance of this model over other possible sources of conductivity that are not included in our simulations: for instance, charge or electron transfer phenomena which require a quantum mechanical description are incompatible with long simulation timescales.

In the future, it would be interesting to study the interaction between LiTFSI and small POE oligomers and polymers using this synergistic approach, and then build a larger model to finely understand ion mobility and study how composite structure and composition affect ion mobility and transport.

CONCLUSIONS

Through this study, the origin of the conductivity of the LiTFSI in close interaction with a inorganic filler was scrutinized. By combining experimental measurements and in silico models, we were able to unambiguously ascribe differences in conductivity to the degree of dissociation of the Li + /TFSI -ion pair, which is itself modulated by interactions with the functionalized silica surface. It is now clear that the functionalization of oxygen atoms on the silica NPs plays a critical role in conduction properties. The replacement of labile protons with lithium induces a conductivity jump of one decade, revealing that conductivity is mediated by the relative strengths of the Li + /O silanol and Li + /TFSI -interactions. Conversely, by hindering the dissociation of LiTFSI, the grafting of hydrophobic methylsilane fragments prevents the interaction of Li with the silica surface and leads to a temperature-independent conductivity curve. Finally, the computational model employed here has been validated by its good agreement with the experimental results. It would be interesting to extend this approach to study other interactions such as lithium salt with different PEO-based oligomers/polymers or to predict the surface functionalization needed to optimize conductivity at a given temperature. These aspects could thus serve as a basis for building a larger model that could prove useful as a preliminary step in the future design of hybrid electrolytes in next generation batteries.

EXPERIMENTAL SECTION

Reagents and Chemicals

Chemicals and solvents such as tetraethylorthosilicate (TEOS), ammonia solution (30 wt%), lithium bis(trifluoromethylsulfonyl)imide salt (LiTFSI), 1,1,1,3,3,3hexamethyldisilazane (HMDS), lithium hydroxide and ethanol were purchased form Sigma Aldrich as reagent grade or better and used as received. Toluene was predried over sodium and distilled prior to use. HMDS was purchased dried and was stored under argon in a glovebox.

Instrumentation

N 2 -adsorption isotherms were measured at liquid nitrogen temperature with a Micrometric ASAP 2020 porosimeter. Prior to analysis, the sample surface was outgassed at 120 °C for 3 h under vacuum (ca. 1 mm Hg). The Brunauer-Emmett-Edward (BET) surface area was calculated from the adsorption part of the isotherm between 0.03 < P/P 0 < 0. (1 °C•min -1 ) with a temperature stabilization of 15 min before impedance measurement every 20 °C. Modelling and simulation of the EIS data were performed using an equivalent circuit model described in the discussion section. Quantitative analyses were done using the Zview software (Scribner Associates, Inc.). Conductivity measurements were realized three times (second measurement on the same pellet and one measurement on a new pellet) in order to ensure reproducibility.

Synthesis of silica nanoparticles

Silica nanoparticles were obtained using the Stöber process, briefly described as follows. [START_REF] Stöber | Controlled growth of monodisperse silica spheres in the micron size range[END_REF] A solution of absolute ethanol (500 mL) was mixed to a 30 wt% ammonia solution (38 mL, 0.29 mol) and the mixture was stirred for 1 h at 70 °C. TEOS (15 mL, 67.25 mmol) was then added dropwise. The first nanometric objects were observed after a few minutes, thus forming a translucent colloidal suspension. The solution was stirred for 12 h to completely hydrolyze the Si-precursor. Silica nanoparticles were recovered by centrifugation (8000 rpm) and washed several times with absolute ethanol and distilled water.

The inorganic nanoparticles were calcined at 500 °C (5 °C/min heating rate) for 6 h in order to remove residual solvent and organic species remaining on the silica surface. After being cooled to room temperature, silica nanoparticles were hydrated in a concentrated (37 %) HCl solution at 100 °C for 12 h. The silica nanoparticles (named OH-SiO 2 ) were then washed with water by centrifugation until neutral pH and finally dried under vacuum at 100 °C before use.

LiOH modification of silica surface (Li-SiO 2 )

Considering the silica specific surface area (64 m².g -1 ) and considering a maximum of two OH per nm² at the SiO 2 surface, 3 equivalents of LiOH were added to 1 g of the synthesized OH-SiO 2 suspended in water. The obtained mixture was stirred at room temperature overnight and then washed five times by centrifugation with water. The nanoparticles (Li-SiO 2 ) were finally recovered after solvent evaporation and dried overnight under vacuum at 100 °C.

Hydrophobic capping of silica nanoparticles (MS-SiO 2 )

The synthesized silica nanoparticles OH-SiO 2 (1 g) were dried under vacuum at 100 °C overnight before being suspended in 20 mL of dry toluene. An excess of dried HMDS (1 mL) was added to the suspension and stirred overnight at room temperature under inert condition. The obtained mixture was washed five times by centrifugation with toluene then the resulting powder (MS-SiO 2 ) was dried under vacuum at 120°C overnight.

Suppression of silica surface hydroxyl groups (SiO 2 )

OH-SiO 2 were calcined at 500 °C (5 °C/min heating rate) overnight in order to remove as many hydroxyl groups of the silica surface as possible. After calcination, the material (SiO 2 )

was transferred to an Argon-filled glovebox to avoid any adsorption of moisture.

LiTFSI impregnation

Depending on the LiTFSI weight percentage, a desired amount of LiTFSI dissolved in 20 ml of water (OH-SiO 2 and Li-SiO 2 ) or anhydrous acetonitrile (SiO 2 and MS-SiO 2 ) was added under inert atmosphere to the unmodified and modified silica nanoparticles. The suspension was then homogenized by sonication. The solvent was removed by a slow evaporation with vigorous stirring at 70 °C overnight.

Simulation details

All molecular dynamics simulations were carried out using GROMACS 2018.3. [START_REF] James | ScienceDirect GROMACS : High performance molecular simulations through multi-level parallelism from laptops to supercomputers[END_REF] In a first step, equilibrated structures of the different silica slabs were obtained from NPT simulations at 500 K, which were subsequently used as starting points for stepwise temperature annealing simulations. Equilibrated initial frames for production runs at 250 K, 300 K, 350 K, 400 K and 450 K were thus obtained. Finally, 200 ns production simulations, from which the diffusion and conductivity values were measured, were performed at each of these temperatures for all silica slabs. These simulations used a 2 fs integration timestep; temperature in the NVT ensemble was regulated using a Nose-Hoover thermostat with a time constant of 1.3 fs and separate couplings to the silica slab, cations and anions. All-atom models of silica slabs (surface-capped and calcinated) including geometry and force field parameters were taken from reference [START_REF] Emami | Force Field and a Surface Model Database for Silica to Simulate Interfacial Properties in Atomic Resolution[END_REF]. An amorphous surface with 4.7 silanol groups per nm 2 is suited to model medium Stöber silica particles, the surface area being approximately of 4 nm × 4 nm and the slab thickness 2.4 nm. Force field parameters for LiTFSI were taken from references [START_REF] Sambasivarao | Development of OPLS-AA force field parameters for 68 unique ionic liquids[END_REF][START_REF] Doherty | Revisiting OPLS Force Field Parameters for Ionic Liquid Simulations[END_REF]; as explained in these works, respective charges of 0.8e and -0.8e for the cation and anion were used. The silica-LiTFSI boxes sizes were in the 4 nm × 4 nm × 8 nm range and the number of salt ion pairs were calculated to reproduce the experimental LiTFSI bulk density (1.33 g.cm -3 ). The deprotonated surfaces were obtained by deleting hydrogen atoms from the Si-OH groups and adding chloride counterions. The hydrophobic surface was obtained by replacing the hydrogen atoms of the Si-OH groups by a Si(CH 3 ) 3 function, whose parameters were taken from the AMBER99SB force field [START_REF] Hornak | Las instituciones de ciencia y tecnología en los procesos de aprendizaje de la producción agroalimentaria en Argentina[END_REF] and the atomic charges obtained using REDserver. [START_REF] Vanquelef | Server: A web service for deriving RESP and ESP charges and building force field libraries for new molecules and molecular fragments[END_REF]

Figure 1 : 2 nanoparticles of diameter 40 -

 1240 Figure 1: (a) FTIR spectroscopy in diffuse reflectance mode of OH-SiO 2 nanoparticles of diameter 40-45 nm. (b) Bright field TEM image of silica nanoparticles. Size distribution measured on a sample of 300 OH-SiO 2 nanoparticles are available in ESI Fig S1. (c) Nitrogen adsorption (red) and desorption (black) isotherms of synthesized OH-SiO 2 nanoparticles of diameter 40-45 nm.

Figure 2 :

 2 Figure 2: Surface modifications of hydroxylated SiO 2 NPs (OH-SiO 2 ) by surface capping with HMDS (MS-SiO 2 ), Lithiation (Li-SiO 2 ) and hydroxyl-free (SiO 2 ).

Figure 3 :

 3 Figure 3: FTIR spectra of OH-SiO 2 nanoparticles (black curve) and surface-modified silica nanoparticles through calcination (SiO 2 , green curve), methyl-silane capping (MS-SiO 2 , red curve) and surface lithiation (Li-SiO 2 , blue curve).

1 )Figure 4 :

 14 Figure 4: a) Nyquist diagrams performed at 60 °C, 80 °C, 100 °C of OH-SiO 2 /13.8 wt% in a blocking electrode/solid electrolyte/blocking electrode system and corresponding equivalent circuit. Nyquist diagrams for other samples are available in SI (Figure S3). b) Temperature dependencies of the ionic conductivity of MS-SiO 2 , Li-SiO 2 , OH-SiO 2 and SiO 2 /13.8 wt% LiTFSI, measured between 20 °C and 100 °C.

Figure 5 :

 5 Figure 5: Radial distribution functions at 350 K between surface atoms and Li + /TFSI -ions. The reference surface atoms are silanol oxygens for the OH-SiO2 and Li-SiO2 surfaces, carbon atoms for MS-SiO2 and silicon atoms for SiO2.

Figure 6 :

 6 Figure 6: Number density profiles of Li + and TFSI -at 350 K, along the axis perpendicular to the silica surface. The zero-density region of the curves corresponds to the position of the silica slab.

Figure 7 :

 7 Figure 7: Radial distribution functions at 350 K between a Li cation and the nitrogen atom of the TFSI anion, obtained in the vicinity of the silica surface and in bulk solvent.

Figure 8 :

 8 Figure 8: Static conductivities obtained using Einstein-Helfand fitting.

  3 using a multipoint method. The different synthesized materials were characterized by FTIR spectra in diffuse reflectance mode (DRIFT) using a Nicolet iS10 FTIR spectrometer from Thermo Scientific. The spectra were acquired at room temperature over a range of 4000-400 cm -1 with a resolution of 8 cm -1 . Electrochemical impedance spectroscopy (EIS) measurements were realized with an MTZ-35 frequency response analyzer (BioLogic Company) coupled with an Intermediate Temperature System (ITS) controlling the sample temperature by Peltier effect.The different synthesized materials were cold-pressed into pellets of 10 mm diameter under a pressure of 5 tons with around 70 mg of powder per pellet. Thicknesses of about 0.60 mm were obtained. The pellets were sandwiched between two aluminium foils (sheet thickness: 0.006 mm) enhancing the electrical contacts between the pellet surface and the controlled environment sample holder (CESH) gold disks. The combined aluminium foil and gold disk ensured an ion blocking electrode. The electrolyte pellet was transferred under argon (in a glovebox) into an airtight environment asymmetric CESH to be sandwiched between the twoion blocking aluminium-gold electrodes. The CESH was then placed in ITS, and AC impedance spectra was recorded in the frequency range from 30 MHz to 0.1Hz with an excitation signal of 5 mV amplitude and from 20 to 100 °C upon heating and cooling

ACKNOWLEDGMENTS

The authors would like to thank jointly the French Research Network on Electrochemical Energy Storage (RS2E) and the Direction Générale de l'Armement (DGA) for supporting this project through the Ph.D. grant of Jennifer Bidal. The authors would like to thank Benoit Fleutot for his valuable contribution and helpful assistance for electrochemical measurements.

The calculations presented in this work were performed using HPC resources from the MATRICS computing platform of Université de Picardie Jules Verne.

AUTHOR INFORMATIONS

Corresponding authors: Matthieu Becuwe, matthieu.becuwe@u-picardie.fr Orcid: Matthieu Becuwe-0000-0002-1949-3955 ; Benjamin Bouvier-0000-0001-8782-2426

CONFLICT OF INTEREST

The authors declare no conflict of interest.

Keywords: hybrid electrolyte, interaction salt-filler, surface modification, lithium mobility, molecular dynamic


Mehdi Tazi

Madani Hedidi

William Erb

Yury S Halauko

Oleg A Ivashkevich

Vadim E Matulis

Thierry Roisnel

Vincent Dorcet

Ghenia Bentabed-Ababsa

Florence Mongin

Fluoro-and Chloroferrocene: From 2-to 3-substituted Derivatives

Ferrocene halogen 'dance' is deemed hardly controllable. Here, we describe the first efficient examples of this reaction. Conversion of 1-fluoro-2-iodo-(or 1-chloro-2-iodo-) to 1-fluoro-3-iodo-(or 1-chloro-3-iodo-) ferrocenes was ensured by suitably protecting the free 5 position.

Ferrocene is present in numerous scaffolds for applications such as catalysis, material sciences and medicinal chemistry. 1 However, despite a very rich history, transformation of monosubstituted ferrocenes is challenging when it comes to regioselectively introducing a function at C3. While deprotometallation has been largely developed to introduce a substituent next to a directing group, 2 one can deplore the lack of an equally general method to access 1,3-disubstituted ferrocenes in spite of promising applications. 3 If building the ferrocene core is a way to obtain such structures, 4 ferrocene functionalization avoids the tedious synthesis of the required building blocks. Whereas direct electrophilic substitutions on monosubstituted ferrocenes are hardly made regioselective, 5 current stoichiometric and catalytic remote CH-functionalizations stay restricted to specific examples. 6 Alternative approaches take the central group (Cl, [START_REF] Slocum | Directed Metalation Reactions. Specific Route to 1,2,3-Trisubstituted and 1,3-Disubstituted Ferrocenes[END_REF] Br 8 or sulfoxide 9 ) away from 1,2,3trisubstituted ferrocenes, but remain lengthy. Herein, we report a convenient alternative which relies on the halogen 'dance' (I > Br) known for benzene derivatives. [START_REF] Gronowitz | Recent Advances in the Chemistry of Thiophenes[END_REF] Little is described on this ferrocene base-triggered reaction allowing 2-halogeno substituted compounds to be converted into the 3-halogeno derivatives. Since our early observation of the unintended formation of 1-bromo-3-iodoferrocene in the course of bromoferrocene deprotonation using LiTMP (TMP = 2,2,6,6-tetramethylpiperidino) in the presence of a zinc trap followed by iodolysis, 11 very few investigations have been made. In 2015, Wang, Weissensteiner and co-workers reported a study in which scrambling was noticed upon treatment of 1,2-dihalogenoferrocenes (1,2-diiodo-, 1,2-dibromo-, 1bromo-2-iodo-and 1-chloro-2-iodo-) by LiTMP in THF (tetrahydrofuran) at -30 °C. [START_REF] Zirakzadeh | Halide-Mediated Ortho-Deprotonation Reactions Applied to the Synthesis of 1,2-and 1,3-Disubstituted Ferrocene Derivatives[END_REF] We recently showed that using N,N-dialkyl 2-iodoferrocenecarboxamides, in which a fixed directing group (DG) is present, reduced the complexity in favour of a better controlled migration; however competitive deiodination and formation of diiodo derivatives were not discarded, at the origin of yield loss. [START_REF] Tazi | From 2-to 3-Substituted Ferrocene Carboxamides or How to Apply Halogen "Dance" to the Ferrocene Series[END_REF] These results led us to believe (i) that the ferrocene substrates we used were not acidic enough to ensure the disappearance of LiTMP in the course of the reaction, and (ii) that it is necessary to block the free position next to the DG. As a consequence, we chose fluorine and chlorine as efficient short-range acidifying substituents to stabilize the produced ferrocenyllithiums, [START_REF] Mongin | Substituent Effects on the Relative Rates and Free Energies of Ortho-Lithiation Reactions: Families of Fluorobenzenes as the Substrates[END_REF] and selected trimethylsilyl (a poor DG) as a protecting group for both the 5 and 4 positions (Fig. 1). We first tested this designed halogen 'dance' reaction by using fluorine as DG. To access the required substrate, we initially studied the functionalization of fluoroferrocene (1a), itself prepared from ferrocene. NFSI (N-fluorobenzenesulfonimide), which has a lower redox potential than Nfluoropyridinium triflate and Selectfluor, is a suitable fluorinating agent towards lithioferrocene; 15 optimization of the reaction (see ESI) allowed 1a to be isolated in an average 50% yield at a scale up to 50 mmol. It is worth noting that the sulfone concomitantly formed when trapping with NFSI may become a real issue from sterically hindered ferrocenes such as the N,N-diisopropylcarboxamide (Scheme 1). Scheme 1. Expected and Unexpected Ferrocenes Formed by Deprotolithiation-NFSI Trapping. Conditions a: 1) tBuLi (2 equiv), tBuOK (0.1 equiv), THF, -75 °C, 1 h; 2) NFSI, THF, -20 °C. Conditions b: 1) sBuLi (1.2 equiv), THF, -75 °C, 1 h; 2) NFSI, THF, -20 °C.

In their synthesis of pentafluoroferrocene from 1a by iterative deprotometallation-fluorolysis, Sünkel and co-workers employed LiTMP. 15a In order to achieve a strict monofunctionalization of 1a, we rather turned to alkyllithiums. sec-Butyllithium proved superior to butyllithium and avoided the use of additives such as N,N,N',N'-tetramethylethylenediamine [START_REF] Herbert | Manipulating the Diastereoselectivity of Ortholithiation in Planar Chiral Ferrocenes[END_REF] or potassium tert-butoxide. [START_REF] Schlosser | Superbase Reactions: the Expedient and Selective Metalation of Fluorine-or Trifluoromethyl-Substituted Benzenes[END_REF] After 1 h contact time at -75 °C in THF, the 2-lithio derivative was quantitatively formed, as evidenced by subsequent deuteriolysis (Table 1, entry 1). Its trapping by electrophiles afforded 1,2-dihalides 3b-e (entries 2-5), silane 3f (entry 6), phosphine 3g (entry 7), sulfide 3h (entry 8), alcohols 3i-m (entries 9-13), aldehyde 3n (entry 14), ketone 3o (entry 15) and acid 3p (entry 16) in high to excellent yields.

This effective fluoroferrocene functionalization can be pushed a step further with iterative deprotometallationtrapping sequences. Using sec-butyllithium and two different electrophiles, we managed to develop a one-pot access to 3fb, i.e. the required substrate to attempt our designed halogen 'dance' (Scheme 2, top). Scheme 2. Halogen 'dance' from a Fluoroferrocene Deriving from 1a.

Table 1. Scope of the Deprotolithiation-trapping Sequence on 1a.

a Except for D2O and CO2 which were used in excess. b After purification, as described in the ESI. c >95% D. d By reaction of the crude with BH3•THF, and subsequent regeneration of the free phosphine (see SI). [START_REF] Argouarch | A New Class of Ferrocene-Based 1,2-Bis(phosphanes) Possessing only Planar Chirality[END_REF] e Diastereoisomeric ratio (see ESI). f After acidic hydrolysis.

Whereas it took 14 h to achieve halogen 'dance' from N,Ndiisopropyl 2-iodoferrocenecarboxamide, [START_REF] Tazi | From 2-to 3-Substituted Ferrocene Carboxamides or How to Apply Halogen "Dance" to the Ferrocene Series[END_REF] the reaction from 3fb only required 2 h, providing 3fb-mig in 77% yield after methanolysis and purification (Scheme 2, bottom; Fig. 2). This better result might come from the higher acidity of 3fb at the position next to the DG, leading to a more stable lithio product, and therefore to less LiTMP in the reaction mixture. As a consequence, the deiodinated product 3f, probably formed by LiTMP-mediated iodine/lithium exchange, [START_REF] Mongin | Extensive Halogen Scrambling and Buttressing Effects Encountered upon Treatment of Oligobromoarenes with Bases[END_REF] was only identified in the crude as minor product (<10%). The trimethylsilyl group here perfectly played its role, avoiding competitive deprotonation at the origin of undesired products. As shown by the calculated pKa values in THF, [START_REF] Matulis | Calculations were performed within the DFT framework at the CAM-B3LYP/6-311+G(d,p)//CAM-B3LYP/6-31G(d) level using a theoretical protocol elaborated previously[END_REF] chloroand fluoroferrocene have similar CH acidities (see Fig. 3, top left). In order to synthesize 6fb and test the halogen 'dance' on this chloro analogue of 3fb, we started from chloroferrocene 21 (5a) and carried out two deprotometallation-trapping sequences. If butyllithium 22a,b,7,22c and LiTMP 21,12,22c can be used to deprotonate 5a, we chose the more reactive sec-butyllithium to efficiently access 6f and 6fb. Under the conditions used for fluoride 3fb, chloride 6fb was isomerized to afford 6fb-mig in high yield (Scheme 3, left & bottom).

In order to evaluate the need of a protecting group, 1chloro-2-iodoferrocene (6b) was similarly prepared and treated by the lithium amide. Under the conditions used for silylprotected 6fb (Scheme 3, bottom; Fig. 3, bottom right), 1chloro-3-iodoferrocene (6b-mig) was identified, but in a low 23% conversion (Scheme 3, right). Although further studies are required, this result might come from competitive deprotometallation next to the chlorine substituent. Indeed, 6b show similar pKa values at its positions next to Cl and I (while 3b appears as an even less appropriate substrate; Fig. 3, top right). This study also evidenced remarkably stable 2-halogenoferrocenyllithiums when compared to their 2-halogenophenyl analogues. In particular, 1-chloro-2-lithioferrocene is quite stable at -15 °C whereas chlorobenzene has to be deprotolithiated at about -100 °C to avoid benzyne formation. [START_REF] Iwao | Directed Lithiation of Chlorobenzenes. Regioselectivity and Application to a Short Synthesis of Benzocyclobutenes[END_REF] On the basis of the calculated pKa values for both 2-halogenoferrocenes and benzenes (see Fig. 3, left), one might suggest that the unstability of 2-halogenoferrocenyllithiums is not at the origin of ferrocyne formation. 22a,24 Thus, efficient methods were developed from fluoro-and chloroferrocene to access 2-and 3-substituted derivatives, allowing original ferrocene-based halogenated compounds to be built. As fluorinated aromatic compounds are present in numerous pharmaceuticals and agrochemicals (in which fluo-rine substituent improves both membrane permeability and metabolic stability), [START_REF] Roesner | Continuous-Flow Synthesis of Biaryls by Negishi Cross-Coupling of Fluoro-and Trifluoromethyl-Substituted (Hetero)arenes[END_REF] it is of interest to build ferrocene analogues of such scaffolds in the search for derivatives with improved activity. Further studies on the chemistry of fluoroferrocene derivatives and their use in halogen 'dance' reactions with interception of the rearranged lithioferrocenes by various electrophiles towards tetrasubstituted derivatives are ongoing and will be reported in due course. Scheme 3. Halogen 'dance' from chloroferrocenes.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.XXX.

Experimental procedures, compounds analyses, X-ray crystallographic details, NMR spectra (PDF) Crystallographic data (CIF)

Figure 1 .

 1 Figure 1. Strategy used to achieve efficient ferrocene halogen 'dance' reactions.

Figure 2 .

 2 Figure 2. ORTEP diagrams (30% probability level) of silylprotected 1-fluoro-2-iodo-ferrocene (3fb) before (left) and after (right) halogen migration.

Figure 3 .

 3 Figure 3. Calculated pKa values in THF for the halogenoferrocenes 1a, 5a, 3b, 6b and 6fb, fluorobenzene and chlorobenzene.

Accession Codes CCDC

  1841221-1841234. The crystallographic data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

ACKNOWLEDGMENTS

This work was supported by the Ministère de l'Enseignement supérieur, de la Recherche et de l'Innovation (grant to M.T.), the Université de Rennes 1, the Ministère de l'Enseignement supérieur et de la Recherche scientifique Algérien. We also thank the Centre National de la Recherche Scientifique and Rennes Métropole. We acknowledge the Fonds Européen de Développement Régional (FEDER; D8 VENTURE Bruker AXS diffractometer) and Thermofisher (generous gift of 2,2,6,6-tetramethylpiperidine).

AUTHOR INFORMATION

Corresponding Author * E-mail for W.E.: william.erb@univ-rennes1.fr. * E-mail for Y.S.H.: hys@tut.by.

ORCID

William Erb: 0000-0002-2906-2091 Thierry Roisnel: 0000-0002-6088-4472 Vincent Dorcet: 0000-0001-9423-995X Florence Mongin: 0000-0003-3693-8861

Notes

The authors declare no competing financial interest.


Mehdi Tazi

Thierry Roisnel

Florence Mongin

William Erb
email: william.erb@univ-rennes1.fr

Ferrocene carboxylic anhydride: identification of a new polymorph Authors

Keywords: crystal structure, ferrocene, acid anhydride, hydrogen bonds, polymorph

A new polymorph of ferrocenecarboxylic anhydride was obtained, characterized and its structural features were compared to the other cyclopentadienyl carboxylic anhydrides.

Introduction

Anhydrides are a well-known class of acylation reagents widely used in organic synthesis. While they are usually prepared intentionally for synthetic use, serendipity sometimes joins the game and leads to unexpected new compounds, as it is the case for ferrocene carboxylic anhydride (1, Scheme 1). It was first reported in 1959 by Lau and Hart during studies to access ferrocene peroxide (Lau & Hart, 1959).

By refluxing ferrocene carboxylic chloride (FcCOCl) with sodium peroxide, they isolated 1, which was characterized by its melting point, IR spectrum and elemental analysis. The same year, Acton and Silverstein obtained 1 during the synthesis of ferrocene carboxamides from ferrocene carboxylic acid (FcCO2H) in the presence of dicyclohexylcarbodiimide (Acton & Silverstein, 1959). Reference samples were further prepared by the reaction of FcCOCl with water in the presence of pyridine. In 1966, Kupchik and Kiesel reported the unexpected formation of 1 during the reaction of FcCOCl with tributyl-tin hydride, supposed to deliver the corresponding ferrocenecarboxaldehyde or ester (Kupchik & Kiesel, 1966). A reference sample was also prepared, that time by reacting FcCOCl with FcCO2Na. It is almost 40 years later that Runqiu and Qingmin reported the formation of 1 (up to 47%) during the synthesis of FcCOCl in the conditions described by Lau and Hart (Runqiu & Qingmin, 2001). In addition to the previously reported melting point, IR spectrum and elemental analysis, the 1 H NMR spectrum and mass analysis of ferrocene carboxylic anhydride were described. In 2004, Schetter and Speiser also reported the formation of 1 during the reaction of FcCO2H with various carbodiimides (Schetter & Speiser, 2004). Finally, in 2016, McAdam and Simpson reported that 1 was isolated during the reaction of FcCO2H with triphosgene as a by-product (McAdam & Simpson, 2016). In tis case the crystals suitable for X-ray analysis were obtained and the solid-state structure has been determined.

Apart from these synthesis reports, 1 was sporadically used as a reagent. Wenzer was the first one to use it for the esterification of cholesterol in the presence of pyridine (Hoffmann et al., 1980). In 1990, Eckert reported the amidation of H-Phe-OtBu in the presence of trimethylamine and the reaction of 1 with bovine serum albumin in buffered solution (Eckert & Koller, 1990). In 2004, Kihara and Takata used 1 in the synthesis of a redox-active rotaxane (Kihara et al., 2004). A ferrocene stopper was introduced by the esterification of an alcohol in the presence of catalytic amounts of nBu3P. The same year, Schetter and Speiser reported the amidation of 3-(triethoxysilyl)propylamine with 1 in the presence of a carbodiimide in 37% yield (Schetter & Speiser, 2004). Finally, in 2012, Burrows and Marken reported the post-functionalization of metal-organic frameworks with 1 to access new redoxactive materials (Halls et al., 2012). It is to note that, apart from the work of Kihara and Tanaka, the yields of ester and amide formations remain surprisingly low (20 to 47%). This lack of reactivity was also noticed by Runqiu and Qingmin who failed to react 1 with primary or secondary amines and with hydrazines (Runqiu & Qingmin, 2001).

It appears that 1 is usually obtained as a by-product in the synthesis of FcCOCl with various chlorinating reagents or during the formation of amides in the presence of carbodiimides. In the course of a program dedicated to the synthesis of new ferrocene carboxamides (Palabindela et al., 2016), we reacted FcCO2H with thionyl chloride to access FcCOCl. During the purification steps, column chromatography afforded an unknown product, and 1 H and 13 C NMR studies revealed the presence of a monosubstituted ferrocene with a carboxyl group, which was not an acid, an acyl chloride, nor an ester. While the possibility of a stable anhydride raised in our minds, slow crystallization of the pure product occurred, and the solidstate structure obtained by X-ray diffraction confirmed our hypothesis. However, instead of the known crystal structure already described by McAdam and Simpson (McAdam & Simpson, 2016), we managed to obtain a previously unknown polymorph of 1. Its spectroscopic data and crystal structure will be described here and compared to other cyclopentadienyl (Cp) carboxylic anhydrides.

Experimental

Synthesis and crystallization

Dimethylformamide (0.1 ml) and SOCl2 (6.4 ml, 88 mmol, 2.2 eq.) were added dropwise to a suspension of FcCO2H (9.2 g, 40 mmol, 1.0 eq.) in dry dichloromethane (110 ml) at room temperature (Scheme 2).

After 15 min, the reaction mixture was concentrated under vacuum and the residue was dissolved in dry dichloromethane (150 ml) before diisopropylamine (17 ml, 120 mmol, 3.0 eq.) was added dropwise.

The reaction mixture was then stirred at room temperature until reaction completion. HCl (1M) was added until pH 1 and the reaction mixture was extracted with EtOAc. The combined organic layers were dried over MgSO4, filtrated and concentrated under vacuum to give the crude product. Purification by column chromatography, eluting with heptane/EtOAc (80:20) afforded the desired FcCOCl as the main product and the new ferrocene carboxylic anhydride polymorph 1B by crystallization upon standing (0.18 g, 2%). Mp 113-116 °C.

Spectroscopic analyses

IR spectra were taken on a Perkin-Elmer Spectrum 100 spectrometer. 1 H and 13 C Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker Avance III at respectively 500 MHz and 125

MHz. 1 H chemical shifts (δ) are given in ppm relative to the solvent residual peak and 13 C chemical shifts are relative to the central peak of the solvent signal. Peak assignments were based on twodimensional experiments. νmax (film)/cm -1 2919, 1768 (C=O, sym), 1713 (C=O, asym), 1444, 1243, 1067, 1044, 1007. 1 H NMR (500.0 MHz, CDCl3): δ (ppm) 4.88 (t, J = 1.9 Hz, 4H, 4 x Fc CH), 4.54 (t, J = 1.9 Hz, 4H, 4 x Fc CH), 4.35 (s, 10H, 10 x Fc CH). 13 C NMR (125.7 MHz, CDCl3): δ (ppm) 167.9

(2 x C=O), 72.9 (4 x Fc CH), 71.0 (4 x Fc CH), 70.4 (10 x Fc CH), 69.6 (2 x Fc C).

Refinement

The crystal structure was solved in the non-centrosymmetric Cc space group by dual-space algorithm using the SHELXT program (Sheldrick, 2015), and then refined with full-matrix least-square methods based on F 2 (SHELXL) (Sheldrick, 2015). The use of combined TWIN and BASF instructions led to a Flack parameter value of 0.21(3), corresponding to a mixture of ~80% of a particular enantiomer and ~20% of the opposite one. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were included in calculated positions. In the absence of restraints or constraints on ADP's of C51-C55 ring atoms, some B and C alerts appeared in the CHECKCIF procedure, due to large values of ADP max/min ratios, especially on C52 (ratio = 4.2) and C55 (ratio=3.5). Introduction of restraints by RIGU instructions applied on these C51-C55 atoms in SHELXL refinement led to decrease these ADP max/min ratios (4.0 and 3.2 respectively) and provided more realistic anisotropic atomic displacements on this Cp ring. No constraints were used to refine the other rings. CCDC 1553140.

Results and discussion

The molecular structures of previously described by Simpson orthorhombic polymorph (1A, Figure 1, top) obtained and the monoclinic polymorph (1B, Figure 1, bottom) feature various differences and the crystallographic data of both polymorphs are disclosed in Table 1. The crystal packing of 1B is shown in Fgure 2. Only one molecule is present in the asymmetric unit of 1A, while three independent molecules crystallized in the unit of 1B. It can be seen that the anhydride molecules comprising the Fe1/Fe2 and Fe5/Fe6 atoms appear nearly identical by a 180 ° rotation around the b axis. This is confirmed by their molecular overlay (Mercury automatic overlay module was used), with the metal atom (Fe1/Fe2 and Fe5/Fe6) located on the same side of the anhydride (Figure 3, bottom, RMSD of 0.1726). However, much larger difference can been seen with the remaining anhydride molecule in which the two metal atoms (namely Fe3 and Fe4) are at the opposite sides on the anhydride, as confirmed by the overlay of molecules containing Fe1-Fe2 and Fe3-Fe4 (Figure 4, top right, RMSD 2.0862) and molecules containing Fe3-Fe4 and Fe5-Fe6 (Figure 4, top left, RMSD 2.1374). A packing similarity search (Mercury Solid Form module used, 20% distance tolerance, 20 ° angle tolerance, Hposition and bound type ignored, structure inversion allowed) revealed one hit between the known orthorhombic polymorph 1A and the anhydride molecules containing the Fe1 and Fe2 atoms of the new monoclinic polymorph 1B. Indeed, as shown in figure 4, a moderate overlap can be seen with a RMSD of 0.624. ) contacts, supporting the zig-zag chains of molecules along the a axis, were observed by the authors in 1A. In 1B, although no traditional H-bond donors are present, each carbonyl group can act as a weak hydrogen bond acceptor (Steiner, 2002;Desiraju, 2011). Although no zig-zag chains as present in 1A could be seen in 1B, we here identified a set of weak hydrogen bonds (Table 2). Along the b and c axes, four hydrogen bonds structure the packing (C85-H85 5).

•••O42, C49-H49•••O72, C31- H31•••O12, C5-H5•••O45) whereas three other ones (C6-H6•••O15, C39-H39•••O45 and C69-H69•••O75) link molecules along the a axis [symmetry codes: (i) x-1/2, -y+1/2, z+1/2; (ii) x, -y+1, z-1/2; (iii) x-1, y, z; (iv) x+1, y, z.] (Figure
It is difficult to assess if the large Fc-Fc torsion angle (146.2°), placing the two ferrocene moieties at opposite sides of the anhydride group in one of the molecules of 1B, is it the result or the origin of the C31-H31•••O12 hydrogen bond. Therefore, to get more information on this large angle, we also compared the new polymorph 1B to the five other examples of Cp-based carboxylic anhydrides for which a solid-state structure was reported (Scheme 3): trovacenecarboxylic anhydride (2, Elschenbroich et al., 1997, NEWFIE, CCDC 1219005), 1,1'-((tert-butoxycarbonyl)amino)ferrocenecarboxylic anhydride (3, Siebler et al, 2010, KAJBUU, CCDC 769441), ruthenocenecarboxylic anhydride (4, Micallef et al., 2011, IVOYOI, CCDC 794550) and the ferrocenecyclophane 5 (Liu et al., 2015, HOVYEY, CCDC 1038498). Our goal was therefore to see if the structure of 1B was closer to the free rotating anhydrides 2 and 4 or to geometrically restrained ones such as 3 (due to the presence of a hydrogen bond between the two tert-butoxycarbonyl groups) and 5 (cyclophane structure).

In 1B, the torsion angles measured for the three anhydride molecules (O12-C11

•••C14-O15, O42- C41•••C44-O45 and O72-C71•••C74-O75
) are lower than the those observed for the free rotating anhydrides 1A, 2 and 4 (Table 3). Furthermore, the torsion angles between the two ferrocenyl cores of each anhydride molecule of 1B are outside the usual range of values. This suggests that the polymorph 1B is closer to geometrically restrained metallocene anhydrides than to the polymorph 1A.

Intramolecular dihedral angles between the Cp rings are in good agreement with those observed for the other derivatives. However, two Cp rings of one ferrocenyl core of 1B are clearly eclipsed with a C40- 21.7°). Such a torsion might be viewed as another mechanism to relieve steric strain. The torsion angles measured on the three molecules that crystallize in 1B are between the ones of the unconstrained anhydrides (1A, 2, 4) and those of the constrained 3 and 5. Accordingly, this suggests that steric strain exists at a higher degree in the new monoclinic polymorph, when compared to the orthorhombic one.

Cg3b•••Cg3a-C35
Therefore, this higher strain in 1B might have an effect on the thermodynamic stability of the polymorph.

Indeed, the density rule, as introduced by Burger and Ramberger (Burger & Ramberger, 1979), states that, at absolute zero, the most stable polymorph will have the highest density. However, the density difference between 1A and 1B (1.661 and 1.667 g/cm 3 for 1A and 1B, respectively) is too small to apply this rule. Therefore, it appears difficult to clearly state which polymorph is here the more stable, although the lower melting point of 1B (113-116 °C), when compared to 1A (140-141°C), suggests a higher stability for the orthorhombic polymorph 1A. (Sheldrick, 2015), SHELXL2016/6 (Sheldrick, 2015), SXGRAPH (Farrugia, 1999), Mercury (CSD, 2016), CRYSCALC (T. Roisnel, local program, 2017).

Table 2 Hydrogen bonds geometry (Å, º) for 1B.

D-H•••

A D-H (Å) H•••A (Å) D•••A (Å) D-H•••A (°)
C49-H49

(i) x-1/2, -y+1/2, z+1/2; (ii) x, -y+1, z-1/2; (iii) x-1, y, z; (iv) x+1, y, z.

Supporting information

Ferrocenecarboxylic anhydride: identification of a new polymorph

Mehdi Tazi a , Thierry Roisnel b , Florence Mongin a and William Erb a *

Computing details

Data collection: Bruker APEX2 (Bruker, 2014). Cell refinement: Bruker APEX2 (Bruker, 2014). Data reduction: Bruker APEX2 (Bruker, 2014). Program(s) used to solve structure: SHELXT (Sheldrick, 2015). Program(s) used to refine structure: SHELXL2016/6 (Sheldrick, 2015). Molecular graphics: SXGRAPH (Farrugia, 1999), Mercury (CSD, 2016). Software used to prepare material for publication: CRYSCALC (T. Roisnel, local program, 2017).

Ferrocenenecarboxylic anhydride, monoclinic polymorph (1B)

Crystal data

C22H18Fe2O3 F( 000

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2 (F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Crystal structure was solved in the non-centrosymmetric Cc space group by dual-space algorithm using the SHELXT program (Sheldrick, 2015), and then refined with full-matrix least-square methods based on F 2 (SHELXL) (Sheldrick, 2015). The use of combined TWIN and BASF instructions led to a Flack parameter value of 0.21(3), corresponding to a mixture of ~80% of a particular enantiomer and ~20% of the opposite one. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions. A final refinement on F 2 with 10804 unique intensities and 683 parameters converged at ωR(F 2 ) = 0.1201 (R(F) = 0.0500) for 10350 observed reflections with I > 2σ(I). In the absence of restraints or constraints on ADP's of C51-C55 ring atoms, some B and C alerts appeared in the CHECKCIF procedure, due to large values of ADP max/min ratios, especially on C52 (ratio = 4.2) and C55 (ratio=3.5). Introduction of restraints with the help of RIGU instructions applied on these C51-C55 atoms in SHELXL refinement lead to decrease these ADP max/min ratios (4.0 and 3.2 respectively) and provided more realistic anisotropic atomic displacements on this CP ring.

No constraints were used to refine the other cycles. CCDC 1553140.

Fractional

Figure 4

 4 Figure 4 clearly shows why an intramolecular hydrogen bond occurred on 1A (C206-H206•••O1) and not on 1B (C2-H2•••O15): both H•••A distances (1A: 2.56 Å, 1B: 3.34) and D-H•••A angles (1A: 148.9°, 1B: 140.9) are more favorable to develop these weak interactions in 1A. In addition to this intramolecular hydrogen bond, three intermolecular (C205-H205•••O2, C204-H204•••O1, C207-H207•••O2) contacts, supporting the zig-zag chains of molecules along the a axis, were observed by the

  torsion angle of 17.1°. This results from the hydrogen bond C31-H31•••O12, which mimics the interactions observed in the 1,1'-disubstituted ferrocene-based anhydride 3. For all six ferrocenyl cores, the angles between Cp ring planes are within the expected range of values, indicating the almost coplanarity of the Cp rings. The angles between each carbonyl group and their respective Cp ring were finally compared. A comparison of the torsion angles of 1A-5 clearly shows that, as the geometry is constrained (intramolecular hydrogen bond in 3, cyclophane structure in 5), high values are observed (3: 19.4°, 5:

Scheme 1

 1 Scheme 1 Structure of ferrocenecarboxylic anhydride 1.

Figure 1

 1 Figure 1 Structures of the orthorhombic polymorph (1A, top) and the monoclinic polymorph (1B, bottom). Thermal ellipsoids shown at the 30% probability level.

Figure 2

 2 Figure 2 Partial packing view of the monoclinic polymorph 1B showing the hydrogen bonds as dashed red lines. Hydrogen not implicated in hydrogen bonds were omitted for clarity.

Figure 3

 3 Figure 3 Overlay of molecules containing Fe3-Fe4 (light grey) and Fe5-Fe6 (light blue), top left; Overlay of molecules containing Fe1-Fe2 (light purple) and Fe3-Fe4 (light grey), top right; Overlay of molecules containing Fe1-Fe2 (light purple) and Fe5-Fe6 (light blue), bottom.

Figure 4

 4 Figure 4 Overlay of the orthorhombic polymorph 1A (light blue) and the monoclinic polymorph 1B(black and grey). Hydrogen were omitted for clarity.

Figure 5

 5 Figure 5 Chains of molecules of the monoclinic polymorph 1B along the a axis showing the hydrogen bonds as dashed red lines. Hydrogen not implicated in hydrogen bonds were omitted for clarity.

  ) = 2712 Mr = 442.06 Dx = 1.661 Mg m -3 Monoclinic, Cc Mo K radiation,  = 0.71073 Å a = 5.9758 (5) Å Cell parameters from 9522 reflections b = 36.456 (3) Å  = 2.2-27.5° c = 24.363 (2) Å  = 1.66 mm -1  = 91.997 (3)° T = 150 K V = 5304.4 (8rotation images, thin slices scans max = 27.5°, min = 3.4° Absorption correction: multi-scan [Sheldrick, G.M. (2014). SADABS Bruker AXS Inc., Madison, Wisconsin, USA] h = -6→7 Tmin = 0.486, Tmax = 0.608 k = -46→41 16323 measured reflections l = -31→31 Refinement Refinement on F 2 Hydrogen site location: inferred from neighbouring sites Least-squares matrix: full H-atom parameters constrained R[F 2 > 2(F 2 )] = 0.049 w = 1/[ 2 (Fo 2 ) + (0.0346P) 2 + 44.5361P] where P = (Fo 2 + 2Fc 2

Table 1

 1 Experimental details. Data taken from McAdam & Simpson, 2016. Computer programs: Bruker APEX2 (Bruker, 2014), SHELXT

		1B a	1A b
	Crystal data
	Chemical formula	C22H18Fe2O3	C22H18Fe2O3
	Mr	442.06	442.06
	Crystal system, space group	Monoclinic, Cc	Orthorhombic, Pbca
	Temperature (K)	150	100
	a, b, c (Å)	5.9758 (5), 36.456 (3), 24.363 (2)	14.8773 (2), 12.2499 (2), 19.3288 (3)
	β (°)	91.997 (3)
	V (Å 3 )	5304.4 (8)	3522.59 (9)
	Z	12	8
	Radiation type	Mo Kα	Cu Kα
	µ (mm -1 )	1.66	13.38
	Crystal size (mm)	0.56 × 0.36 × 0.30	0.16 × 0.13 × 0.11
	Data collection
	Diffractometer	APEXII, Bruker-AXS	Agilent SuperNova, Dual, Cu at zero, Atlas
		Multi-scan	Gaussian
	Absorption correction	[Sheldrick, G.M. (2014). SADABS Bruker AXS Inc.,	(CrysAlis PRO; Agilent, 2014)
		Madison, Wisconsin, USA]
	Tmin, Tmax	0.486, 0.608	0.267, 0.456
	No. of measured, independent and observed [I > 2σ(I)] reflections	16323, 10804, 10350	13584, 3468, 3262
	Rint	0.023	0.040
	(sin θ/λ)max (Å -1 )	0.649	0.621
	Refinement
	R[F 2 > 2σ(F 2 )], wR(F 2 ), S	0.049, 0.120, 1.11	0.030, 0.090, 0.84
	No. of reflections	10804	3468
	No. of parameters	730	244
	No. of restraints	32	0
	H-atom treatment	H-atom parameters constrained	H-atom parameters constrained
	Weighting scheme	w = 1/[σ 2 (Fo 2 ) + (0.0346P) 2 + 44.5361P] where P = (Fo 2 + 2Fc 2 )/3	w = 1/[σ 2 (Fo 2 ) + (0.089P) 2 + 0.2107P] where P = (Fo 2 + 2Fc 2 )/3
	Δρmax, Δρmin (e Å -3 )	0.96, -0.87	0.31, -0.68
	Absolute structure	Flack x determined using 4113 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013).
	Absolute structure parameter	0.204 (11)

a This work. b

Table 3

 3 Selected geometric parameters of the Cp-based carboxylic anhydrides compared (°).

	Compound Anhydride torsion angle Fc/Fc torsion angle	Cp/Cp eclipse angle	Cp/Cp planes angle	O-C-C-C torsion angle
	1A	57.4 (2)	72.2	6.3 -5.3	2.84 -1.75	12.1 (3) -2.1 (3)
	2	62.4 (5)	75.1	n.a.	1.96 -2.52	11.7 (9) -6.2 (9)
	2	66.7 (5)	103.7	n.a.	2.61 -2.71	9.8 (9) -6.2 (9)
	3	57.0 (2)	85.9	18.9 -7.0	2.31 -4.55	0.7 (4) -19.4 (5)
	4	62.3 (3)	72.9	12.8 -3.2	2.19 -2.03	3.6 (5) -4.2 (5)
	5	27.5 (2)	1.5	0.6 -0.4	2.51 -2.51	8.3 (4) -21.7 (4)
	1B	50.9 (7)	33.6	7.3 -2.5	1.33 -1.72	16 (1) -1 (1)
	1B	46.6 (7)	146.3	17.1 -0.7	0.89 -2.51	14 (1) -7 (1)
	1B	47.0 (7)	30.1	3.9 -6.2	0.39 -2.22	2 (1) -16 (1)
	n.a. Not applicable.

Scheme 3 Exemples of Cp-based carboxylic anhydrides. Boc = CO2tBu.