Metal Atom Clusters as Building Blocks for Multifunctional Proton

Publication Date (Web): July 30, 2018 ... The luminescent [Mo6L 8 i(OH) 6 a]2−/4− cluster units are building blocks for the synthesis of three pro...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Metal Atom Clusters as Building Blocks for Multifunctional ProtonConducting Materials: Theoretical and Experimental Characterization Gilles Daigre,† Jérôme Cuny,*,‡ Pierric Lemoine,† Maria Amela-Cortes,† Serge Paofai,† Nathalie Audebrand,† Annie Le Gal La Salle,¶ Eric Quarez,¶ Olivier Joubert,¶ Nikolay G. Naumov,§,∥ and Stéphane Cordier*,† Downloaded via UNIV OF READING on July 30, 2018 at 11:53:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Univ. Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F-35000 Rennes, France Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse (UPS) and CNRS, 118 Route de Narbonne, F-31062 Toulouse, France ¶ Institut des Matériaux Jean Rouxel (IMN), CNRS-Université de Nantes, 2 rue de la Houssinière, BP32229 44322 Nantes Cedex 3, France § Nikolaev Institute of Inorganic Chemistry, 3 Acad. Lavrentiev pr., 630090 Novosibirsk, Russia. ∥ Novosibirsk State University, Pirogova str.2, 630090 Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: The search for new multifunctional materials displaying protonconducting properties is of paramount necessity for the development of electrochromic devices and supercapacitors as well as for energy conversion and storage. In the present study, proton conductivity is reported for the first time in three molybdenum cluster-based materials: (H) 4 [Mo 6 Br 6 S 2 (OH) 6 ]-12H 2 O and (H)2[Mo6X8(OH)6]-12H2O (X = Cl, Br). We show that the self-assembling of the luminescent [Mo6Li8(OH)a6]2−/4− cluster units leads to both luminescence and proton conductivity (σ = 1.4 × 10−4 S·cm−1 in (H)2[Mo6Cl8(OH)6]-12H2O under wet conditions) in the three materials. The latter property results from the strong hydrogen-bond network that develops between the clusters and the water molecules and is magnified by the presence of protons that are statistically shared by apical hydroxyl groups between adjacent clusters. Their role in the proton conduction is highlighted at the molecular scale by ab initio molecular dynamics simulations that demonstrate that concerted proton transfers through the hydrogen-bond network are possible. Furthermore, thermogravimetric analysis also shows the ability of the compounds to accommodate more or less water molecules, which highlights that vehicular (or diffusion) transport probably occurs within the materials. An infrared fingerprint of the mobile protons is finally proposed based on both theoretical and experimental proofs. The present study relies on a synergic computational/experimental approach that can be extended to other proton-conducting materials. It thus paves the way to the design and understanding of new multifunctional proton-conducting materials displaying original and exciting properties.

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INTRODUCTION Protonic conductors are key materials for the development of devices for applications in energy conversion and storage, electrochromic devices, or supercapacitors.1 Many solid-state compounds are proton conductors, and there exist many ways to classify them considering either their chemical compositions, their proton-conduction mechanisms, or their shapings.2 For instance, hydrates can be classified into two categories: particle hydrates and framework hydrates.3 In the former, proton conductivity occurs at the surface of particles, while, in framework hydrates, proton conduction occurs within or throughout the entire particle. Recently, on the basis of structural features, porous crystalline proton-conducting materials were classified in four families: metal−organic © XXXX American Chemical Society

frameworks (MOFs), coordination polymers (CPs), polyoxometalates (POMs), and covalent organic frameworks (COFs).4 Advantages of MOFs and CPs compared to other inorganic or hybrid solid-state compounds is that several properties can be associated within one material by incorporating complementary building-blocks via solution chemistry route.5−7 As an illustration, the chromium terephthalate metal−organic framework MIL-101 is a multifunctional material that combines many properties relevant for gas storage applications8 or catalysis.9 Impregnation of MIL101 with molybdenum cluster units leads to improved Received: February 7, 2018

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DOI: 10.1021/acs.inorgchem.8b00340 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry hydrogen storage capacity10 or provides photoactive nanocarrier properties useful for biological and medical applications,11 while impregnation with H3PO4 leads to protonic conductivity.12 Intrinsic properties of the building blocks, their structural framework organization in the solid state, and the functionalities of impregnate moieties are the keystone at the origin of the macroscopic properties of the final multifunctional material. Metal atom cluster-based halides, oxides and chalcogenides constitute a family of compounds that exhibit fascinating structures and an outstanding variety of physical properties with potential applications in the fields of energy conversion and storage, lighting, displays, or biotechnologies.13−16 Most of them are built from the molybdenum octahedral cluster unit, [Mo6Li8La6], in which Mo6 is face-capped through covalent bonds to eight Li ligands (Li; i = inner) to form a Mo6Lim+ 8 cluster core that is further stabilized by six La ligands (La; a = apical; m = oxidation state). The number of electrons available for Mo−Mo bonding and the strength of electronic interactions between adjacent [Mo6Li8La6] units give rise to a wide range of inorganic solid-state compounds, hybrid organic nanomaterials, and supramolecular frameworks.17,18 For instance, in the case of chalcogenides, [Mo6Si8Sa6] cluster units can connect by double Si−a/Sa−i bridges to form a a−i [Mo6Si2Si−a 6/2S6/2] extended polymeric framework. Compounds exhibiting such structural arrangement can host countercations that can be reversibly deintercalated. For instance, the 2+ 2+ a−i Chevrel−Sergent phases, Ax[Mo6Si2Si−a 6/2S6/2] (A = Mg , Ca , 3+ and Al ), have been studied for a decade for their application as fast cathode materials for rechargeable magnesium bat te ri es . 1 9 − 2 1 Electronic delocalization on the a−i Ax[Mo6Si2Si−a 6/2S6/2] framework also leads to transport properties and even superconductor behavior.22 Last but not least, cluster-based molybdenum selenides are also studied as promising thermoelectric materials.14 In the solid state, molecular compounds can be obtained by substituting chalcogen by halogen ligands to form Ax[Mo6Xi8Xa6] inorganic salts, wherein the [Mo6Xi8Xa6]2− cluster units are held together by Coulombic interactions with surrounding counter cations.23 This leads to a range of original properties. For instance, the Cs2[Mo6Xi8Xa6] series is known to exhibit phosphorescent properties in the deep red−NIR region (NIR = near-infrared) upon excitation in the visible.16 The functionalization of [Mo6Xi8Xa6]2− can be performed in solution by exchanging Xa ligands by inorganic or organic ones. This allows to further incorporate [Mo6X8iX6a]2− units into various organic and inorganic frameworks.17,18,24 With the view to search for new building blocks for the design of multifunctional materials, the present study shows that the self-assembling of luminescent [Mo6Xi8(OH)a6]2− or [Mo6Bri6Si2(OH)a6]4− cluster units in solution yields to a novel and original class of proton-conducting hydrates. Three compounds are under investigation: first, the newly synthesized (H)4[Mo6Br6S2(OH)6]-12H2O aquahydroxo complex (1) is reported; second, the synthesis and the crystal structures of (H)2[Mo6X8(OH)6]-12H2O (X = Br (2), Cl (3)), initially published by Brosset,25−27 are also revisited in the light of the present discussion. The structures of (1), (2), and (3) are strongly related one to each other and are based on [Mo6Br6S2(OH)6]4− and [Mo6X8(OH)6]2− cluster units, respectively. All three compounds display phosphorescent properties. Protonic conductivity was measured in a 25−31 °C temperature range under wet, ambient, and dry atmospheres

revealing protonic conduction in the three compounds. Their thermal stability is discussed in the light of thermogravimetric analyses performed in the same conditions. They suggest that vehicular transport may occur in the three compounds. Furthermore, ab initio molecular dynamics simulations also demonstrate that concerted proton transfers can take place along the hydrogen-bond network. This mechanism is made possible by the statistical distribution of H+ located between the apical hydroxyl groups of adjacent clusters. Finally, the infrared (IR) fingerprint of those mobile protons was theoretically and experimentally highlighted. Those materials open new and exciting opportunities for the development of multifunctional molybdenum cluster-based compounds and bridge the gap with more conventional proton-conducting materials.

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EXPERIMENTAL SECTION

Materials and Synthesis. The Cs2Mo6Br14 ternary compound was synthesized from a stoichiometric amount of CsBr and MoBr2 binary halides according to a procedure previously described.28 Cs2Mo6Cl14 was synthesized from a stoichiometric mixture of CsCl and (H3O)2[Mo6Cl14]·7H2O.29 Cs4[Mo6Br12S2] was synthesized following a previously reported procedure.30 NaOH solution (1 M) was prepared by solubilizing 10 g of NaOH (Prolabo) in 250 mL of water. Ammonia solution (28%, Prolabo) was used as purchased. (H)4[Mo6Bri6Si2(OH)a6]-12H2O (1). Cs4[Mo6Br12S2] (100 mg) was solubilized in water (20 mL). pH was adjusted at 12 by addition of few drops of NaOH (1 M). Then, the solution was stirred for 5 min at 75 °C. The solution was then filtrated and allowed to stand at room temperature for 24 h resulting in the crystallization of (1). The supernatant was extracted, and the crystals were washed with aliquots of water. Yield: 30 mg (35 mmol; 41%). The synthesis was repeated ∼30 times to obtain a sufficient amount of material (800 mg) to prepare pellets of powdered samples (200 mg each) for protonconductivity measurements. IR (cm−1): 463, 775, 980, 1269 (ν mobile H+), 1630, 3270. Energy-dispersive X-ray spectroscopy (EDS) analysis, heavy atoms in atom %: Mo, 41; Br, 44; S, 15. Calcd: Mo, 42.9; Br, 42.9; S, 14.2. (H)2[Mo6Bri8(OH)6a]-12H2O (2). Cs2[Mo6Br14] (100 mg) was introduced in 25 mL of water in which few drops of ammonia (28%) were added (pH ≈ 12). The powder was solubilized after heating (75 °C) and stirring for 2 min. Then, the solution was gently evaporated at 75 °C without stirring. (2) was obtained as single crystals within 2 h. The supernatant was extracted, and the crystals were washed with aliquots of water. Yield: 75 mg (48.8 mmol; 96%). The synthesis was repeated ∼12 times to have a sufficient amount of material (800 mg) to prepare pellets of powdered samples (200 mg each) for proton-conductivity measurements. IR (cm−1): 473, 758 1031, 1323 (ν mobile H+), 1635, 3263. EDS analysis, heavy atoms in atom %: Mo, 42; Br, 58. Calcd: Mo, 42.9; Br, 57.1. (H)2[Mo6Cl8i(OH)6a]-12H2O (3). Cs2[Mo6Cl14] (100 mg) was solubilized in 10 mL of water in which few drops of ammonia (28%) were added (pH ≈ 12). The hydrolysis immediately occurred, and (H)2[Mo6Cl8(OH)6]-12H2O precipitated. The supernatant was extracted, and the resulting compound was washed with aliquots of water. Yield: 60 mg (50.9 mmol) (68%). The synthesis was repeated ∼15 times to have a sufficient amount of material (800 mg) to prepare pellets of powdered samples (200 mg each) for protonconductivity measurements. IR (cm−1): 484, 782, 1055, 1400 (ν mobile H+), 1627, 3155. EDS analysis, heavy atoms in atom %: Mo, 41; Cl, 59. Calcd: Mo, 42.9; Cl, 57.1. Elementary Analyses. Elementary analyses were done by EDS with a scanning electron microscope JSM 7100F. Infrared Spectroscopy. Infrared spectra were recorded on a Bruker Equinox 55 FTIR spectrometer on transmittance with KBr as reference. B

DOI: 10.1021/acs.inorgchem.8b00340 Inorg. Chem. XXXX, XXX, XXX−XXX

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systems.47−49 For instance, it yields a less structured liquid water than the well-known PBE50 and BLYP51,52 functionals. Electronic wave functions and densities were expanded with a plane-wave basis-set defined by an energy cutoff of 80 and 320 Ry, respectively. Valence− core interactions were described by use of Troullier−Martin normconserving pseudopotentials.53 All the pseudopotentials were taken from the extended library package provided with the CPMD code. The Brillouin zone integration was done at the Γ-point only. The structures of (1), (2), and (3) being extremely similar, we focused our molecular dynamics studies on (H)2[Mo6Br8(OH)6]12H2O only. Of course, similar conclusions are expected from the simulations of (1) and (3). The simulations consisted of a unique unit cell of (H)2[Mo6Br8(OH)6]-12H2O with cell parameters equal to those obtained from single-crystal measurements. Each simulation cell consisted in three cluster units and 36 water molecules. The position of the hydrogen atoms is unknown from the crystal structure refinements from X-ray diffraction data; therefore, models must be considered based on chemical intuition. The initial hydrogen atom positions were placed by-hand by ensuring the following three conditions: (i) no dangling hydrogen atom is present, (ii) hydrogen bonds (HB) form a network as continuous as possible, and (iii) no H3O+ species is initially present in the structure. Of course, the amount of possible configurations fulfilling these three constraints is very large, and the study of the whole of them is cumbersome. Furthermore, any choice of configuration inevitably involves symmetry breaking of the system. In the present study, we generated two distinct initial configurations and discuss the results for only one of them. Geometry optimizations were performed using 1 × 10−8 and 1 × 10−5 atomic units (au) as convergence criterions for the largest element of the gradient of the wave function and ions, respectively. Furthermore, the whole atomic positions were allowed to relax, while the cell parameters were kept fixed. Optimized geometries were further used as initial configurations in the AIMD simulations. Twenty picosecond equilibration runs were first performed in the canonical ensemble. The temperature was controlled using a colored-nose Langevin thermostat especially designed for CPMD simulations and tuned to optimally sample all the frequencies of the system up to 1000 cm−1.54 Production runs were subsequently conducted during 80 ps in the microcanonical ensemble. All the AIMD simulations were conducted using a fictitious electron mass of 310 au and a 3 au time step. These parameters avoid any drift in fictitious kinetic energy of the electrons and allow to get a good energy conservation during the simulation length of time. Metadynamics (MetaD) simulations were performed on (H)2[Mo6Br8(OH)6]-12H2O using the CPMD code v.3.17 in combination with the PLUMED package v.1.2.55 All the molecular dynamics parameters were set identical to those of the nonbiased simulations. The well-tempered formulation of MetaD was used,56 with an initial hill height of 1 kcal·mol−1 and a deposit stride of 18.75 fs. The collective variable (CV) was sampled at a fictitious temperature of 15 500 K. The proton transfer mechanism occurring in (H)2[Mo6Br8(OH)6]-12H2O was described using a unique CV, which is the distance between one hydroxyl oxygen atom and its covalently bounded hydrogen atom. The Gaussian width used for this CV was 0.08 Å.

Crystal Structure Determination of (1). Single-crystal X-ray diffraction data were collected at 150 K on a D8 VENTURE Bruker AXS diffractometer and processed with the APEX 3 program suite.31 The X-ray wavelength used was the Mo Kα (λ = 0.710 73 Å). Frames integration and data reduction were performed with the program SAINT.32 The program SADABS was then employed for multiscantype absorption corrections.33 The structural model was determined by direct method using the SHELXT program34 and refined with fullmatrix least-squares methods based on F2 (SHELXL-2014)35 with the aid of the WinGX plateform.36 The final refinements included anisotropic displacement parameters for the non-hydrogen atoms. Crystallographic data, details on data collections, final atomic positions, and refinement parameters of the crystal structure of (1) are summarized in the Supporting Information (see Tables S1 and S2). Structure drawings were performed with Diamond 3, supplied by Crystal Impact.37 Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data of (1)−(3) were collected at room temperature using a Bruker D8 Advance two circle diffractometer (θ−2θ Bragg−Brentano mode) using the Cu Kα radiation (λ = 1.540 56 Å) equipped with a Ge(111) monochromator and a Lynx Eye detector. The extraction of the peak positions was performed with the WinPlotr38 software in the FullProf suite package.39 Pattern indexing and refinement of the unit-cell parameters were performed with the DICVOL06 program.40 Integrated intensities were extracted with the Le Bail iterative pattern decomposition algorithm available in FullProf. Structure model of (1) obtained on single crystal reported above and those already published for (2)41 and (3)25−27 were used as initial models in the Rietveld refinements made using the FullProf package. A pseudo-Voigt function was selected to describe the individual line profiles. Backgrounds were represented with a linear interpolation between refined intensity points. The refinements involved the following parameters: atomic coordinates, isotropic atomic displacement parameters, scale factor, zero-point, two unit-cell parameters, three half-widths parameters, two line asymmetry parameters, and two variables for the angular variation of the line shape factor as well as refined intensity points for background interpolation. Crystal data and Rietveld structure refinement parameters are reported in Table S3 of the Supporting Information. The final Rietveld plots (see Figures S1 to S3 in the Supporting Information) show the good agreement between experimental and calculated patterns. Measurement of Luminescence Properties. Luminescence spectra were recorded with a Nikon 80i polarized microscope equipped with a Linkam LTS420 hot stage, a Nikon Intensilight irradiation source, a Nikon DS-FI2 digital camera, and an ocean optics QE6500 photodetector connected by optical fiber. Thermogravimetric Analyses. Thermogravimetric analyses (TGA) in air were performed on a Netzsch STA 449 F3 Jupiter thermobalance. A powder sample of 60 mg was loaded in an alumina crucible and heated in 3% or 2% water vapor air gas or in dry air gas flow at a rate of 0.5 °C·min−1 up to 120 °C before quick cooling to room temperature. Measurement of Proton Conductivity. Samples used for impedance measurements were obtained by compacting powder into pellets. Dimensions of the pellets are provided in Table S4 of the Supporting Information. Silver paste was deposited on the two external sides of pellets. Then, they were tested by electrochemical impedance spectroscopy (EIS). Spectra were recorded at Udc = 0 V, with a signal amplitude of 100 mV and with 76 points scattered in a frequency range from 4 MHz to 0.1 Hz, with a frequency response analyzer Solartron 1260. It was checked that the amplitude of the perturbation signal is small enough to meet the linearity requirement of the transfer function.42 The impedance data were analyzed using the ZView2-Software.43 Quantum Chemical Calculations. Ab initio molecular dynamics (AIMD) simulations were performed with the Car−Parrinello formalism as implemented in the CPMD code v.3.17.44,45 The same code was used for geometry optimizations. The exchange-correlation interaction was described using the HCTH/120 functional,46 which was shown to provide a good description of hydrogen-bonded

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RESULTS AND DISCUSSION Structural Description of the Compounds. (1) crystallizes in the trigonal system, space group R3m (No. 166) with unit-cell parameters refined using single-crystal Xray diffraction data a = 15.220(2) Å, c = 11.130(1) Å, V = 2233.0(5) Å3, and Z = 3 (see Table S1). The structure is built from [Mo6Bri6Si2(OH)a6]4− cluster units represented in the Figure 1. In this motif, Mo1 and O1 (i.e., the apical ligands) fully occupy 18h Wyckoff positions (Cs point symmetry), while face-capping ligand positions, corresponding to 18h (L1) and 6c (L2) Wyckoff positions (C3v point symmetry), are randomly C

DOI: 10.1021/acs.inorgchem.8b00340 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Representation of the [Mo6Bri6Si2(OH)a6]4− cluster unit in (1). The inner positions (L1 and L2) represented in yellow are randomly occupied by two Si and six Bri ligands. The O2 position does not belong to the cluster unit and is represented in Figure 2. Oxygen atoms are represented in red, and molybdenum atoms are in black.

occupied by Br and S (see Table S2). This coloring problem is often encountered in Re6 and Mo6 octahedral clusters bonded to two kind of ligands. Cordier et al. demonstrated that only the composition [Mo6Bri6Qi2(L)a6]2− (Q = S or Se and La = Br or CN) is obtained after ligand-exchange reaction from the Cs4Mo6Bri6Qi2Bra6 solid-state precursor.30 Furthermore, it was also observed for the [Mo6Bri6Si2(CN)a6]2− cluster unit in Cs0.4K0.6(Et4N)11·[(Mo6Br6S2)(CN)6]3·16H2O and for the [Mo6Bri6Si2Bra6]2− cluster unit in Cs4Mo6Bri6S2iBra6 that no significant Br/S ratio was found even in lower symmetry space group on the inner positions. This suggests a full orientational disorder of the [Mo6Bri6Si2(OH)a6]2− units in (1). Finally, crystallization water molecules (O2) fully occupy a general 36i Wyckoff position (C1 point symmetry). These latter are represented in the Figure 2. (2) and (3) are isostructural to (1) except for the L1 and L2 positions that are fully occupied by bromine and chlorine atoms, respectively. The cell parameters of (2) and (3) are equal to a = 15.2455(8) Å, c = 11.1440(8) Å, and V = 2243.1(3) Å3 and a = 15.15 Å, c = 11.02 Å, and V = 2190.47 Å3, respectively.25−27,41 The cell parameters of (1) and (2) are comparable, which highlights the weak effect of the sulfur atoms on those parameters. In contrast, the structure of (3) is more compact in agreement with Br−/Cl− anionic radius. Selected bond lengths of (1), (2), and (3) are gathered in the Table 1. The Mo−Mo distances are similar in the three compounds. In the same way, the Mo−Li distances are comparable in (1) and (2), highlighting the weak impact of the random occupation by the sulfur atoms, while they are significantly shorter in (3). A relation exists between the Mo− O1 distance and the nature of the corresponding inner ligand, as the more ionic the chemical bond is, the longer the bond is. Considering the oxygen atoms only, the O1−O1 distance is clearly shorter in (1) than in (2) and (3) by ∼0.1 Å. In contrast, the other oxygen−oxygen distances, that is, O1−O2 and O2−O2, are shorter in (3). This suggests that the smaller

Figure 2. View along the c (a) and the b (b) axis in (1) of the connection between a given [Mo6Br6S2(OH)6]4− cluster unit and the surrounding six cluster units. Blue, red, and green dashed lines represent HB between O1−O1, O1−O2, and O2−O2 oxygen atoms, respectively. O2 positions are highlighted. Molybdenum and oxygen atoms are represented by black and red spheres, respectively. Bromine/sulfur and hydrogen atoms are omitted for clarity. (b) The two groups of three cluster units belonging to the layer located above and below the considered cluster unit are highlighted.

cell parameters in (3) result from shorter O1−O2 and O2−O2 distances, that is, a more compact water molecule network. From a three-dimensional point of view, the structure of (1) can be depicted as discrete [Mo6Bri6Si2(OH)a6]4− cluster units arranged into layers according to an A−B−C−A stacking type along the c direction (see Figure 3). The same arrangement is observed for (2) and (3) with the stacking of [Mo6X8(OH)6]2− (X = Cl, Br) cluster units. In both compounds, the structural cohesion is ensured by a strong HB network represented in Figures 3 and 2 (additional representations are provided in the Supporting Information). Three distinct HB types are observed. First, each [Mo6Bri6Si2(OH)a6]4− cluster unit interacts with six surrounding cluster units via HB through O1−O1 contacts. They are represented by dashed blue lines in Figures 2 and 3 as well as in Figures S4, and S6−S8 of the Supporting Information. Three belong to the layer located above the considered cluster unit, while the three others belong to the unit layer located below (see Figure 2b). For the three compounds, the O1−O1 D

DOI: 10.1021/acs.inorgchem.8b00340 Inorg. Chem. XXXX, XXX, XXX−XXX

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crystal data refinement) for (1), (2), and (3), respectively. They are represented by dashed red lines in Figure 2 as well as in Figures S5−S8 of the Supporting Information. The O1−O2 distance in (1) is between those found in (2) and (3). Finally, each O2 from crystallization water molecules interacts with three other O2 (dashed green lines in Figure 2 and Figures S7−S8 of the Supporting Information). Whatever the compound, the longest oxygen−oxygen distance in the structure is always O2−O2 (see Table 1). Figure 2a,b highlights that consecutive apical oxygen atoms (O1) in a given plane of a given cluster unit are connected by two water molecules. Consequently, in the (ab) plane, each cluster unit is sandwiched between two nine-remembered rings of oxygen atoms. Additional representation of the HB network of (H)4[Mo6Br6S2(OH)6]-12H2O are provided in Figures S4− S8 in the Supporting Information. Localization of Protons in (1), (2), and (3). Each O2 atom carries two protons, and each O1 atom carries one proton. To counterbalance the charge of [Mo6Br6S2(OH)6]4− or [Mo6X8(OH)6]2−, it is necessary to consider the additional presence of four or two protons per cluster unit, respectively. At this stage, the positioning of those protons within the structure remains an open question. Since the work of Brosset,25 (2) and (3) are formulated as [Mo6X8(OH)4(H2O)2]-12H2O. In this model, protons are assumed to belong to a neutral Mo6X8(OH)4(H2O)2 cluster unit meaning a random distribution of four hydroxyl groups and two water molecules on apical positions. Following this model for (1), the cluster unit would be Mo6Br6S2(OH)2(H2O)4, and the compound formula should be written [Mo6Br6S2(OH)2(H2O)4]-12H2O. Sheldon latter reported the existence of the [Mo6Bri8(OH)a6]2− species in solution by titration.26 Recently, the existence of such species was confirmed by mass spectroscopy experiments.41 From the latter study, the formulation (H3O)2[Mo6Bri8(OH)a6]-10H2O was proposed in which apical positions are only occupied by hydroxyl groups, while H3O+ species are distributed into the structure. In that case, the formula of (1) should be written (H3O)4[Mo6Br6S2(OH)6]-8H2O. As we discussed above, there are only two crystallographic positions for oxygen atoms: O1 for apical ligands and O2 for crystallization water molecules. In the three compounds, the O1−O1 distance is a striking feature of the structures. Indeed, if compared to the average oxygen−oxygen distance reported in the literature for liquid water (between 2.74 and 2.80 Å),57,58 the O1−O1 distances are significantly shorter. Furthermore, it is much shorter in (1) than in (2) and (3), which supposes that the size of the unit cell is not the unique parameter controlling this distance. In contrast, the O1−O2 and O2−O2 distances are shorter in (3). Consequently, this suggests the existence of a particular bonding mode between adjacent [Mo6Li8(OH)i6]4− (and [Mo6Xi8(OH)i6]2−) cluster units that is influenced by the number of excess proton within the structure. To provide a clear picture of the localization of the excess protons in those compounds, nonbiased ab initio molecular dynamics and metadynamics simulations were conducted. Quantum Chemical Calculations. In the present article, only simulations on (H)2[Mo6Br8(OH)6]-12H2O (2) are considered and discussed. However, the structural similarity between the three compounds under investigation suggests that similar results would be obtained for (1) and (3). Four different bonding motifs can exist between two clusters

Table 1. Selected Intra-Unit and Oxygen−Oxygen Bond Lengths (Å) for (1) (Present Work), (2),41 and (3)25−27 (H)4[Mo6Br6S2(OH)6]-12H2O (1) #3,4

Mo1−Mo1 Mo1−Mo1#1,2 Mo1−L1 Mo1−L1#3,4 Mo1−L2 Mo1−L2#1,2

2.6251(2) Mo1−O1 2.6212(3) O1−O1 2.5957(2) O1−O2 2.6115(2) O2−O2 2.6123(2) O2−O2 2.6123(2) O2−O2 (H)2[Mo6Br8(OH)6]-12H2O (2)

2.151(7) 2.5118(2) 2.7337(2) 2.7809(2) 2.7744(3) 2.8044(3)

Mo1−Mo1 Mo1−Mo1 Mo1−Br1 Mo1−Br1#3,4 Mo1−Br2 Mo1−Br2#3,4

2.6329(5) Mo1−O1 2.6259(5) O1−O1 2.6133(8) O1−O2 2.6181(5) O2−O2 2.5999(6) O2−O2 2.6000(6) O2−O2 (H)2[Mo6Cl8(OH)6]-12H2O (3)

2.102(3) 2.6135(1) 2.7800(1) 2.7506(1) 2.7664(2) 2.8235(1)

Mo1−Mo1 Mo1−Mo1 Mo1−Cl1 Mo1−Cl1#3,4 Mo1−Cl2 Mo1−Cl2#1,2

2.636 2.624 2.501 2.564 2.615 2.615

Mo1−O1 O1−O1 O1−O2 O2−O2 O2−O2 O2−O2

2.289 2.594 2.667 2.651 2.759 2.778

Symmetry transformations used to generate equivalent atoms: #1 1 − x + y, 1 − x, z ; #2 1 − y, x − y, z ; #3 1/3 + x − y, 2/3 − y, 2/3 − z ; #4 1/3 − x, 2/3 − x + y, 2/3 − z. a

Figure 3. Projection along the b axis of the unit cell of (1) highlighting the A−B−C−A stacking type of the [Mo6Bri6Si2(OH)a6]4− cluster units. Molybdenum, bromine/sulfur, and oxygen atoms are represented by black, yellow, and red spheres, respectively. Dashed blue lines represent HB between apical oxygen atoms (i.e., O1−O1 contacts) of adjacent [Mo6Br6S2(OH)6]4− cluster units. Water molecules and hydrogen atoms are omitted for clarity.

distances are the shortest oxygen−oxygen distances in the structure. These are equal to 2.5118(2), 2.6135(1), and 2.594 Å (from single-crystal data refinement) in (1), (2), and (3), respectively. Interestingly, the O1−O1 distances are significantly shorter in (1) than in (2) and (3), although (3) is the most compact structure. This point would be further discussed below in the text. Second, each O1 atom is also involved in two HB with crystallization water molecules (O1−O2 distances equal to 2.7337(2), 2.7800(1), and 2.667 Å (from singleE

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Figure 4. Representation of the four bonding motifs that can exist between two adjacent [Mo6Bri8(OH)i6]2− cluster units in (2): H2O−H2O (a), H2O−HO (b), HOH−OH (c), and OH−OH (d).

OH appears as the only bonding motif able to explain the short experimental O1−O1 distance. Furthermore, the central proton involved in this particular motif (latter referred to as H*), although closer to one of the oxygen atoms, was observed in one case to be localized between the two oxygen atoms (latter referred to as O1 and O2 in the discussion, see Figure 5 for atom labeling). The four

depending on the protonation state of the ligand, that is, aqua or hydroxyl. They are represented in Figure 4 and can be described as H2O−H2O (a), H2O−HO (where OH is the HBdonor) (b), HOH−OH (where H2O is the HB-donor) (c), and OH−OH (d). In a preliminary structure model for (H)2[Mo6Br8(OH)6]-12H2O, the HB network was constructed in such a way to include those four bonding modes. After geometry optimization, two important features were observed: (i) the H2O−H2O motif leads to the longest O1− O1 distances; (ii) some H2O−HO motifs display a noteworthy behavior as one of the OH distances of the aqua ligand significantly lengthens. In several cases, the motif even undergoes partial deprotonation, which results in the formation of a H3O+ species adjacent to the initial position of the aqua ligand. This demonstrates the strong tendency of double HB-donor aqua ligands to deprotonate, which highlights the weak stability of the H2O−HO and H2O−H2O motifs. Consequently, a final structural model for (H)2[Mo6Br8(OH)6]-12H2O was constructed that contains four HOH−OH and OH−OH motifs and only one H2O−HO motif within the unit cell. The O1−O1 distances obtained from the corresponding geometry optimization are reported in Table 2. From those values, the HOH−OH bonding mode leads to the three shortest O1−O1 distances. One OH−OH motif leads to one rather short distance of 2.64 Å, while the others are significantly longer. The unique H2O−HO motif leads to the longest O1−O1 distance. Consequently, HOH−

Figure 5. Representation of the HOH−OH bonding mode between two adjacent [Mo6Bri8(OH)a6]2− cluster units in (2) with labeling of the atoms as used in the text.

O1H*/O2H* distances are 1.17/1.31, 1.00/1.72, 0.99/1.68, and 0.99/1.70 Å. Those noteworthy distances suggest a more complex behavior of the HOH−OH motif than a simple HB. If one considers the AIMD simulations performed at 300 K and starting from the optimized structure, additional important features appear. The average O1−O1 distances (averaged over time for each of the nine motifs) are reported in Table 2. The values confirm that only the HOH−OH motif is able to explain the short experimental O1−O1 distance as the two other motifs lead to distances longer by more than 0.2 Å. The O1−O1 distance in H2O−HO remains the longest one. Interestingly, although the four optimized distances for HOH− OH were somewhat scattered, the time average leads to four values that are similar. This demonstrates that all the motifs behave similarly and are not affected by local asymmetries. A closer look at the time evolution of one of the O1−H*/H*−O2

Table 2. Nine O1−O1 Distances Obtained in the Optimized Structure of the (H)2[Mo6Br8(OH)6]-12H2O (2) Modela (H)2[Mo6Br8(OH)6]-12H2O (Å) H2O−HO HOH−OH OH−OH

2.85 2.94 2.47/2.66/2.68/2.70 2.52/2.51/2.55/2.52 2.64/2.74/2.74/2.79 2.78/2.80/2.78/2.94

a

The corresponding time-averaged distances (Å) extracted from AIMD simulation at 300 K are indicated in bold for each bonding motif. F

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and OH−OH motifs is equal to 2.67 Å, which is rather close to the experimental value of 2.61 Å. The dynamics of the HO−H*−OH bonding motif and its effects on the surrounding HB network were further investigated by looking at two different correlations occurring around H*. First, Figure 6b displays the correlation between the O1−O2 distance and the symmetrized proton transfer coordinate ν (ν = rO1−H* − rH*−O2) in (H)2[Mo6Br8(OH)6]12H2O (2). The latter variable was used in a number of previous studies to characterize proton transfer processes.61,64,65 As already observed,61,64,65 the proton transfer between O1 and O2 is made possible by the compression of the oxygen−oxygen distance that originates from thermal fluctuations. This transient compression reduces the energetic barrier for the transfer.1 Second, Figure 6c displays the correlation between the O1−H* distance and the distance between the adjacent HB donor hydrogen H1 and O1 (see Figure 5 for labeling). Although rather weak, a slight correlation exists that supposes that the delocalization of H* has a local effect on the surrounding HB network. Those local dynamical aspects are of fundamental importance, as the ability of H* to jump from O1 to O2 suggests that proton conduction can occur in those compounds. To further support this hypothesis, a MetaD simulation was performed on (H)2[Mo6Br8(OH)6]-12H2O (2) at 300 K. Figure 7 displays various snapshots taken along the

distances (the same is observed for the three other motifs) shows that H* jumps from one to the other oxygen atom (see Figure 6a). Consequently, the bonding mode between adjacent

Figure 6. (a) Time evolution of the O1−H*, H*−O2, and O1−O2 distances along a 300 K AIMD simulation of (H)2[Mo6Br8(OH)6]12H2O (2). (b) Correlation between the O1−O2 distance and the symmetrized proton transfer coordinate ν in (H)2[Mo6Br8(OH)6]12H2O (2) at 300 K. (c) Correlation between the O1−H* and the H1−O1 distances in (H)2[Mo6Br8(OH)6]-12H2O (2) at 300 K.

clusters is not a simple HOH−OH hydrogen bond but a HO− H*−OH motif that displays a proton equally shared between two hydroxyl groups. This bonding motif is closely related to a Zundel-like motif well-known in the field of proton transfer in liquid water49,59−63 and that also exists in various solid-state compounds such as acid hydrates.64 The local positive charge created by the excess proton H* induces a strong electrostatic interaction and an enhanced density polarization, which make this bonding pattern much stronger than a simple water−water HB. This explains the short O1−O1 distance between apical oxygen atoms determined from X-ray diffraction. It is worth highlighting that the experimentally observed O1−O1 distance is a statistical average over all the bonding motifs, mainly HOH− OH and OH−OH. Consequently, the more excess proton there is in the structure, the more HOH−OH bonding motifs exist, and the smaller the average O1−O1 distance is. This explains the shorter O1−O1 distance in (H) 4 [Mo 6 Br 6 S 2 (OH) 6 ]-12H 2 O (1) as compared to (H)2[Mo6Br8(OH)6]-12H2O (2) and (H)2[Mo6Cl8(OH)6]12H2O (3) (see Table 1) and the weak influence of the unitcell parameters on this distance. In the present simulation, the time-averaged O1−O1 distance obtained from the HOH−OH

Figure 7. Snapshots of the proton transfer mechanism taken along a metadynamics simulation of (H)2[Mo6Br8(OH)6]-12H2O (2) at 300 K.

corresponding trajectory, where the distance between H2 and O2 was used as the CV (see Figure 7 for labeling). During 1.97 ps, nothing occurs but an increase in the O2−H2 distance fluctuations. At 2.07 ps, the bond breaks, and H2 is transferred to the adjacent water molecule (O3). In a concerted manner, H* is transferred to O2. During the following 0.04 ps, two other proton transfers occur. In total, over 0.14 ps, four concerted proton transfers are observed involving four oxygen atoms. Although it was initially stated that proton transfer from a proton donor to a proton acceptor is the rate-limiting step of proton conduction,1 it later appears that long-range structural diffusion of protons is more complex; in particular, it necessitates molecular reorientations involving HB breaking and forming.49,66−68 In dense water-containing compounds, this reorientation mechanism is facilitated, which promotes structural (or Grotthus-type)1,62 transport over diffusion (or G

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although this was not looked at in the present study. The second part of the curves corresponds to the loss of the remaining five weakly bound water molecules along with those originating from the hydroxyl groups of the clusters. An EDS analysis of products after TGA experiments shows that the Mo/Cl ratio (6:8) is preserved, whereas the content of oxygen is measured to be less than 4%, that is, less than 0.5 oxygen atom per Mo6. This is in agreement with the full loss of water molecules and hydroxyl groups deduced from the TGA. Note that the low oxygen content measured from EDS probably corresponds to adsorbed oxygen when preparing the samples for analysis. However, the presence of 0.5 oxygen atoms per 6 Mo and 8 Cl in the final products cannot be fully excluded. The X-ray powder patterns (see Figure S15 in the Supporting Information) evidence that the final compounds recorded after TGA analysis from (H)2[Mo6Cl8(OH)6]-12H2O are amorphous. Under dry air, (H)2[Mo6Br8(OH)6]-12H2O (2) displays a similar behavior as that of (H)2[Mo6Cl8(OH)6]-12H2O (3), with a first weight loss between 20 and 36 °C corresponding roughly to five water molecules and a second weight loss of ∼12−13 water molecules between 36 and 120 °C. As for (3), the EDS and X-ray analysis of the final products of (2) confirm a full loss of water molecules and hydroxyl groups (Mo/Br = 6:8; content of oxygen less than 4%, that is, less than 0.5 oxygen atom per Mo6). The X-ray powder pattern (see Figure S15 in the Supporting Information) also evidence that the final compound obtained after TGA of (2) under dry atmosphere is almost amorphous. In contrast, the behavior of (2) under wet atmosphere recorded in our experimental condition is quite different than that recorded in wet conditions. Indeed, only the second-step weight loss is visible on TGA curves. The weight loss corresponds to ∼13 water molecules. Consequently, the TGA curve of (2) under wet atmosphere suggests that part of the weakly bound water molecules was evaporated during the equilibrium period initiating the analysis. For (1) only the second weight loss could be observed under both wet and dry atmospheres, and it corresponds to the loss of ∼12−13 water molecules in a one-step process. As for (2) under wet conditions, the TGA curve of (1) under dry and wet atmospheres suggests that part of the weakly bound water molecules were evaporated during the equilibrium period initiating the analysis. The EDS analysis of the final products highlights a full loss of water molecules and hydroxyl groups after TGA of (1). The Mo/Br/S is close to 6:6:2, and the oxygen content is again less than 4%. The X-ray powder pattern evidences that the final compound obtained after TGA of (1) is amorphous (see Figure S15 in the Supporting Information). On the basis of these experimental results, in the following, we discuss proton-conduction measurements performed within the 25−31 °C temperature range under ambient and wet atmospheres. Thermal stability of the compounds in this temperature range is of course subject to appropriate and careful manipulation of the samples. In the case of dry atmosphere, despite that some water molecules are expected to be lost along the measurements, proton conductivity was also measured for the three compounds for the sake of comparison. Table S5 of the Supporting Information collects the different values of conductance and resistance obtained for the three materials at 25 °C under the three relative humidity conditions discussed for the TGA measurements: ambient atmosphere (2% H2O), wet (3% H2O), and dry air. Figures S9 and S11−

vehicle) transport as highlighted in a study of proton diffusion in Nafion.69 In the present study, reorientations of water molecules were not observed in the unbiased and MetaD simulations. Although they can not provide a full picture of the proton transfer mechanism in (H)2[Mo6Br8(OH)6]-12H2O (2) (and subsequently in (1) and (3)), this lack suggests that long-range structural diffusion does not necessarily prevail despite the occurrence of concerted proton transfers along the HB network. However, the fact that successive proton transfers stop when the excess proton is bound to an apical hydroxyl group (see last snapshot of Figure 7) suggests that the hydroxyl ligands are proton defects within the network that behave as natural resting places for the excess proton. To conclude this part, although limited in time-scale, the present MetaD simulation demonstrates that concerted proton transfers can occur in (1), (2), and (3), which suggests that proton conductivity may be observed in those compounds. To confirm this assumption, proton-conduction measurements were subsequently conducted. Proton-Conduction Properties. To perform protonconduction measurements in a meaningful range of temperatures for the three considered compounds, we first conducted a TGA to probe their stability with respect to relative humidity and temperature. Figure 8 presents TGA curves obtained for

Figure 8. TGA of (1) (dashed line), (2) (dotted line), and (3) (dashed dotted line) under various relative humidity conditions: dry (black), ambient (2% H2O) (red), and wet (3% H2O) (blue) atmosphere. All curves were obtained with a heating rate of 0.5 °C· min−1.

(1), (2), and (3) under dry and wet (3% H2O) atmosphere as well as under ambient (2% H2O) atmosphere for (3) only. Various features can be observed from the comparison of these seven curves. The two curves for (H)2[Mo6Cl8(OH)6]-12H2O under ambient and wet air are qualitatively the same: (i) a first weight loss of approximately seven water molecules occurs between 20 and ∼50 °C and (ii) a second weight loss of 11 additional water molecules takes place between 50 and 120 °C, where a plateau is reached. In both curves, the weight loss starts at ∼35 °C highlighting the thermal stability of (H)2[Mo6Cl8(OH)6]-12H2O up to this temperature. Under dry air, a similar two-step behavior is observed, although the first step is much faster and takes place as soon as the experiment is started. Regardless of the humidity conditions, (H)2[Mo6Cl8(OH)6]-12H2O loses ∼18 water molecules when heated to 120 °C. The initial loss of seven water molecules suggests that only part of the weakly bound molecules are lost, that is, water molecules corresponding to the O2 atoms, which is confirmed by a faster decrease of the dry air curve. This loss could be associated with a structural rearrangement in (3), H

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temperature on R3 values, suggesting that only the first resistance value is associated with a protonic conduction as already described for particle hydrates.3 Vibrational Fingerprint and Luminescence. To support the structural hypothesis of the excess proton location, the power spectrum of (H)2[Mo6Br8i(OH)6a]-12H2O (2) was extracted from the AIMD simulation at 300 K, and experimental IR measurements were conducted. The experimental IR spectra are presented in Figure 9a along with the total theoretical power spectrum in Figure 9b. The inset of Figure 9b highlights the signal originating from H*.

S13 report the corresponding Nyquist diagrams. It is worth highlighting that, for the three compounds, R2 decreases slowly when the atmosphere is moistened, as shown in Figure S9 for (H)2[Mo6Cl8(OH)6]-12H2O. For the three compounds, stabilization is reached after 3 days of equilibration and was checked during 21 days. This ensures a proper control of the hydration state of the compounds. Under ambient and wet atmospheres, (H)2[Mo6Cl8(OH)6]-12H2O (3) displays the lowest R2 value and so the highest conductivity. For instance, after equilibration and taking into account the thickness e of the pellet (1.13 mm), its conductivity under wet air, calculated from σ = e/(R2 × S), is 1.4 × 10−4 S·cm−1. This is higher than values commonly reported for inorganic particle hydrates proton conductors.3,70,71 Under the same conditions, for samples (1) and (2), the conductivity values, both equal to 2.5 × 10−6 S·cm−1, are drastically lower. This can be interpreted in terms of average oxygen−oxygen distances, as (H)2[Mo6Cl8(OH)6]-12H2O (3) displays the shortest O1−O2 and O2−O2 distances. According to Kreuer, this leads to a significant decrease of energy barriers for proton transfer.1 For the three samples, the capacitance value C3 is comprised between 5 × 10−9 and 1 × 10−7 F·cm−2, suggesting that the second semicircle of the impedance diagram results from grain boundary resistance, which thus depends on the pressing conditions of the pellet, and also on electrode contributions. When the atmosphere is dried, the resistance associated with the first semicircle of the diagram increases drastically, as quantified in the Table S5 of the Supporting Information, and the second semicircle disappears from the impedance diagram. This suggests a degradation or a structural transformation of the samples under dry air most likely associated with the loss of properly organized HB network resulting from the loss of water molecules. This is supported by the TGA measurements discussed above that show that a loss of water molecules can occur in all three compounds already at 25 °C under dry atmosphere. To discuss TGA and proton-conduction measurements in parallel, the influence of temperature on the proton conductivity of the three compounds was also measured. In that case, experiments were performed under wet atmosphere only to preserve as much as possible the structure of the three materials. The temperature increase induces an R2 decrease up to 35 °C for sample (3) and 46 °C for samples (1) and (2). The tendency of the compounds to easily lose water molecules suggests that the water molecules are quite mobile in the temperature range considered for proton-conduction measurements. Furthermore, the decrease of R2 with increasing temperature also suggests that vehicular transport is responsible for the proton conductivity. This does not demonstrate that structural diffusion does not occur, but additional experiments such as pulse-field-gradient NMR or quasielastic neutron scattering would be required to probe it and to determine their relative contribution to the whole conductivity. From these measurements, and considering the effect of the temperature on sample (3), activation energy was evaluated in wet atmosphere from an Arrhenius law in the 25−31 °C range, that is, in conditions in which dehydration does not occur, and is equal to 0.34 eV. A similar evaluation was made for samples (1) and (2) and leads to respective values of 0.49 and 0.21 eV, that is, comparable to other particle hydrates proton conductors.70 In the corresponding temperature range, and as shown, for instance, for sample (3) (see Figure S14 of the Supporting Information), there is no significant influence of

Figure 9. (a) IR spectra of (1) (black), (2) (red), and (3) (blue) highlighting the vibration signal of the H*. (b) Power spectrum of (H)2[Mo6Br8(OH)6]-12H2O (2) obtained from the Fourier transform of the atomic velocities autocorrelation functions extracted from a MD simulation at 300 K. (inset) Highlighting the signal of the H* protons.

The power spectrum is dominated by the signal of the water molecules: a large band centered at 3375 cm−1 and a narrower band centered at 1650 cm−1 corresponding to O−H stretching and HOH bending, respectively. The region below 600 cm−1 encompasses both Mo−Mo, Mo−Br, and Mo−O vibrations. The large band between 1200 and 600 cm−1 is more complex to analyze, as it represents both H2O libration and Mo−O−H bending. The inset of Figure 9b reveals that the H* proton vibrates at a frequency of ∼1200−1250 cm−1, which falls between the bending and libration modes of water molecules. This H* vibration is also higher in energy than the Mo−O−H bending. Experimentally, the IR spectra of (1), (2), and (3) are very similar and are in good agreement with the power spectrum of (H)2[Mo6Br8(OH)6]-12H2O (2): they exhibit a large band centered at 3270, 3263, and 3155 cm−1, respectively, corresponding to O−H stretching; a narrower band centered at 1630, 1635, and 1627 cm−1, respectively, corresponding to I

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instance, the very recent study of a CP zirconium phosphate,73 and will be useful for the design of future proton-conducting materials. The present proton conductivities can be compared to other molybdenum-based proton conductors such as heteropolyoxometallate acids. In this family, H3PMo12O40·29H2O is known to display a very high proton conductivity originating from both Grotthus and vehicle-type transport mechanisms.1 Of course, the present materials would need further optimization to reach similar performances, in particular, in terms of thermal stability, which would need to be improved over much larger temperature range. Once achieved, this would allow for more accurate and systematic characterizations of their proton-conduction properties. This work is now in progress. Nevertheless, the luminescent properties of the isolated [Mo6L8(OH)6]2−/4− building blocks are conserved in the three materials, which makes them photoluminescent proton-conductors. It is worth pointing out that materials combining photoluminescence with protonic conductivity expand the range of possible applications, as they should be used as component devices for water splitting. For this reason, molybdenum-cluster materials represent a new and original class of proton conductors deserving further investigations.

water bending; a narrow band centered at 980, 1031, and 1055 cm−1, respectively, corresponding to water libration. At 775, 758, and 782 cm−1 for (1), (2), and (3), respectively, the Mo− O−H bending modes are observed. The narrow bands below 500 cm−1 correspond to Mo−O vibrations. Finally, comparison with the theoretical power spectrum allows to attribute the narrow signal at 1269 cm−1 for (1), 1323 cm−1 for (2), and 1400 cm−1 for (3) to the vibration of H* as it falls between bending and libration modes of water. In contrast to the vibrational modes associated with the water molecules, this signal appears to be quite sensitive to the nature of the compound and the cluster unit. This particular signal is thus a vibrational fingerprint of the HO−H*−OH bonding motif that can be readily identified in the spectra. Finally, Figure S11 of the Supporting Information displays the luminescence spectra of (1), (2), and (3) recorded at −180 °C. All compounds exhibit luminescence properties, and their emission is between 550 and 900 nm. The shape of the spectra are typical of those observed for Mo6Xi8 cluster-core-based compounds.16,72 The maximum of emission is located at 667, 674, and 681 nm for (2), (1), and (3), respectively. This demonstrates that the maximum of emission is shifted to red with the electronegativity of the inner ligand (χCl > χBr/S > χBr) and that the luminescent properties of the [Mo6X8(OH)6]2− (or [Mo6Br6S2(OH)6]4−) cluster units are conserved within the materials. This makes them luminescent proton-conducting materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00340. Crystal structure determination of (H)4[Mo6Br6S2(OH)6]-12H2O, crystal data and structure refinement parameters, atomic coordinates, site occupancy, and equivalent isotropic displacement parameters, Rietveld refinement parameters, PXRD data, illustrated projections of hydrogen-bond networks, proton conduction measurements, Nyquist diagrams, schematic representation of electrical equivalent circuit used to fit the Nyquist diagrams, comparison of resistance and capacitance values determined from Nyquist diagrams, luminescence measurements, CIF and Checkcif files (PDF)

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CONCLUSIONS In the present article, three compounds based on the discrete [Mo6L8(OH)6]2−/4− building blocks have been studied: (H)4[Mo6Br6S2(OH)6]-12H2O, (H)2[Mo6Br8(OH)6]-12H2O, and (H)2[Mo6Cl8(OH)6]-12H2O. Combining AIMD simulations and proton-conduction measurements, proton conductivity is highlighted in the three compounds. Those materials thus represent the first examples of proton conductors based on molybdenum clusters. MD simulations are in line with the occurrence of concerted proton transfers, which are facilitated by a well-organized HB network. The existence of HO−H*−OH bridges between adjacent cluster units is proposed to explain the experimentally observed short oxygen−oxygen distances in the crystal structures. They are natural resting places for the excess protons, which results in a shorter oxygen−oxygen distance for (H)4[Mo6Br6S2(OH)6]12H2O as compared to the two other compounds. Nevertheless, (H)2[Mo6Cl8(OH)6]-12H2O displays the highest proton conductivity resulting from overall lower oxygen− oxygen distances. The combination of TGA and protonconduction measurements demonstrates that vehicular transport occurs in the three compounds, which suggests that the conduction mechanism is more complex than the one inferred by the theoretical simulation only. However, the full characterization of the conduction mechanism would necessitate further measurements such as pulse-field-gradient NMR or quasielastic neutron scattering. An infrared fingerprint for the HO−H*−OH bridges is also proposed from MD simulations, which is effectively observed experimentally. The present study is thus original in two respects: first, it presents for the first time molybdenum cluster-based materials that are proton conductors; second, it involves an MD description of the proton conduction process that efficiently complements the experimental observations. Such synergy between theory and experiments has already been used in other studies, for

Accession Codes

CCDC 1823855 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (J.C.) *E-mail: [email protected] Phone: +33 (0)5 61 55 68 36; +33 (0) 2 23 23 65 36. Fax: +33 (0)5 61 55 60 65. (S.C.) ORCID

Jérôme Cuny: 0000-0002-7882-9156 Pierric Lemoine: 0000-0002-3465-7815 Eric Quarez: 0000-0001-7887-892X Nikolay G. Naumov: 0000-0002-7531-6291 Stéphane Cordier: 0000-0003-0707-3774 J

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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the supercomputing facility of CALMIP for generous allocation of computer resources (Project P1320). Authors thank CNRS, Univ. of Toulouse, Univ. Rennes 1, INSA, ENSCR, and RFBR (No. 17-53-16015) for the funding of LIA No. 1144 CLUSPOM: Innovative Materials and Nanomaterials Based on Tailor-Made Functional Building Blocks between France and Russia. G.D. thanks Rennes Métropole for funding a three-month stay in Nikolaev Institute of Inorganic Chemistry (Novosibirsk). N. Dumait is strongly acknowledged for preparation of starting materials, T. Roisnel from CDIFX for X-ray diffraction data collection, and B. Lefeuvre for measurement of IR spectra.

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DOI: 10.1021/acs.inorgchem.8b00340 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00340 Inorg. Chem. XXXX, XXX, XXX−XXX