Lewis Acidic Behavior of MoOF4 towards the Alkali Metal Fluorides in Anhydrous Hydrogen Fluoride Solutions

: Previous studies performed on samples of MoOF 4 dissolved in anhydrous hydrogen fluoride (aHF) solutions have indicated the presence of the [Mo 2 O 2 F 9 ] – anion. Building upon this earlier work, MoOF 4 , and M F ( M = Li–Cs) were dissolved in aHF solutions to produce M [Mo 2 O 2 F 9 ] ( M = Li–Cs) salts. Structural analysis of the obtained compounds was performed using single-crystal X-ray diffraction. This study provides the first sin-Introduction lated for a polycrystalline powder sample (total isotropic intensity in arbitrary units). The Raman spectra were obtained by using a pseudo-Voigt band profile (50:50 Lorentzian:Gaussian) and an FWHM of 8 cm –1 . The Raman spectra were simulated taking into account the experimental setup ( T = 298.15 K, λ = 532 nm). For the IR spectra, a Lorentzian lineshape and an FWHM of 8 cm –1 was used. The band assignments were carried out by visual inspection of the normal modes using the J mol program package. [28]


Introduction
Trifluorido trioxido molybdate(VI) anions, [MoO 3 F 3 ] 3-, such as those in K 3 MoO 3 F 3 and Cs 2 RbMoO 3 F 3 , have been shown to display many applicable physical properties, most notably ferroelectricity. [1,2]In addition, molybdenum oxide fluoride anions, in which the molybdenum atoms have d 0 electronic configurations, may exhibit second-order Jahn-Teller distortion. [3]This distortion, characterized by a displacement of the Mo atom from the center of its octahedral coordination environment, and alternating atomic distances between the Mo and neighboring bridging fluorine/oxygen atoms, can yield compounds displaying metallic, ferroelectric, and magnetic behaviors. [3][6][7][8] However, little remains known regarding the chemical properties of the simplest Mo VI molybdenum oxide fluoride, MoOF 4 .
MoOF 4 was first synthesized in 1907 by Ruff and Eisner, whereby aHF was condensed onto molybdenum oxide tetrachloride (MoOCl 4 ). [9]MoOF 4 is a white solid having a melting point of 97 °C and a boiling point of 186 °C (under ambient atmospheric pressure). [9]The crystal structure of MoOF 4 , which is related to that of VF 5 , [10] is characterized by essentially octahedrally-coordinated Mo atoms linked into infinite chains by cisbridging fluorine atoms. [11]The solid-state structure of MoOF 4 3672 gle crystal structures for the [Mo VI 2 O 2 F 9 ] -anion.Additionally, IR and Raman spectroscopy were used to characterize each salt.These spectra were compared to calculated ones for the solidstate structure of each salt using the DFT-PBE0 density functional method.The calculated spectra were used to give band assignments in the experimentally obtained spectra.
features Mo atoms in distorted octahedral surroundings and alternating Mo-(μ-F) atomic distances, see Figure 1. [3]gure 1.Section of the infinite, one-dimensional chains observed in the solid-state structure of MoOF 4 .Atomic distances between Mo and the μ-F atoms are displayed to show their alternating pattern.Displacement ellipsoids are shown at a 70 % probability level at 100 K. MoOF 4 has been described in the literature as being a weak Lewis acid and fluoride ion acceptor, with WOF 4 being, comparatively, both a stronger Lewis acid and fluoride ion acceptor. [12,13]A detailed understanding of the Lewis acid and fluoride ion acceptor behavior of MoOF 4 in aHF was given by Bougon and co-workers, who studied aHF solutions containing MoOF 4 and fluoride ion donors using Raman and 19 F NMR spectroscopy. [12]Their study determined that MoOF 4 forms a chemical equilibrium in aHF solutions between the [Mo 2 O 2 F 9 ] -, [MoOF 5 ] -, [MoOF 6 ] 2-anions, depending on the [HF 2 ] -concentration available in solution, as shown in Equations 1-3. [12]It was predicted that higher concentrations of [HF 2 ] -drives the chemical equilibrium of each reaction to the right, and thus pushes the chemical equilibrium from the left side of Equation 1 to the right side of Equation 3. [12] Similarly, a study aimed at determining the interaction of MoOF 4 with acetylacetone in acetonitrile solution also reported evidence for the formation of the [Mo 2 O 2 F 9 ] -anion. [14]Additionally, work performed by Holloway and Schrobilgen described the formation of KrF 2 •nMoF 4 (n = 1-3) and XeF 2 •nMoOF 4 (n = 1-4) adducts, in which a structurally related anion, MoO 2 F 8 , to which an F atom from a XeF 2 ligand is bound, was observed. [13,15][14][15][16][17][18][19][20][21][22] Therefore, the goal of this work was to expand the current knowledge of the chemical behavior of MoOF 4 in aHF solutions by reacting it with the alkali metal fluorides.As a result, the series of M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts was obtained.Structural analysis of these salts, performed using single-crystal X-ray diffraction, provides the first crystal structures containing the [Mo VI 2 O 2 F 9 ] -anion.The structures of these anions will be compared to the previously reported [Mo V 2 O 2 F 9 ] 3-anions.Furthermore, vibrational spectro-

Results and Discussion
Synthesis of the M[Mo 2 O 2 F 9 ] (M = Li-Cs) Salts First, MoOF 4 was synthesized through the hydrolysis of MoF 6 by silicon dioxide in aHF, see Equations 4 and 5. [23] After pumping off the unreacted aHF and the SiF 4 formed during the reaction, the MoOF 4 product was stored in an inert atmosphere glovebox until further use.Next, a 1:1 molar ratio of MoOF 4 and the respective alkali metal fluoride was reacted in an aHF solution to obtain the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts, see Equation 6. Single crystals of the salts were grown directly from aHF solution.However, as evidenced by X-ray powder diffraction, as well as IR and Raman spectroscopy, the compounds could not be obtained phase-pure.Evidence suggests the impurities to be the respective adducts of [F(HF) n ] (n = 1,2)], and other, yet unidentified, compounds.

Single Crystal X-ray Analysis
All Selected crystallographic data and details of the single crystal structure determination are given in Table 1.The single crystal structures of each salt will be discussed individually; comparisons, when appropriate, will be noted.A comparison of the salts reported here to the previously reported •H 2 O salts will be presented at the end of this section.S1).On the other hand, Li(2) experiences disorder due to space group symmetry (center of inversion) and its site occupancy factor was fixed to a value of 0.5.The Li(2) cation is coordinated by four F t atoms also belonging to four [Mo 2 O 2 F 9 ] - anions, forming a tetrahedral coordination polyhedron (Figure S2a).The coordination tetrahedron around Li(2) shares an edge with the coordination tetrahedron generated by the crystallographic inversion center (Figure S2b).Similar edge-sharing tetrahedra containing disordered Li + cations have been described previously in compounds such as Li 3 InF 6 , [24] Li 7 OsO 6 , [25] and Li 9 B 19 S 33 . [26]Moreover, the coordination of the [Mo 2 O 2 F 9 ] -anions to the lithium cations form two-dimensional infinite sheets, which are oriented parallel to the (011) plane, see Figure S3 (for a topological comparison of the crystal structures, see below).When the [Mo 2 O 2 F 9 ] -anion is viewed from the side, so that the respective Mo atoms are eclipsing, it can be seen that the adjacent F t atoms are nearly eclipsing.The smallest of these torsion angles range between 2.393(11)°and 6.781( 12 S1.A list of selected atomic distances and angles observed in the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts is reported in Table 3.
In comparison to the other salts reported here, Li[Mo 2 O 2 F 9 ] has the largest range for its Mo-F t atomic distances, ranging from 1.829( 2

Single Crystal Structure of Na[Mo 2 O 2 F 9 ]
Na[Mo 2 O 2 F 9 ] crystallizes in the monoclinic space group P2/c with the lattice parameters a = 8.14411( 8  The smallest torsion angles between adjacent F t atoms (seen when the [Mo   salts are isotypic to each other, so only the potassium salt will be discussed in detail.Additionally, these salts are also isotypic to the previously reported [H 3 O]W 2 O 2 F 9 salt. [23]n the K[Mo 2 O 2 F 9 ] salt, the K + cation is coordinated by twelve fluorine atoms, ten F t atoms and two μ-F atoms (belonging to four different [Mo 2 O 2 F 9 ] -anions), creating a distorted anti-   6.The torsion angles between adjacent F t atoms (Figure S4 7. The torsion angles between adjacent F t atoms (Figure S4) range between 40.607(13)°to 44.819( 14 2)°, giving rise to an angle larger than those observed in the potassium and rubidium salts, but smaller than those found in the sodium and lithium salts.
Upon comparison of the salts reported here, it is seen that Li[Mo 2 O 2 F 9 ] crystallizes in the triclinic space group P1 ¯, whereas the remaining salts crystallize in the monoclinic space group P2/c.The potassium, rubidium, and cesium salts were observed to be isotypic to one another, as well as the previously reported The Mo-μ-F, Mo-F t , and Mo-O atomic distances found in the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts are comparable to those found in other molybdenum fluorides and oxyfluorides. [11,27]No obvious explanation could be found to explain the trend in Mo-μ-F-Mo angles observed in the salts reported here.It was observed that torsion angles between adjacent F t atoms, however, steadily increased with increasing cation size.

Infrared Spectroscopy
An IR spectrum for each M[Mo 2 O 2 F 9 ] (M = Li-Cs) salt was collected at 25 °C in the region of 4000 to 400 cm -1 .Band positions are listed in Table 8.Calculated IR spectra were obtained for the solid-state structure of each salt using the DFT-PBE0 density functional method; combination bands and overtones bands were not accounted for.Experimental spectra correlated well with the calculated spectra (Figure 6; Figures S10-S13, Tables S4-S8), however, the experimentally obtained spectra show additional bands belonging to by-products.Powder diffraction patterns of each sample were recorded to aid in the identification of the by-products.However, due to the quality of the diffraction patterns, unambiguous identification of the by-products was not possible.Therefore, a discussion of potential by-products will be provided based on the reported IR spectra.Powder diffraction patterns are shown in Figures S15-S19.Band assignments in Table 8, when given, were made by the visual inspection of the calculated normal modes using the J mol software [28] and through direct comparison to the experimentally obtained bands.In each spectrum, no bands were observed beyond 2500 cm -1 .As an example of an IR spectrum typical of this series of salts, the IR spectrum of Na[Mo 2 O 2 F 9 ], in the region of 2500 cm -1 to 400 cm -1 , is shown in Figure 6.The remaining experimentally obtained and calculated IR spectra are shown in Figures S10-S13.The IR spectrum of each salt is dominated by Mo-F stretching in the region of about 400 cm -1 to 700 cm -1 , along with a prominent Mo-O stretching band seen around 1015 cm -1 .
In the IR spectra of Li[Mo 2 O 2 F 9 ] and Na[Mo 2 O 2 F 9 ], the deformation vibration ν 2 and the antisymmetric stretching vibration ν 3 of the bifluoride ion, HF 2 -, are clearly visible (refer to Table 8). [29,30]The presence of bifluoride and other [F(HF) n ] - (n = typically 2 to 4) impurities likely arise from the fact that the alkali metal fluorides were present in excess in the reaction 3678 mixtures.The IR spectrum of K[Mo 2 O 2 F 9 ] (Figure S11) shows bands at 451, 1015, 1037, 1101, 1157, 1743 and 2359 cm -1 .Previous studies report the IR bands of KF•2HF to occur at 480 (ν 1 ), 1030-1150 (ν 2 ), 1780 (ν 3 ) 2000, and 2350 cm -1 . [30,31]The presence of similar bands in the spectrum of K[Mo 2 O 2 F 9 ] may arise from the presence of KF•2HF in the product mixture.However, since the Mo-O stretching bands in the [Mo 2 O 2 F 9 ] -anion occur in the region of 1000-1100 cm -1 , the bands arising from any KF•2HF would be overlapped with these bands.This makes the detection of any possible KF•2HF present difficult since its band intensities cannot be adequately measured in this region.Given the reaction conditions, however, it should be possible to observe the presence of fluorides such as KF•2HF.Moreover, the spectrum of K[Mo 2 O 2 F 9 ] shows bands at 1300, and 1366 cm -1 .These two bands may also arise from higher HF adducts of potassium fluoride, however, direct assignment of these bands to a particular fluoride could not be made.The IR spectra of Rb[Mo 2 O 2 F 9 ] and Cs[Mo 2 O 2 F 9 ] showed no evidence of bifluoride or higher HF adduct formation.
The spectra of all M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts show bands in the region of about 850 to 990 cm -1 which do not belong to the [Mo 2 O 2 F 9 ] -anion.In the work of Beuter and Sawodny, in which Rb[Mo 2 O 2 F 9 ] was reported, they observed a series of bands in the IR region of 930 to 975 cm -1 . [16]They attributed the observation of these bands to hydrolysis products of the Rb[Mo 2 O 2 F 9 ] salt. [16]However, since the samples presented in this work were handled under strictly-dried atmospheres, and because there is no evidence of water or hydroxides in the spectra presented here, it seems highly unlikely that the bands observed in this work arise because of hydrolysis products.Moreover, in the work of Bougon and co-workers, NO[Mo 2 O 2 F 9 ] and ClOF 2 [Mo 2 O 2 F 9 ] were synthesized and characterized through IR spectroscopy.The bands arising from the [Mo 2 O 2 F 9 ] - anion in the NO[Mo 2 O 2 F 9 ] and ClOF 2 [Mo 2 O 2 F 9 ] salts correlate well to those observed in this work.However, the authors observed bands at 910, 935, 967, 973, and 983 cm -1 in the spectrum of NO[Mo 2 O 2 F 9 ] which do not belong to the [Mo 2 O 2 F 9 ] - anion.They assigned the band at 983 cm -1 to NO[MoOF 5 ] but gave no further discussion on the occurrence of the remaining bands. [12] determine if the [MoOF 5 ] -anion is responsible for the observation of bands arising between 850 and 990 cm -1 in the spectra reported here, an IR spectrum of a C 4v [MoOF 5 ] -anion was calculated using the DFT-PBE0 density functional method and the def2-TZVP basis set (Figure S19, Table S9).The calculated spectrum shows a band at 1037 cm -1 , correlating to Mo-O stretching and bands below 700 cm -1 correlating to Mo-F stretching (and other vibrational modes), but does not show any bands in the region of about 700 to 1000 cm -1 .This calculated IR spectrum correlates well to previously reported IR spectra of salts containing the [MoOF 5 ] -anion. [12,32]Moreover, the calculated IR spectrum of the [MoOF 5 ] -anion is quite similar to the calculated IR spectra of the [Mo 2 O 2 F 9 ] -anion.Therefore, it would be hard to determine the presence of the [MoOF 5 ] - anion in the sample mixtures with IR spectroscopy alone since any bands arising from this anion would likely overlap with those arising from the [Mo 2 O 2 F 9 ] -anion.Based on the calcu-lated IR spectrum, though, it can be said that the bands arising between 850 to 970 cm -1 in the spectra reported in this work are not observed because of a possible presence of the [MoOF 5 ] -anion.The fact that similar bands are seen in the IR spectra of previously reported [Mo 2 O 2 F 9 ] -anions, though, may provide insight into the complex equilibria that seemingly exists during the formation of the [Mo 2 O 2 F 9 ] -anion.

Raman Spectroscopy
Raman spectra were obtained for each M[Mo 2 O 2 F 9 ] (M = Li-Cs) salt at 25 °C from about 0 to 1200 cm -1 .Except for K[Mo 2 O 2 F 9 ], all spectra were recorded at an excitation wavelength of 532 nm.The K[Mo 2 O 2 F 9 ] salt showed strong fluorescence at this wavelength and so Raman data for this salt was collected at an excitation wavelength of 785 nm.Band locations for these experimental spectra are listed in Table 9. Calculated Raman spectra from 0 to 1200 cm -1 were obtained for the solid-state structures of each salt using the DFT-PBE0 density functional method.Experimentally obtained spectra correlated well with the calculated spectra, however, the experimentally obtained spectra show additional bands belonging to by-products (Figure 7; Figures S20-S23, Tables S10-S14).Powder diffraction patterns of each sample were recorded to aid in the identification of the by-products.However, due to the quality of the diffraction patterns, unambiguous identification of the by-products was not possible.Therefore, a discussion of potential by-products will be provided based on the reported Raman spectra.
Band assignments in Table 9, when given, were made by the visual inspection of the calculated normal modes using the J mol software [28] and through direct comparison to the experimentally obtained bands.As an example of a typical spectrum Table 9. Experimental Raman bands observed in the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts.When given, a description of vibrational modes is as follows: ν = stretching, δ = deformation, ρ w = wagging, ρ r = rocking, ρ t = twisting, s = symmetric, as = asymmetric.F μ refers to the bridging fluorine atoms and F t refers to terminal fluorine atoms.salts. [12]The Raman spectra of each salt, in the region of about 0 to 400 cm -1 , is dominated by various Mo-O and Mo-F deformation, wagging, rocking, and twisting modes.Similar to the IR spectra, the Raman spectra show bands in the 400 to 700 cm -1 region, arising from Mo-F stretching modes and the Mo-O stretching modes were observed around 1020 cm -1 .
In the IR spectra of Li[Mo 2 O 2 F 9 ] and Na[Mo 2 O 2 F 9 ], the presence of the HF 2 -anion is clearly visible (Figure 7; Figure S20).However, evidence for the presence of the bifluoride anion in the Raman spectra is not as obvious.The bifluoride anion has two reported Raman active modes: the F-H-F -symmetric stretching mode ν 1 (A 1g ) and the librational lattice mode (E g ), which occur at about 630 and 145 cm -1 , respectively. [33]Since the backgrounds in the early region of the Raman spectra are large, it is hard to determine if any bands are present around 145 cm -1 .Additionally, bands arising from the Mo-F stretching modes of the [Mo 2 O 2 F 9 ] -anion occur in the region where the symmetric stretching mode of the HF 2 -anion is expected to occur.It is, therefore, difficult to identify the presence of the bifluoride anion in these Raman spectra.Moreover, the presence of the H 2 F 3 -anion was evidenced in the IR spectrum of K[Mo 2 O 2 F 9 ].However, the Raman spectrum of K[Mo 2 O 2 F 9 ] shows no foreign bands (i.e.all bands could be assigned to the [Mo 2 O 2 F 9 ] -anion).Reasons for this observation may stem from the fact that this Raman spectrum was collected at 785 nm and it was noticed that the background in this spectrum was much higher than those seen in the other spectra.It could be that the noise in this spectrum masks any bands arising from the H 2 F 3 -anion.
Just as in the IR spectra, the Raman spectra of all salts, with the exception of K[Mo 2 O 2 F 9 ], show a series of bands around 900 to 1000 cm -1  ] was synthesized, they described an IR band at 983 cm -1 , which was sometimes present in the Raman spectrum as a weak band. [12]They attribute this band to the presence of NO[MoOF 5 ].
Again, to determine if the [MoOF 5 ] -anion was present in our samples, a Raman spectrum of a C 4v [MoOF 5 ] -anion was calculated using the DFT-PBE0 density functional method and the def2-TZVP basis set and is shown in Figure S24 (band locations and intensities reported in Table S15).This spectrum showed a band at 1038 cm -1 corresponding to Mo-O stretching and several other bands below 700 cm -1 corresponding to Mo-F stretching (and other vibrational modes).This calculated Raman spectrum correlates well to previously reported Raman spectra of salts containing the [MoOF 5 ] -anion. [12,32]Moreover, the calculated Raman spectrum of the [MoOF 5 ] -anion is some-what similar to the calculated Raman spectra of the [Mo 2 O 2 F 9 ] - anions calculated here.Therefore, as was the conclusion in the IR spectroscopy analysis, it would be hard to determine the presence of the [MoOF 5 ] -anion in the sample mixtures with Raman spectroscopy alone.Based on the calculated Raman spectrum, though, it can be said that the bands arising between 900 to 970 cm -1 in the Raman spectra reported in this work are not observed because of a possible presence of the [MoOF 5 ] - anion.

Conclusions
A series of M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts was synthesized through the reaction of MoOF 4 with the respective alkali metal fluoride in an aHF solution.The crystal structures of these salts were studied using single-crystal X-ray diffraction.All crystal structures show a layer structure motif.The potassium, rubidium and cesium salts are isotypic.In the case of the lithium salt, disorder was shown by one of the lithium atoms.In all compounds, the [Mo 2 O 2 F 9 ] -anion is observed, which can be thought of as two MoOF 4 units bridged by a fluorine anion.IR and Raman spectroscopy were employed to obtain the vibrational spectra of the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts.Calculated IR and Raman spectra were obtained for the solid state of each salt using the DFT-PBE0 density functional method in order to aid in the assignment of bands in the experimentally obtained spectra.The calculated IR and Raman spectra agree nicely with the experimentally obtained spectra.Evidence for hydrogen fluoride adducts, such as [F(HF) n ] -(n = 1,2) were observed in some cases.The synthesis of phase-pure M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts was not realized, although their phase-pure synthesis may be possible in solvents which would not directly interact with the starting materials.
It is clear that the reaction conditions leading to the production of the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts is quite complex, as indicated by the variety of by-products observed within the samples (as evidenced by the IR and Raman spectra and powder X-ray diffraction patterns).Studies in the past which have tried to produce the [MoOF 5 ] -anion have observed that, in systems with relatively low free F -concentration, the [Mo 2 O 2 F 9 ] -anion is preferably produced.If the concentration of free F -is increased, by increasing the concentration of the fluoride salt(s) used, then the [MoOF 5 ] -anion and, subsequently, the [MoOF 6 ] 2-anion may be successfully produced.

Experimental Section
General Procedures and Materials: All operations were performed in either stainless steel (316 L) or Monel metal vacuum lines, which were passivated with 100 % fluorine at various pressures before use.Preparations were carried out in an atmosphere of dry and purified Argon (5.0, Praxair).Molybdenum hexafluoride (99 %, ABCR) was distilled once prior to usage.Anhydrous HF (Fluka Analytical, > 99.9 %), which was stored over K 2 [NiF 6 ], was distilled twice before use.Fluorinated ethylene propylene, FEP (15.90x19.05mm -3/4"), was used to produce reaction vessels.To produce the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts, an approximately 1:1 molar ratio of MoOF 4 and MF (M = Li-Cs) was mixed using an agate mortar and pestle in an inert atmosphere glovebox and transferred to an FEP reaction vessel.The reaction vessel was attached to a metal vacuum line and evacuated and flushed with argon 3 times.4 to 6 mL of aHF was condensed onto the reactants at -196 °C.The reaction vessels were warmed to room temperature, producing colorless solutions in all cases.Once all reactants had dissolved, the aHF was slowly pumped off to produce colorless, crystalline samples.The products were transferred to an inert atmosphere glovebox for storage.It was noticed after months of storage the samples began to turn gray-blue.

Synthesis of Li[Mo 2 O 2 F 9 ]:
Under inert atmosphere, 123.91 mg of MoOF 4 (0.66 mmol) and 17.90 mg of LiF (0.69 mmol) were placed in an FEP reaction vessel.4 mL of aHF were condensed on top of the reaction mixture.After warming to room-temperature the aHF was slowly pumped off over a period of 5 hours.Product was stored in a PTFE container under argon in a glovebox.

Synthesis of Na[Mo 2 O 2 F 9 ]:
Under inert atmosphere, 125.07 mg of MoOF 4 (0.67 mmol) and 27.90 mg of NaF (0.66 mmol) were placed in an FEP reaction vessel.4 mL of aHF were condensed on top of the reaction mixture.After warming to room-temperature the aHF was slowly pumped off over a period of 5 hours.Product was stored in a PTFE container under argon in a glovebox.

Synthesis of K[Mo 2 O 2 F 9 ]:
Under inert atmosphere, 126.15 mg of MoOF 4 (0.67 mmol) and 39.08 mg of KF (0.67 mmol) were placed in an FEP reaction vessel.4 mL of aHF were condensed on top of the reaction mixture.After warming to room-temperature the aHF was slowly pumped off over a period of 5 hours.Product was stored in a PTFE container under argon in a glovebox.

Synthesis of Rb[Mo 2 O 2 F 9 ]:
Under inert atmosphere, 127.82 mg of MoOF 4 (0.68 mmol) and 69.91 mg of RbF (0.67 mmol) were placed in an FEP reaction vessel.6 mL of aHF were condensed on top of the reaction mixture.After warming to room-temperature the aHF was slowly pumped off over a period of 5 hours.Product was stored in a PTFE container under argon in a glovebox.

Synthesis of Cs[Mo 2 O 2 F 9 ]:
Under inert atmosphere, 128.78 mg of MoOF 4 (0.69 mmol) and 102.82 mg of CsF (0.68 mmol) were placed in an FEP reaction vessel.6 mL of aHF were condensed on top of the reaction mixture.After warming to room-temperature the aHF was slowly pumped off over a period of 5 hours.Product was stored in a PTFE container under argon in a glovebox.lated for a polycrystalline powder sample (total isotropic intensity in arbitrary units).The Raman spectra were obtained by using a pseudo-Voigt band profile (50:50 Lorentzian:Gaussian) and an FWHM of 8 cm -1 .The Raman spectra were simulated taking into account the experimental setup (T = 298.15K, λ = 532 nm).For the IR spectra, a Lorentzian lineshape and an FWHM of 8 cm -1 was used.The band assignments were carried out by visual inspection of the normal modes using the J mol program package. [28] 3673 scopy was studied using IR and Raman techniques and the obtained spectra were compared to quantum chemically calculated solid-state spectra.A discussion on the synthesis of the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts is presented, along with suggestions as to the possible synthesis of M[MoOF 5 ] and M 2 [MoOF 6 ] (M = Li-Cs) salts.
)°. Upon comparison of the torsion angles between F t atoms in the M[Mo 2 O 2 F 9 ] (M = Na-Cs) salts, a trend became apparent: the larger the respective cation, the larger the respective torsion angles.That is, nearly eclipsing F t atoms were observed in Li[Mo 2 O 2 F 9 ] while a more staggered conformation of F t atoms was observed in Cs[Mo 2 O 2 F 9 ].A schematic showing the differences in torsion angle can be seen in Figure S4.A complete list of torsion angles observed in Li[Mo 2 O 2 F 9 ] is reported in Table ) to 1.895(2) Å.Additionally, the shortest Mo-μ-F atomic distances were observed in Li[Mo 2 O 2 F 9 ], ranging from 2.110(2) to 2.114(2) Å.The Mo-O atomic distances in Li[Mo 2 O 2 F 9 ] have values of 1.650(3) and 1.653(3) Å and the Mo-F μ -Mo angle in the lithium salt was observed to be 158.89(12)°; the largest Mo-μ-F-Mo angle in the series.A section of the crystal structure of Li[Mo 2 O 2 F 9 ] is shown in Figure 3.To the best of our knowledge, the compound represents a new structure type.

Figure 3 .
Figure 3. Section of the crystal structure of Li[Mo 2 O 2 F 9 ].Displacement ellipsoids are shown at a 70 % probability level at 100 K, grey atoms indicate Li atoms with site disorder.
), b = 6.5008(4), c = 8.6741(8) Å, β = 113.025(7)°,V = 422.65(7)Å 3 , Z = 2, T = 100 K. Atomic coordinates and equivalent isotropic displacement parameters for Na[Mo 2 O 2 F 9 ] are reported in Table 4.The coordination number of the sodium cation is seven as a result of six coordinating F t atoms and one coordinating μ-F atom (belonging to four different [Mo 2 O 2 F 9 ] -anions), creating a distorted pentagonal bipyramidal coordination polyhedron (Figure S5).The coordination of the [Mo 2 O 2 F 9 ] -anions to the sodium cations again leads to the formations of two-dimensional infinite sheets which are parallel to the b-c plane, see Figure S6.
2 O 2 F 9 ] -anion is viewed along the Mo-Mo axis) range between 9.087(21)°and 15.491(22)°.These angles are larger than those observed in Li[Mo 2 O 2 F 9 ] but smaller than those observed in K[Mo 2 O 2 F 9 ].A list of all torsion angles observed in Na[Mo 2 O 2 F 9 ] is reported in Table S2.Na[Mo 2 O 2 F 9 ] has the longest Mo-O atomic distance in the series, having a value of 1.660(5) Å. Mo-F t atomic distances range between 1.828(4) and 1.871(4) Å.The Mo-μ-F atomic distance is 2.1489(15) Å and the Mo-μ-F-Mo angle is 150.7(3)°.A section of the crystal structure of Na[Mo 2 O 2 F 9 ] is shown in Figure 4. To the best of our knowledge, the compound represents a new structure type.

Figure 4 .
Figure 4. Section of the crystal structure of Na[Mo 2 O 2 F 9 ].Displacement ellipsoids are shown at a 70 % probability level at 100 K.

Figure 5 .
Figure 5. Section of the crystal structure of K[Mo 2 O 2 F 9 ].Similar crystal structures are seen in the isotypic rubidium and cesium salts.Displacement ellipsoids are shown at a 70 % probability level at 100 K.

F 9 ] 3 -
anions are the values of the Mo-μ-F-Mo angles.In the salts reported here, the Mo-F μ -Mo angles range from 137.98(11)°to 158.89(12)°.These angles are much larger in the previously reported Mo +V salts, having values of 169.4(3)°forK 2 (NMe 4 )[Mo V 2 O 2 F 9 ]•H 2 O and 180.0°for (NH 4 ) 3 [Mo (V) 2 O 2 F 9 ].This observation is expected since the repulsion arising from the additional negative charge found in the [Mo (V) 2 O 2 F 9 ] 3-salts can be mitigated by larger Mo-F μ -Mo angles.Furthermore, upon comparison of atomic distances in the [Mo V 2 O 2 F 9 ] 3-anions to the [Mo VI 2 O 2 F 9 ] anions reported here, it was shown that the Mo-O distances are comparable, whereas the Mo-F atomic distances are, on average, longer in the [Mo V 2 O 2 F 9 ] 3-anions.Again, such an observation is expected because the presence of additional negative charge on the [Mo V 2 O 2 F 9 ] 3-anion leads to increased electronic repulsion between the F and Mo atoms, which subsequently increases Mo-F atomic distances.

Figure 7 .
Figure 7. Experimentally obtained (above) and calculated (below) Raman spectrum of Na[Mo 2 O 2 F 9 ].obtained from these salts, the Raman spectrum of Na[Mo 2 O 2 F 9 ] is given in Figure 7.The remaining experimentally obtained and calculated Raman spectra are shown in Figures S20-S24.
Since the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts produced in this work were synthesized using a 1:1 molar ratio of MoOF 4 to MF (M = Li-Cs), which gave moderate concentrations of free F -in aHF solution, it is understandable that the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts were preferably obtained over species such as M[MoOF 5 ] or M 2 [MoOF 6 ] (M = Li-Cs).The M[MoOF 5 ] and M 2 [MoOF 6 ] (M = Li-Cs) salts may be obtainable if the starting concentration of the respective alkali metal fluoride is increased substantially in comparison to MoOF 4 .

2 O 2 F 9 ]
(M = Li-Cs): First, MoOF 4 was synthesized through the hydrolysis of MoF 6 in aHF using quartz wool.For this reaction, 0.56 grams of quartz wool, SiO 2 , (9.32 mmol) was added to an FEP vessel and attached to a metal vacuum line and evacuated and flushed with argon three times.3 mL of aHF and 5 grams of MoF 6 (23.82 mmol) were condensed onto the quartz wool at -196 °C.The reaction vessel was warmed to room temperature and stored for 1 week.After the reaction was completed, as indicated by complete solvation of the SiO 2 wool, all volatile products were removed and the MoOF 4 was stored under argon in a glovebox.IR and powder X-ray diffraction showed no evidence of impurities in the MoOF 4 sample.

Table 3 .
Selected interatomic distances and angles in the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts.In the case of Li[Mo 2 O 2 F 9 ], all atomic distances reported have a multiplicity of 1; for M[Mo 2 O 2 F 9 ] (M = Na-Cs), all reported atomic distances have a multiplicity of 2.

Table 2 .
The unit cell contains two symmetrically inequivalent [Mo 2 O 2 F 9 ] -anions coordinating to two distinct Li + cations.Li(1) has full site occupancy and is octahedrally coordinated by six terminal fluorine atoms (F t ) belonging to four [Mo 2 O 2 F 9 ] -anions (Figure

Table 2 .
Atomic coordinates and equivalent isotropic displacement parameters U iso for Li[Mo 2 O 2 F 9 ].

Table 4 .
Atomic coordinates and equivalent isotropic displacement parameters U iso for Na[Mo 2 O 2 F 9 ].

Table 5 .
Atomic coordinates and equivalent isotropic displacement parameters U iso for K[Mo 2 O 2 F 9 ].

Table 6 .
Atomic coordinates and equivalent isotropic displacement parameters U iso for Rb[Mo 2 O 2 F 9 ].

Table 7 .
Atomic coordinates and equivalent isotropic displacement parameters U iso for Cs[Mo 2 O 2 F 9 ].

Table 8 .
Experimental IR bands observed in the M[Mo 2 O 2 F 9 ] (M = Li-Cs) salts.When given, a description of vibrational modes is as follows: ν = stretching, s = symmetric, as = asymmetric.F μ refers to bridging fluorine atoms while F t refers to terminal fluorine atoms.All band locations are given in cm -1 .
which are not explained by the presence of the [Mo 2 O 2 F 9 ] -anion.In the spectra of Li[Mo 2 O 2 F 9 ], Rb[Mo 2 O 2 F 9 ], and Cs[Mo 2 O 2 F 9 ], these bands have considerable intensities when compared to the bands arising from the [Mo 2 O 2 F 9 ] -anion.In the spectrum of Na[Mo 2 O 2 F 9 ], however, these bands are quite weak in comparison to the [Mo 2 O 2 F 9 ] - anion bands, and in the spectrum of K[Mo 2 O 2 F 9 ] these bands are missing entirely.In the work of Bougon and co-workers, in which NO[Mo 2 O 2 F 9