Syntheses and structures of two new lithium-heptamolybdates

Abstract The synthesis, crystal structures, IR, UV–vis, 7Li NMR spectra, electrochemical investigations, and conductivity studies of two new lithium-heptamolybdates, (NH4)4[Li2(H2O)7][Mo7O24]·H2O (1) and (NH4)3[Li3(H2O)4(μ6-Mo7O24)]·2H2O (2), are reported. In 1 the (NH4)+ and [Li2(H2O)7]2+, cations are charge balanced by the heptamolybdate anion. In 2, the [Mo7O24]6− anion is coordinated to three unique Li+ ions via a μ6-hexadentate-binding mode resulting in the formation of a two-dimensional (2-D) [Li3(H2O)4(μ6-Mo7O24)]3− anionic complex, charge neutralized by three (NH4)+ ions. The cations, anions, and the lattice water molecules in 1 and 2 are linked by weak H-bonding interactions.

anion for the organic or metal-organic cation. Heptamolybdate can also function as a pure inorganic ligand, binding to metals via the terminal oxygens (entries 2 to 33 in table S1). In a recent report, we have shown that the acidic nature of heptamolybdate can be exploited for the synthesis of new s-block metal heptamolybdates by reacting it with an appropriate base. Using this strategy, we recently reported the synthesis of a heptamolybdate-bridged dimagnesium compound [Mg(H 2 O) 5 [10]. Since no structurally characterized lithium compound containing [Mo 7 O 24 ] 6− is reported, we have used the same synthetic methodology, reaction of heptamolybdate with a basic reagent (LiOH) for the synthesis of the first examples of lithium-heptamolybdates. The results of these investigations describing the synthesis, crystal structure, spectral characteristics, and electrochemistry of (NH 4 ) 4 [ [Li 3 (H 2 O) 4 (μ 6 -Mo 7 O 24 )]·2H 2 O (2) are described in this report.

Materials and methods
All chemicals were used as purchased from commercial sources without purification. Infrared spectra of the samples diluted in KBr were recorded from 4000 to 400 cm −1 using a Shimadzu (IR Prestige-21) fT-IR spectrometer at a resolution of 4 cm −1 . Raman spectra were recorded using 785 nm radiation for excitation on an Agiltron PeakSeeker Pro Raman instrument from 4000 to 200 cm −1 . UV-visible absorption spectra were recorded using a UV-3600 Shimadzu UV-vis spectrophotometer. X-ray powder patterns were measured on a Rigaku Miniflex II powder diffractometer using Cu-K α radiation with a Ni filter. Thermal studies of 1 and 2 were carried out in a temperature-controlled electric furnace at 600 °C. 7 Li NMR spectra of 1 and 2 were recorded in D 2 O with lithium chloride as a reference in a Bruker 500 MHz fT-NMR spectrometer. Conductivity measurements were carried out at 30 °C using a Digital conductivity meter model-LT-16 LABTRONICS with a standard conductometric cell composed of two platinum black electrodes calibrated with KCl solution. Cyclic voltammetry was performed in an electrochemical Workstation-CH Instrument (Inc. CHI6107) under inert atmosphere by using platinum as working electrode, platinum wire as counter electrode and saturated calomel electrode (SCe) as the reference. The redox properties of aqueous solutions of 1 and 2 were studied using 0.2 M KCl solution as supporting electrolyte at a scan rate of 0.

Synthesis of (NH 4 ) 4 [Li 2 (H 2 O) 7 ][Mo 7 O 24 ]·H 2 O (1) and (NH 4 ) 3 [Li 3 (H 2 O) 4 {μ 6 -Mo 7 O 24 }]·2H 2 O (2)
Ammonium heptamolybdate (1.236 g, 1 mmol) was crushed with lithium hydroxide (0.083 g, 2 mmol) using a mortar and pestle for ~5 min, resulting in the evolution of ammonia. The reaction mixture was then transferred into a beaker containing 20 mL of distilled water and heated on a water bath till the volume of the solution is reduced to half. At this stage the pH of the solution was ~5. The reaction mixture was then filtered and the colorless filtrate was kept aside for crystallization at room temperature. Colorless crystals separated after a week and when kept for further crystallization, 1 was obtained in 74% yield.
The use of 3 mmol of LiOH in the above procedure with 1 mmol of ammonium heptamolybdate followed by work up afforded crystals of 2 in 78% yield.

Crystal structure determination
Intensity data for 1 and 2 were collected with an Image Plate Diffraction System (IPDS-1) from STOe. The structures were solved with direct methods using SHeLXS-97 [35] and refinement was done against F 2 using SHeLXL-97 [35]. All non-hydrogen atoms were refined anisotropically. The O-H and N-H, hydrogens were located in a difference map, their bond lengths set to ideal values and refined using a riding model. A numerical absorption correction was performed (T min/max : 0.5782/0.6971) (1) (T min/max : 0.5742/0.6545) (2). One lattice water molecule (O38 in 1 and O24 in 2) is disordered and was refined using a split model. The hydrogens on O38 and O24 could not be located. Technical details of data acquisition and selected refinement results are listed in table 1.

Synthetic aspects
The syntheses of lithium-heptamolybdates 1 and 2 were carried out by a base-promoted cation-exchange reaction between (NH 4 ) 6  and then bringing it into aqueous solution followed by crystallization (equations 1 and 2). The reaction involves replacement of (NH 4 ) + cation by Li + with displacement of the weaker base ammonia by the strong base lithium hydroxide keeping intact the {Mo 7 O 24 } core. Despite the use of a strong base LiOH, due to the removal of ammonia, the final reaction medium is acidic pH ~5, which is essential for isolation of heptamolybdate. The slow evaporation of aqueous solutions results in the formation of crystalline products 1 and 2.
The phase purity of 1 and 2 was confirmed by comparing their respective calculated and experimental powder patterns (figure S1). The presence of lithium in both compounds was initially identified by flame test and the ammonium and molybdenum content were determined gravimetrically following standard procedures [36].

Structural aspects of heptamolybdates
Based on a comparative study of the structural features of thirty-one heptamolybdates, we had shown that the heptamolybdate anion is structurally flexible and all heptamolybdates isolated from acidic media contain at least one lattice water molecule [4]. In the present analysis of structural characteristics covering a total of fifty-four heptamolybdates (table S1), the same property is observed namely that all heptamolybdate compounds listed in table S1 contain at least one lattice water molecule. A majority of these compounds crystallize in centrosymmetric space groups with only four (entries 1-4) crystallizing in noncentrosymmetric space groups. In all the compounds listed in table S1, the heptamolybdate functions as a charge balancing anion. In addition the heptamolybdate is a ligand coordinating to s, d, or f-block metals in many compounds (entries 2-33). In these heterometallic compounds, the denticity of [Mo 7 O 24 ] 6− varies from monodentate (entries 2-5) to hexadentate in the Li-heptamolybdate 2. Of the two mixed cationic compounds described in this work, in the ammonium-rich compound 1 (four (NH 4 ) + ions) the heptamolybdate functions as a counter anion. In the alkali-metal-rich heptamolybdates like Na 6 [38]. The symmetric stretching vibration ν 1 of the MoO 6 unit is observed as an intense band in the Raman spectrum at 936 cm −1 [39] whereas the doubly degenerate asymmetric stretching mode ν 2 occurs as an intense signal at 884 and 893 cm −1 in 1 and 2, respectively (figure S10). The IR spectra of the residues obtained by pyrolysis of 1 and 2 indicate the disappearance of signals due to water and ammonium cations (figure S11).

IR, UV-vis, and 7 Li NMR spectral studies
The nearly identical UV-vis spectra of 1, 2 and (NH 4 ) 6 [Mo 7 O 24 ]·4H 2 O showing a signal centered at 208 nm also confirms the presence of the heptamolybdate core in 1 and 2 (figure S12). Both 1 and 2 exhibit nearly identical 7 Li NMR spectra in D 2 O (figure 8). Both compounds exhibit a single 7 Li chemical shift at 0.1376 and 0.1385 ppm, respectively, for 1 and 2 which is in agreement with the reported 7 Li chemical shift (0.006 ppm) for Li 6 [α-P 2 W 18 O 62 ] [40]. The chemical shift data indicate that in 1 and 2 the Li + ions are equivalent and do not have different chemical surroundings as observed in the solid state structures. Hence, the observation of a single chemical shift can be explained due to hydration of Li + in solution (D 2 O). This explanation gains more credence from the electrochemical and conductivity studies described below.

Electrochemistry, conductivity measurements and photochemical studies
The cyclic voltammograms of 1 and 2 as well as (NH 4 ) 6   electrochemical event centered on the anionic heptamolybdate which is formed due to hydrolysis of 1 and 2, in addition to the hydrated Li + ions. 1 and 2 described in the present study differ from the recently reported (hmtH) 2 [10] in that the latter compound exhibits the same electrochemical event at −0.780 V. However, no other electrode process is observed till −1.0 V for 1 or 2, indicating that the hydrated Li + cations do not undergo any electrochemical change.
The formation of hydrated Li + ions is also revealed by the conductivity measurements for various concentrations of 1 and 2 (table 3) It is well documented that heptamolybdates charge balanced by organic cations exhibit interesting photochemistry [4,34]. Recently, we have shown that the dimagnesium-heptamolybdate (hmtH) 2 [10] can be irradiated by exposure to sunlight and thus can be used as a photocatalyst. In order to study the photochemical properties, similar experiments were performed by irradiation of 1 or 2 in solid state (or in aqueous solution), but no photochemically induced changes were observed. This differing behavior can be explained by the presence of an organic cation (hmtH) + in the dimagnesium-heptamolybdate unlike the Li-heptamolybdates 1 or 2.  Table 3. specific conductivity (Ҡ) and molar conductivity (λ m ) data.