Solid-Phase “Self-Hydrolysis” of [Zn(NH3)4MoO4@2H2O] Involving Enclathrated Water—An Easy Route to a Layered Basic Ammonium Zinc Molybdate Coordination Polymer

An aerial humidity-induced solid-phase hydrolytic transformation of the [Zn(NH3)4]MoO4@2H2O (compound 1@2H2O) with the formation of [(NH4)xH(1−x)Zn(OH)(MoO4)]n (x = 0.92–0.94) coordination polymer (formally NH4Zn(OH)MoO4, compound 2) is described. Based on the isostructural relationship, the powder XRD indicates that the crystal lattice of compound 1@2H2O contains a hydrogen-bonded network of tetraamminezinc (2+) and molybdate (2−) ions, and there are cavities (O4N4(μ-H12) cube) occupied by the two water molecules, which stabilize the crystal structure. Several observations indicate that the water molecules have no fixed positions in the lattice voids; instead, the cavity provides a neighborhood similar to those in clathrates. The @ symbol in the notation is intended to emphasize that the H2O in this compound is enclathrated rather than being water of crystallization. Yet, signs of temperature-dependent dynamic interactions with the wall of the cages can be detected, and 1@2H2O easily releases its water content even on standing and yields compound 2. Surprisingly, hydrolysis products of 1 were observed even in the absence of aerial humidity, which suggests a unique solid-phase quasi-intramolecular hydrolysis. A mechanism involving successive substitution of the ammonia ligands by water molecules and ammonia release is proposed. An ESR study of the Cu-doped compound 2 (2#dotCu) showed that this complex consists of two different Cu2+(Zn2+) environments in the polymeric structure. Thermal decomposition of compounds 1 and 2 results in ZnMoO4 with similar specific surface area and morphology. The ZnMoO4 samples prepared from compounds 1 and 2 and compound 2 in itself are active photocatalysts in the degradation of Congo Red dye. IR, Raman, and UV studies on compounds 1@2H2O and 2 are discussed in detail.


Introduction
The molybdates of the first-row 3D transition metals, including (non)-stoichiometric coordination polymers [1,2], are widely used as photocatalysts [3,4] and safe and high-density storage materials for the controlled release of ammonia in selective catalytic reduction of NO x in exhaust gases or as fuel source for fuel cells [5].Their synthesis with controlled composition, however, is a difficult task because their composition strongly depends on the synthetic conditions, as their layered structure permits building up various structural motifs and the formation of defector/substituted structures [6].The M I M II (OH)MoO 4 and M I (H)M II 2 (OH) 2 (MoO 4 ) 2 (M I = Na, K, NH 4 + , M II = Cu, Mg, Co, Ni, Zn) type compounds were first prepared by Pezerat [7] with variable compositions, including compounds containing ammonium and zinc ions (e.g., NH 4 Zn(OH)MoO 4 ) [8].
Among these oxometallates, only limited information is available on tetraamminezinc(II) molybdate dihydrate ([Zn(NH 3 ) 4 ]MoO 4 @2H 2 O (Compound 1@2H 2 O) [9] (we use this unusual notation to express that, as it will turn out, the two water molecules do not occupy regular water-of-crystallization position in the crystal lattice).This salt is an unstable material, which, upon heating first releases 1 mol of NH 3 and 0.6 mol of H 2 O at 50 • C, then 2 mol of ammonia and 1.3 mol of H 2 O at 120 • C, respectively.The loss of coordinated ammonia in the first decomposition step before completion of the elimination of water molecules is rather curious, because disruption of the coordination sphere should result in the collapse of the crystal lattice and so water from the crystallization should already be completely eliminated in the first decomposition step.To resolve this puzzle, we studied this process experimentally and performed quantum-chemical calculations to understand the order of elimination of water and ammonia ligands.
In this paper, we present our results on the formation of polymeric basic ammonium zinc molybdate (NH 4 ) x (H) 1−x Zn(OH)MoO 4 with x = 0.92-0.94(compound 2) and show that this product forms from [Zn(NH 3 ) 4 ]MoO 4 @2H 2 O in solid-phase reaction routes either driven by external humidity or "self-hydrolysis" of compound 1@2H 2 O. Detailed spectroscopic and thermal analysis of the coordination polymer product and its precursor material, [Zn(NH 3 ) 4 ]MoO 4 @2H 2 O, are also discussed.The photocatalytic activity of the thermal decomposition product of the hydrated compound 1@2H 2 O and compound 2 has also been evaluated in detail.Tetraamminezinc(II) molybdate was prepared as dihydrate (1@2H 2 O) by the method of Zelikman et al. [9] in a reaction of ZnMoO 4 and stoichiometric aq.ammonia solution with subsequent cooling to 5-10 • C and drying under passing in NH 3 stream.Another synthesis route for 1@2H 2 O was also designed, which starts from freshly prepared basic zinc molybdate as a precursor (formed from aq. sodium molybdate and aq.zinc sulfate in 1:1 molar ratio).The latter was dissolved in excess of the concentrated ammonia solution and 1@2H 2 O was precipitated by adding absolute ethanol.An excess of ethanol and ammonia is essential to prevent the hydrolysis of compound 1.The product was always contaminated due to fast decomposition on drying.In open air, the amount of the decomposition product increased with increasing drying time (Figure 1) and was identified as compound 2, (NH 4 ) x (H) 1−x Zn(OH)MoO 4 (x = 0.92-0.94)(Card No.: 73-2389) with XRD and chemical analysis.To perform thermal, XRD, and IR studies and density determinations, compound 1 was precipitated as a powder from its aqueous solution with ethanol followed by washing the ethanol with diethyl ether.The thermal decomposition product of the hydrated compound 1 in air is ZnMoO 4 [9] (see ESI Figure S1, Card.No.: 35-0765), with a mass loss of 31.2%, which corresponds to a loss of 2 moles of water in addition to all ammonia (the theoretical mass loss for the process is 31.62%).According to powder X-ray diffraction, the colorless microcrystalline compound 1 that formed has a cubic lattice with a = 10.345Å (space group: F23 (no.196)) (ESI Figure S2, ESI Table S1).The cell volume is 1107.11Å 3 , while the pycnometric density determined experimentally at 25 • C is 1.8612 g/cm 3 .The formula weight calculated for Z = 1 is M = 346 g/mol, and with the cell parameters, one obtains d theor = 1.867 g/cm 3 .The difference of the pycnometric and theoretical densities corresponds to the presence of two molecules of water in the hydrated compound 1.This stoichiometry is supported by the thermogravimetric measurement, presented later.Even in the absence of humid air, it slowly hydrolyzes, utilizing its own water content.Five days after preparation, the main final decomposition product was identified as (NH 4 )Zn(OH)(MoO 4 ) (compound 2, ESI Figure S3) in both cases.Section 3 below is devoted to the description of the similarities and differences in the mechanism of hydrolysis with an external or only internal water source.

Crystal Structure of Compounds 1 and 2
The structure of compound 1 has not yet been identified.Our efforts to grow single crystals to perform single-crystal X-ray diffraction measurements also remained unsuccessful.Compound 1 decomposes faster in its aqueous solutions even if cooled enough for the crystals to grow.It does not dissolve in non-polar organic solvents, whereas polar ones like DMSO or DMF or inorganic solvents like liquid NH 3 or SO 2 react with it [8].Therefore, only powder XRD data could be used to solve the structure of compound 1.To find appropriate starting coordinates, an analogous isostructural compound would be needed.We could not find compounds with known crystal structures of compositions [M(NH 3 ) 4 ]XO   [10,11] or [Zn(NH 3 ) 4 ](ClO 4 ) 2 [12], were found to be analogous to those of the hydrated compound 1.This is indeed surprising because neither the number of univalent negative anions nor their charge matches the composition of hydrated compound 1.The structure of the permanganate and perchlorate compounds isostructural with 1@2H 2 O is unique in the sense that one perchlorate/permanganate anion is built into a network with the complex [M(NH 3 ) 4 ] 2+ cations, making voids (O 4 N 4 (µ-H 12 )cube) that are occupied by the other perchlorate/permanganate anion.
Since 1@2H 2 O is isostructural with the salt of monovalent cations, in the crystal lattice of 1, there is also a cation-anion network with cavities.However, since the cation:anion ratio is 1:1 in 1@2H 2 O, all molybdate ions are located in the cation-anion network, and there is no further anion in the compound that would, analogous to one of the permanganate ions, occupy the voids of the lattice.On the other hand, instead of the second anion, in 1@2H 2 O, there are two water molecules in the lattice that have no room in the cation-anion network, and it seems obvious that they are located in the cavities of the lattice, in the form of either two individual water molecules or a dimer.
In other words, a charged anion is substituted with two non-charged water molecules.Since the water molecules are embedded in lattice voids (Figure 2), they are neither coordinated nor are present as water of crystallization; instead, they are enclathrated (this is what the @ symbol refers to in the abbreviation 1@2H 2 O).Since the water molecules located in the lattice voids are weakly bound, they can act the same way as water vapor [13].An indication of the presence of some weakly bound components in the lattice of 1@2H 2 O is provided by the low-temperature DSC study of 1@2H 2 O, performed between −150 and 25 • C. Two endothermic effects have been detected at −103 and −17 • C with heats of 1.17 and 5.67 kJ/mol, respectively (ESI Figure S4).The temperature is very low, and the energy absorption is small, so it seems reasonable to assign these effects to some phase transitions and/or re-arrangement of the enclathrated water molecules.These might involve the change of the position of the two water molecules due to the change of their interaction with each other or with the atoms of the cage wall.
The crystal structure of compound 2 consists of sheets in which distorted ZnO 6 octahedra are connected by tetrahedral molybdate groups [2].The layers are stacked along the direction of the crystallographic c axis and are held together by hydrogen bonds.The ammonium ions are oriented through hydrogen bonds, and two different orientations for the ammonium ions are possible with equal probability.The ammonium ions cannot rotate freely.The zinc atoms in the structure are linked to each other through double OH bridges [2].
Further insight into the structure of 2 is provided by ESR spectroscopy.Since Zn and Mo are diamagnetic, we doped the compound with Cu 2+ ions, which are expected to substitute Zn, and can provide information on its environment.The EPR spectrum of the powdered compound 2#dotCu 2+ formed this way suggests the presence of two different copper (and accordingly Zn) environments characterized by g ⊥ = 2.1135 and 2.2300 with A ⊥ 62.2 and 27.5 G, as well as g = 2.3252 and 2.0221 with A = 139.0 and 147.6 G. Surprisingly, one component has a "normal" (g > g⊥) and the other an "inverse" (g⊥ > g ) copper spectrum with a ratio of about 1:4.(ESI Figure S5).It seems reasonable to assume that, on one hand, the two different signals originate from the Cu 2+ (Zn 2+ ) ions in the regular coordination environment [2], and on the other, from ions in environments distorted as a consequence of the ammonia deficiency.

Hydrolysis Reactions of Compound 1
As mentioned in Section 1, 1@2H 2 O easily hydrolyzes in the solid state.The hydrolysis takes place even when no external source of water is available, and in aqueous solutions as well.The main final product in both cases is compound 2. Heating speeds up the hydrolysis in water.Upon boiling the aqueous solution of compound 1@2H 2 O for ≈ 10 min or 2 h in open air, the hydrolysis not only yielded compound 2, but in addition, basic zinc carbonate (Zn 5 (OH) 6 (CO 3 ) 2 ) and ammonium heptamolybdate ((NH 4 ) 6 Mo 7 O 24 ) were also formed.Since compound 2 in an aqueous suspension is stable toward hydrolysis even on prolonged boiling, the latter two by-products must be formed from the hydrolysis of the starting compound 1.
These experimental observations on the aqueous phase hydrolysis can be interpreted in terms of the following elementary steps [14,15]: (a) The tetraamminezinc cation releases some of its ammonia ligands; (b) the liberated ammonia is protonated with the formation of ammonium and hydroxide ions, making the pH basic; and (c) Zn 2+ and MoO 4 2− ions get into the solution.(d) The least soluble basic salts precipitate.These are {[NH 4 Zn(OH)(MoO 4 )] 2 } n , compound 2, the main product, as well as Zn 5 (OH) 8 (CO 3 ) 2 , which we identified as coming from the carbon dioxide absorbed by the alkaline solution.e) As a result of the removal of Zn 2+ and the majority of the OH − ions, only NH 4 + and MoO 4 2− ions remain available.The latter at the basic pH condensate into heptamolybdate ions known from the molybdate-polymolybdate equilibrium system [10], and then, together with the ammonium ions, they crystallize in the form of ammonium heptamolybdate (ESI Figure S6).
When 1@2H 2 O is left to stand in a closed, dry container, it slowly degrades and compound 2 is formed.This means that ammonia and water leave the lattice.As both are volatile, in principle, both could depart spontaneously.However, thermogravimetry shows that the rate of mass loss is negligible at temperatures below 80 • C.This suggests that the initial steps involve chemistry.This assumption seems to be confirmed by the XRD patterns taken after a day of reaction time, plotted in Figure 3.One can see that in addition to the reactant 1@2H 2 O and product 2, there are some intermediates in the system.Moreover, from the comparison of the top panels of Figure 3, it is also visible that the intermediates differ when external water is available compared to when the system is closed.Considering that the products are the same, and that the two water molecules located in the cavities of the lattice behave almost as if they were in the gas phase, it seems reasonable to assume that the reaction in the absence of an external water source is also hydrolysis, which is conducted by the lattice water molecules.To the best of our knowledge, this is the first example of a solid phase "quasi-intramolecular" self-hydrolysis reaction in a solid hydrate.
To understand the possible mechanism for the removal of ammonia ligands and the formation of ammonium ligands, we recall that the mechanism of a similar process has been determined in the case of anhydrous tetraamminecopper (II) molybdate [1].It was shown that that compound, insoluble in water, undergoes a solid-state reaction when water molecules are available.The process was experimentally shown to start by NH 3 release.The proposed mechanism is that the ammonia ligands are successively substituted by water molecules, and even the identity and structure of the intermediate mixed-ligand salts have been determined experimentally [1].The final ammonia ligand was found to be so strongly bound to the transition metal ion that instead of leaving, it gets protonated by a water ligand coming originally from ambient humidity.It is likely that the mechanism for the decomposition of 1@2H 2 O in open, humid air is very similar, namely, stepwise formation of [Zn(NH 3 ) n (H 2 O) 4-n ] complexes, where n decreases from 4 to 1 (compounds 3 (n = 3), 4 (n = 2), and 5 (n = 1)), and finally the last ammonia ligand in compound 5 is protonated by a water molecule in the lattice, yielding ammonium and hydroxide ions and with that, the formation of compound 2 (Figure 4).In the absence of humidity, only the two water molecules located in the voids of the crystal lattice of 1@2H 2 O are available for the hydrolysis.Obviously, one of them is incorporated into the lattice of compound 2. Since the H 2 O/Zn ratio is 2, no intermediates can primarily be formed with more than two H 2 O molecules per Zn.However, there are several cavities containing easily mobilizable water molecules in the close neighborhood of each coordinated Zn ion, and the constrained H 2 O/Zn ratio may make certain reaction channels more favorable and others less so.The basic mechanism is probably similar in the absence of external water molecules in comparison to when water is abundant.

Quantum Chemical Considerations on Self-Hydrolysis/Vapor-Phase Hydrolysis of Compound 1
As mentioned, the [Zn(NH 3 ) 4 ] 2+ ions in 1@2H 2 O are located in the crystal lattice.The evolution of the compound in humid air or in a closed environment including any possible ligand exchange has to take place in the solid phase.Direct ligand exchange of the stronger bound ammonia ligands by water is not favorable.It seems rather reasonable to assume that the ligand exchange is initiated by coordination of a water ligand to the tetraamminezinc ions, and loss of an ammonia ligand if the energetics permits.The first step then is coordination of a water molecule coming from the environment or from the crystal lattice with the formation of the [Zn(NH 3 ) 4 (H 2 O)] 2+ (6 2+ ) complex ion.If abundant water is available, hexacoordinated [Zn(NH 3 ) 4 (H 2 O) 2 ] 2+ ions (cis and trans-9 2+ ) can also be formed.These ions, however, do not initiate new reaction pathways because the products of water or ammonia release are the same pentacoordinated ions (6 2+ ) formed by direct water addition.
To explore the possibilities for the further steps of hydrolysis, guided by the mechanism determined for the hydrolysis of the copper molybdate analog, we performed electronic structure calculations using density functional theory.Here we present the results obtained at the M05-2X/LANL2DZ level of theory.The purpose is to gain a qualitative picture of the structures and relative energies of the intermediates possibly involved in the reaction.We determined the structures and energies of all possible pure and mixedligand complexes of the Zn 2+ ion from the mono-up to the hexacoordinated ones.Their most important structural parameters are listed in ESI Table S2.The geometry of the ions is as follows: Hexacoordinated-distorted octahedral; pentacoordinated-trigonal bipyramidal (except Zn(NH 3 )(H 2 O) 4 , which has a metastable square pyramidal conformer); tetracoordinated-tetragonal; tris-coordinated-planar trigonal.In the pentacoordinated 1:4 mixed-ligand ions, the unique atom can be in axial or equatorial position, while in those with 3:2 ligand ratio, the two identical ligands can be both equatorial, both axial, or one equatorial, one axial.The relative energies of the conformers of the pentacoordinated complex ions with NH 3 :H 2 O ratios of 4:1, 3:2, 2:3, and 1:4 are also very small, being as little as 0.21 kcal/mol (for the NH 3 :H 2 O = 1:4), and not more than 4.1 kcal/mol (for NH 3 :H 2 O = 2:3).It seems to generally hold that in the most stable conformer, the ammonia ligand is in an equatorial position.We note that the absolute magnitude of the energy differences may be overestimated, but the tendencies/trends can be considered to be right.An indication that the calculated ligand binding energies are too high is that according to thermogravimetry, both water and ammonia loss can be observed at as low temperatures as 130 • C, while the calculated binding energies are in the order of several tens of kcal/mol.An additional complication is that our calculations are performed for the molecules in vacuo, while in the real system, the neighborhood of the ions certainly influences the energetics, but the magnitude of the effect is hard to estimate.For the purpose of orientation, the tendency of the calculated energy differences seems to be a reasonable starting point.
The addition of a water ligand to the [Zn(NH 3 ) 4 ] 2+ ion is an exothermic reaction by about 27 kcal/mol.This supports the assumption that the initial step of the substitution of the ammonia ligands is water addition.The resulting complex can lose a water or ammonia ligand at the cost of 27 kcal/mol or 32 kcal/mol, respectively.Thus, the loss of an ammonia is much less favorable than that of a water ligand, but it is not excluded.The reactions are slow and can lead to equilibrium.The equilibrium can be shifted towards increasing ammonia to water substitution because once an ammonia molecule is liberated, it can leave the crystal lattice.Thus, similarly to the copper molybdate serving as a model for the mechanism, complexes with lower and lower ammonia content can be accumulated.As the number of water ligands increases in the penta-coordinated ions, the removal of any ligand becomes more and more endothermic, with each new water ligand at roughly 5 kcal/mol for the ammonia and 4 kcal/mol for the water loss.The tetracoordinated ligands can also lose a ligand, but the energy to be invested for that process is roughly 20 kcal/mol larger.The energy of ligand removal from tris-coordinated ions is again larger by roughly an additional 20 kcal/mol, and the gap between the energies required for ammonia and water loss increases.From the [Zn(NH 3 )(H 2 O)] 2+ ion, the binding energy of the ammonia and water ligands is 115 and 93 kcal/mol, respectively, which suggests that, similarly to the copper molybdate, the removal of the last ammonia is not favorable.A possible network of elementary steps is displayed in Figure 4.

Vibrational and UV-VIS Spectra of Compounds 1@2H 2 O and 2
As mentioned, compound 1@2H 2 O consists of approximately tetrahedral Zn(NH 3 ) 4 2+ cations with ZnN 4 core, coordinated ammonia molecules (C 3v ), approximately tetrahedral MoO 4 2− anions, and two enclathrated water molecules.Knowing the main crystal structural parameters, a correlation analysis could be performed (ESI Figure S7a-c).The molybdate ion, ammonia, and Zn 2+ ion were taken into consideration.Both the ZnN 4 core and MoO 4 2− ion are tetrahedral, thus the molecular, site, and factor groups are under T d symmetry.There are four normal modes: Symmetric stretching vibration (ν 1 or ν s ); symmetric bending (ν 2 or δ s ); antisymmetric stretching (ν 3 or ν as ); and antisymmetric bending vibration (ν 4 or δ as ).The symmetric bending mode is doubly, while both antisymmetric modes are triply degenerate under T d .
The total number of factor group modes due to the internal MoO 4 2-vibrations is four and corresponds to nine vibrational degrees of freedom, exactly as in the free anion.The hindered rotations and the hindered translations of the molybdate anions are both triply degenerate under T d symmetry and remain so under both site and factor groups (ESI Figure S6a).All four modes are Raman active, but only the F 2 modes (ν as and δ as ) are IR active.The total number of factor-group modes due to the external MoO 4 2-vibrations (hindered translations and hindered rotations) is six vibrational degrees of freedom: Three rotation-like motions and three translation-like motions (ESI Figure S6b).The IR, far-IR, and Raman spectra of compound 1@2H 2 O can be seen in Figure 5 and ESI Figures S8-S10.The IR and Raman data are given in ESI Tables S3-S5.For the molybdate, ion band positions are ν s > ν as , which represents an inversion of the normal behavior of MO 4 -type species (M = Cr, Mn, S, Se, Tc, Re) [16][17][18][19][20][21].The relative positions of deformation bands for the MoO 4 2− anion, δ as /δ s , may be >1, 1, or <1, and the intensities of symmetric modes are higher in the Raman than in the IR spectra [17].The singlet/triplet nature and intensity relations of ν s (Mo-O) ν as (Mo-O) bands in the Raman (>>1) and IR (<<1) spectra allow unambiguous assignment of these modes.Based on these considerations, in compound 1 @2H 2 O, unlike the assignment by Busey [17] for Na 2 MoO 4 but in agreement with the result given by Clark [22] for ZnMoO 4 , the band positions are ν s (Mo-O) > ν as (Mo-O), and δ as (Mo-O) > δ s (Mo-O) The δ as band position is either mixed with or is partly overlapped with that of ν as (Zn-N).The δ as /δ s > 1 situ-ation for compound 1@2H 2 O and ZnMoO 4 unambiguously show that the sensitivity of the symmetric and antisymmetric deformation mode positions depend on the chemical environment around the molybdate ion.
The appearance of the forbidden ν s (Mo-O) and δ s (Mo-O) modes in the IR spectrum of compound 1@2H 2 O confirms the distortion of the molybdate anion probably due to the hydrogen bond interactions with the ammonia hydrogens of the cation or the water molecules embedded in the cages.There is a rather large difference (~40 cm −1 ) between the center of gravity positions (∑ n i h i ν i /∑ n i h i ) of modes in the IR and the Raman spectra.The reasons for such "splitting" are not clear at this moment.
The expected vibrational modes for the ZnN 4 core of the complex cation are analogous to those for the molybdate anion.The correlation analysis of the internal ammonia ligand modes (C 3v molecular and site group, and T d factor group) can be seen in Figure 6.In the assignment of cation modes in compound 1@2H 2 O, the IR and Raman data of the isostructural [Zn(NH 3 ) 4 ](MnO 4 ) 2 , and the vibrational analysis results of the isotopesubstituted Zn( 15 NH 3 ) 4 2+ and Zn( 14 NH 3 ) 4 2+ cations [23] were also considered.The assignments of bands belonging to the ZnN 4 core and ammonia ligands are given in ESI Tables S3 and S4.
The appearance of the IR inactive δ s (Zn-N) mode confirms the distortion of T d geometry around the ZnN 4 core.Similarly, the IR inactive A 1 and E modes (in C 3v ) of ammonia ligands are allowed, due to symmetry lowering induced by the evolution of extended hydrogen bond network involving the ammonia molecules and molybdate ions and the interaction of N-H bonds of the network with the water components in the cages.The bands at 1392 and 1301 cm −1 can be attributed to second-order transitions analogous to the first overtones of the ρ(NH 3 )(E) modes at ~700 and 650 cm −1 .
The vibrational modes of enclathrated water in the spectra of compound 1@2H 2 O are expected to be similar to the monomeric or dimeric water modes in the gaseous state.The monomer water molecules must be disordered, as they occupy sites of T d symmetry.The easiest (we found no better) way to account for this is to add three degrees of freedom for internal vibrations, three others for librations, and another three for hindered translations without employing the correlation method (Figure 7).An alternative approach would be to treat the water molecules as having spherical symmetry.The latter would imply a triply degenerate hindered translational mode T xyz , which is feasible, a triply degenerate librational mode R xyz , which is hardly acceptable, and a triply degenerate internal vibrational mode, which is unfeasible, leading to three additional modes of F 2 symmetry (9 degrees of freedom).The water molecules embedded in the cages may form various dimer structures [18], which may interact with the atoms in the cage wall.Basically, the O-H stretching modes are located on the left side of N-H bands on a wide shoulder.The band system around 1650 cm −1 may be attributed to the scissoring mode of water molecules.A librational mode of water appears at 905 cm −1 [17], rather high for almost free water.[17].The changes in the bonding mode and motions of these water molecules can change some spectral features.The Raman spectra on cooling the compound 1@2H 2 O down to −180 • C (ESI Figure S9) showed better separation of combination/overtone bands in the region below 3000 cm −1 .It suggests changes in the orientation/motion of freely rotating water molecules in the cages, which influences the hydrogen-bonded ammonia molecule's band positions.
There are some second-order transition bands (analogous to combinations and overtones) around 2000 cm −1 , which belong to the water bending mode with other low-wavenumber (<400 cm −1 ) modes [17], and the band system observed around 1060/1082 cm −1 might, again, belong to water modes related to some second-order transitions.
The IR and temperature-dependent Raman spectra of compounds 1@2H 2 O and 2 are given in Figures 5 and 8 or ESI Figures S8-S13.The Raman spectral features are independent of the temperature.A slightly better resolution could be found at −180 • C compared to at room temperature.A comparison of tetrahedral molybdate and ammonium ion vibrational modes with the data of NH 4 Cu(OH)MoO 4 is given in ESI Table S5.The ν s (Mo-O) band could easily be assigned as an intensive singlet in the Raman spectra at various temperatures.The ν as (Mo-O) appears as a triplet, and the difference between the A 1 and B 1 components is ∆ν = 54 cm −1 , lower than that observed in isotypic tetrahedral η 2 -C 2v anions (∆ν ∼ 95−124 cm −1 ) [1].Thus, the molybdate ion is most likely not a chelating one in compound 2, and the coordination polymer is built up from Zn-O-Mo units.The Zn-O-Mo and Zn-OH modes are assigned as a wideband system around 650 cm −1 , whereas the rocking mode of water has appeared at 960 cm −1 .
The presence of a combination band of the symmetric N-H deformation and a lattice vibration around 2000 cm −1 (ν 2 + ν 6 ) unambiguously shows the presence of the same type of hindered rotation as in the NH 4 Cu(OH)MoO 4 [1], i.e., the ammonium is tied in place by relatively strong hydrogen bonds.
The compounds 1@2H 2 O and 2 do not absorb light in the visible light range, but as for other tetrahedral oxoanions, LMCT charge-transfer bands involving the MoO 4 2− ions can be observed in the UV range [24][25][26].Since the environment of the molybdate ions in compounds 1@2H 2 O and 2 is very similar, their UV spectra hardly differ (ESI Figures S14 and S15).Based on symmetry and Laporte's rules, two LMCT transitions are permitted for the tetrahedral molybdate ion: 2e ← t 1 and 3t 2 ← t 1 [25].The higher- energy LMCT band at ~211 nm corresponds to the transfer of an oxygen 2p p electron into the antibonding 2e orbital whose major constituents are the d-orbitals of the Mo atom in MoO 4 2− [25].The second transition (3t 2 ← t 1 ) as in other solid molybdates [26] was found at ~260 nm in both compounds.

Thermal Behavior of Compounds 1@2H 2 O and 2
The thermal decomposition of the samples of compound 1@2H 2 O and compound 2 was conducted both and inert and oxidative atmosphere.The curves suggest that aerial oxygen does not significantly affect the decomposition mechanism (Figure 9A).Compound 1@2H 2 O decomposes at almost the same temperatures in air as in argon (the difference is less than 5 • C) and the mass of the solid residue at 400 • C is only ~1% less in air than in inert atmosphere.The overall mass loss during thermal decomposition of compound 1@2H 2 O up to 400 • C is 30.1% in argon and 31.2% in air, which are in good agreement with the theoretical loss (31.62%) of four ammonia and two water molecules from one dihydrate.In the temperature range from ~70 to ~200 • C, compound 1@2H 2 O decomposes in several overlapped processes with DTG maxima at 134, 142, and 160 • C. The evolution of enclathrated water overlaps with ammonia release.This suggests that the release of the weakly bound water becomes possible after partial disruption of the lattice.To gain better insight into the decomposition mechanism of 1@2H 2 O, online coupled TG-MS measurements were also done.The MS data (Figure 10) proved the simultaneous evolution of both lattice water (m/z 18 and 17) and ammonia (m/z 17 and 16) in the whole range up to 400 • C. The low-intensity peaks of fragments m/z = 14, 15, 28, 30, and 44 (N + , NH + , N 2 + , NO + and N 2 O + ) show that, due to a slight oxidizing ability of molybdate, a small amount of coordinated ammonia is oxidized by MoO 4 2− .Up to 220 • C, the overall mass loss is 27.4 and 28.4% in argon and air, respectively, and corresponds to the loss of two water and three ammonia molecules (26.45%) per a 1@2H 2 O unit.The mass losses are somewhat higher than the calculated value, because a small part of the oxygen content of the molybdate ions also leaves in the form of a small amount of ammonia oxidation products.The decomposition processes at about 250 • C in both inert and oxidative atmosphere are followed by an endothermic heat effect (ESI Figure S16).The decomposition of compound 2 is a single-step process with DTG maximum at 245 and 246 • C in inert and oxidative atmosphere (Figure 9B).The reaction is endothermic.The molar decomposition heats were found to be 79.66 and 75.86 kJ/mol in air and argon, respectively.The thermal decomposition of compound 2 resulted in ZnMoO 4 both in argon and air, and mass loss of 11.1% (argon) and 11.3% (air), which are somewhat less than the calculated mass decrease after loss of NH 4 + and OH − (13.46%).It means that the formula of the hydrolysis product (compound 2) of compound 1 is close to [NH 4 Zn(OH)MoO 4 ], but the "neutralization" of [(NH 4 ) x (H) 1-x Zn(OH)MoO 4 ] is not complete (~88-90%).
TG-MS results show that only traces of molybdate reduction and ammonia oxidation (NO + , m/z = 30) appear in an inert atmosphere.The NO + peak intensity is higher in air than in Ar, which can be attributed to a small extent of the oxidation of the released ammonia (ESI Figures S17 and S18).The intensity ratios of m/z = 17 and m/z = 18 (m/z = 17, NH 3 + and OH + , m/z = 18, H 2 O + ) in inert atmosphere show that ammonia and water formed together [27], whereas, in air atmosphere, the intensity of m/z = 16 peaks is high because of O + (m/z = 16 from O 2 fragmentation) presence.Zinc molybdate can be used as a photocatalyst in the degradation of Victoria Blue [4] and other harmful dyes and chemicals like phenol [4] or nitrobenzene [3], and its photocatalytic efficiency strongly depends on its preparation route [7].We tested the photocatalytic activity of compounds 1 and 2 as precursor molecules, and ZnMoO  S6).From the kinetic curves, we determined the reaction rate and the pseudo-first order rate coefficients by a linear fit to the −ln(A/A 0 )-t.The results obtained in the photocatalytic degradation of Methyl Orange and Congo Red dyes at 375 nm UV irradiation (18 W, pH = 5.6 and 5.7, respectively) are given in ESI Table S6.We have found weak photocatalytic activity (~ca 2-fold acceleration) in methyl orange degradation in the presence of compounds 1, 2, and ZnMoO 4 samples prepared at 350 • C from these precursors (ESI Table S6).
The photocatalytic activity for oxidation of Congo Red, was found to be excellent, k app .S6).The ZnMoO 4 samples prepared at 350 • C showed minimal increasing/decreasing activity in photocatalytic degradation of Congo Red (k app .10 -4 = 9 and 3 min −1 , respectively) comparing that of with the blank sample (ESI Table S6).Compound 1@2H 2 O is soluble in water and slowly hydrolyses, thus the photocatalytically active ingredient might be a dissolved species or formed in situ as well.The BET surface area of solid compound 2 and the ZnMoO 4 samples (compounds 10-220-1, 10-350-1, 10-350-2) were found to be 12, 27, 26, and 18 m 2 /g, respectively.As can be seen, neither the BET values (12-27 m 2 /g) nor the differences between morphologies (Figure 11a,b) explain the difference between the photocatalytic powers of these samples.Therefore, the differences in the photocatalytic activities might be attributed to the valence distribution of each species in the surface layer.Based on the results mentioned above, the ZnMoO4 and NH 4 Zn(OH)MoO 4 derived from compound 1@2H2O are promising, easy-to-prepare photocatalysts.They speed up the photodegradation of Congo red at 325 nm by more than an order of magnitude.
Further XPS study of samples of compounds 10-220-1 and 10-350-1 is in progress to clarify the mechanism of photodegradation enhancement and test the technological power of these materials.
The zinc molybdate was prepared according to Zelikman [9] in the reaction of a stoichiometric amount of ZnO and MoO 3 at 650 • C. ZnO(s) + MoO 3 (s) = ZnMoO 4 (s) To form the ammonia complex according to ZnMoO 4 (s) + 4NH 3 (aq) = [Zn(NH 3 ) 4 ]MoO 4 , ZnMoO 4 was dissolved in 25% aq.ammonia solution at room temperature by intensive stirring.The undissolved residue was removed by filtration, and, by volume, 3 times more ethanol was added to the ammoniacal solution.The formed precipitate was filtered off immediately and washed with abs.ethanol and diethyl ether.
Compound 2 was prepared by contacting compound 1@2H 2 O with air for several days at room temperature.
The transformation and the end of the process was followed by powder XRD.The reaction time was found to be 1-5 days, depending on the humidity in the air.For elemental analysis, the samples of 1@2H 2 O and 2 were dissolved in dilute perchloric acid.The zinc content was measured as oxinate [28], while molybdate content was determined as PbMoO 4 [28].The ammonia content was determined by boiling it off with 0.1 M aq.NaOH and acidimetric titration and checked with gravimetry using H 2 PtCl 6 as the precipitating agent [28,29].
Thermoanalytical studies on compound 1@2H 2 O and compound 2 were performed on a TA Instruments SDT Q600 thermal analyzer coupled to the Hiden Analytical HPR-20/QIC mass spectrometer (TA ISntrument, New Castle, DE, USA and Hidden Co., Warrington, UK) and he Setaram LabsysEvo thermal analyzer (SETARAM, Lyon, France), respectively.DSC tests were performed with a Perkin Elmer DSC 7 instrument (Perkin-Elmer, Waltham, MA, USA).The details of the measurements are given in [22,29].
UV spectra was measured on a Jasco V-670 UV-Vis spectrometer (JASCO Co., Oklahoma City, OK, USA) as it was given previously [22,34] Low-temperature (−196.15• C) N 2 adsorption measurements were performed after 24 h degassing at 110 • C on a NOVA 2000e (Quantachrome, Boynton Beach, FL, USA) automatic volumetric instrument.The apparent surface area S BET was determined using the Brunauer-Emmett-Teller (BET) model [35].Evaluation of the adsorption data was performed with the Quantachrome ASiQwin software (version 3.0).CW-EPR spectra were recorded using a BRUKER EleXsys E500 X-band spectrometer (Bruker, Ettingen, Germany, microwave power 10mW, modulation amplitude 5G, modulation frequency 100 kHz).Measurements were performed at ambient temperature on 1 and 10% doped powder samples.As expected, for the 1% sample, since the average distance of the paramagnetic centers was significantly higher, better resolution spectra were obtained.
To evaluate the photocatalytic activity of the [Zn(NH 3 ) 4 ]MoO 4 and its intermediates formed at 220 • C and 350 • C under air, as well as NH 4 Zn(OH)MoO 4 and its decomposed product formed at 350 • C by isothermal heat under air, 1.0 mg of each compound was put into 3 mL of an aqueous solution of Methyl Orange (4 × 10 −5 M) and Congo Red (2 × 10 −5 M) dyes by using quartz cuvettes.The samples were left standing in dark for 1 h in order to reach the adsorption equilibrium.After that, they were submitted to a UV radiation source by Osram 18 W blacklight lamps (λ = maximum intensity at 375 nm).The cuvettes were placed 5 cm from each lamp and the absorbance was measured every 30 min during four hours by a Jasco V-550 UV-VIS spectroscope.The relative absorbance values of the most intensive peaks for Methyl Orange (464 nm) and Congo Red (497 nm) were considered to evaluate the catalysts' activity in the degradation of dyes.The pseudo-first order reaction rate constants were determined by linear regression of the data considering the Lagergren model.
Density-functional theory was used with the M05-2X and CAM-B3LYP functionals and the LANL2DZ basis set containing pseudopotentials to describe the core electrons of the transitional metal atoms.We report the results of the M05-2X/LANL2DZ calculations as those produced by the other functional are very similar.For each optimized molecular geometry, vibrational frequency analysis was performed to confirm that the stationary points are minimal.The discussion of energetics is based on the classical energy levels because the structures are rather fluxional and the determination of even the zero-point energies (zpe) is incorrect with the harmonic approximation.The simplest anharmonic correction, the hindered rotation approximation, is not of help because the most important factor is not the hindering of the NH 3 and H 2 O rotors but Berry's pseudorotation of the complex ion.All calculations were performed with the Gaussian 09 suite of programs [36].

Conclusions
A detailed analysis of the structure and properties of tetraamminezinc molybdate supplemented by two water molecules, 1@2H 2 O, of its conversion to basic ammonium zinc molybdate, 2 as well as those of the product 2 have been performed.The crystal structure of 1@2H 2 O, hitherto unknown, has been determined by finding isostructural analogs and powder XRD, since single crystals could not be prepared because the hydrolysis of the compound is faster than the crystal growth.There is a network a tetramminezinc cations and molybdate anions surrounding cavities occupied by the water molecules.Although the cage wall contains electronegative O and N atoms, the interaction between the caged water molecules and the wall proved to be weak, possibly due to the symmetry of the cage in which none of the positions are favorable.Thus, the water molecules do not behave as water of crystallization, and their state is close to those in clathrates.Their mobility is reflected in numerous properties such as vibrational spectra or DSC results.Detailed assignment of the IR/Raman spectra and interpretation of thermal decomposition of the compound have been provided.The compound 1@2H 2 O in aqueous solution is converted relatively fast to compound 2 (10 min boiling) and is contaminated by basic zinc carbonate and ammonium heptamolybdate.Rather interesting is the solid-phase hydrolysis of 1@2H 2 O in the presence, and especially in the absence, of aerial humidity.The probable mechanism for this non-trivial reaction is analogous to that determined for the close analog of compound 1@2H 2 O, tetraamminecopper molybdate; namely, successive substitution of ammonia ligands by water.The feasibility of this mechanism for 1@2H 2 O is supported by electronic structure calculations using density functional theory.The curiosity of the hydrolysis of 1@2H 2 O in a closed system is that the hydrolyzing agent in the solid-phase reaction comes from the lattice itself, and the reaction can be classified

Figure 1 .
Figure 1.XRD of compound 1 and its decomposition products on standing in the open air.The dihydrate of compound 1 decomposes very easily on standing in the open air.Even in the absence of humid air, it slowly hydrolyzes, utilizing its own water content.Five days after preparation, the main final decomposition product was identified as (NH 4 )Zn(OH)(MoO 4 ) (compound 2, ESI FigureS3) in both cases.Section 3 below is devoted to the description of the similarities and differences in the mechanism of hydrolysis with an external or only internal water source.

Figure 2 .
Figure 2. Voids for enclathrated water molecules (water dimer) in the lattice of compound 1@2H 2 O his is a figure.Schemes follow the same formatting.(a,b,c-axes direction, o-origo).

Figure 3 .
Figure 3.Comparison of the XRDs of decomposition intermediates formed from 1@2H 2 O in open air and in a closed system.

Figure 4 .
Figure 4. Possible pathways of compound 2 formation by self-hydrolysis/water vapor-mediated hydrolysis of compound 1@2H 2 O (red boxes contain the sources of the tetraammin-monoaqua salt; yellow and blue indicate the penta, whereas colorless boxes the tetracoordinated intermediates; green represents the final decomposition product).

Figure 6 .
Figure 6.Correlation analysis of internal ammonia modes in compound 1*2H 2 O.The total number of factor-group modes due to the internal NH 3 vibrations is 10, which is equal to 24 vibrational degrees of freedom.The F 2 modes are IR active, while A 1 , E, and F 2 are Raman active.The F 1 modes are inactive.The symmetric modes (ν 1 or ν s , and ν 2 or δ s ) are singlets, while the antisymmetric modes (ν 3 or ν as , and ν 4 or δ as ) are doubly degenerate vibrations under C 3v .Regarding external modes of NH 3 molecules, both T xy and R xy are doubly degenerate modes under C 3v .The total number of factor-group modes due to the external vibrations equals 4 × 6 = 24 vibrational degrees of freedom (12 of rotational and 12 of vibrational origin).The F 2 modes are both IR and Raman active.The F 1 ones are inactive.A 1 and E modes are only Raman active.The central zinc cation has one triply degenerate hindered translational mode, T xyz (both site and factor group symmetries are F 2 ).In the assignment of cation modes in compound 1@2H 2 O, the IR and Raman data of the isostructural [Zn(NH 3 ) 4 ](MnO 4 ) 2 , and the vibrational analysis results of the isotopesubstituted Zn( 15 NH 3 ) 42+ and Zn( 14 NH 3 ) 4 2+ cations[23] were also considered.The assignments of bands belonging to the ZnN 4 core and ammonia ligands are given in ESI TablesS3 and S4.The appearance of the IR inactive δ s (Zn-N) mode confirms the distortion of T d geometry around the ZnN 4 core.Similarly, the IR inactive A 1 and E modes (in C 3v ) of ammonia ligands are allowed, due to symmetry lowering induced by the evolution of extended hydrogen bond network involving the ammonia molecules and molybdate ions and the interaction of N-H bonds of the network with the water components in the cages.The bands at 1392 and 1301 cm −1 can be attributed to second-order transitions analogous to the first overtones of the ρ(NH 3 )(E) modes at ~700 and 650 cm −1 .The vibrational modes of enclathrated water in the spectra of compound 1@2H 2 O are expected to be similar to the monomeric or dimeric water modes in the gaseous state.The monomer water molecules must be disordered, as they occupy sites of T d symmetry.The easiest (we found no better) way to account for this is to add three degrees of freedom for internal vibrations, three others for librations, and another three for hindered translations without employing the correlation method (Figure7).An alternative approach would be to treat the water molecules as having spherical symmetry.The latter would imply a triply degenerate hindered translational mode T xyz , which is feasible, a triply