Synthesis and characterization a new polyoxomolybdate C34H114Fe2Mo12N18Na2O66 and study of its catalytic activity in the production of 1,2,3-triazoles

Abstract A new polyoxomolybdate, [(C6H12N4-CH3)2Na2(H2O)8](C6H12N4-CH3)2[(H0.5)N(CH2O)3FeMo6O18.5(OH)2.5]2·10H2O (1), was synthesized. The structure of the synthesized polyoxomolybdate was investigated by single-crystal X-ray diffraction analysis and several other identification techniques such as FTIR and EDX analysis. Each unit cell of 1 contains one cation [(C6H12N4-CH3)2Na2(H2O)8]4+ and two hexamethylenetetramine cations (C6H12N4-CH3)+, and two polyanions [(H0.5)N(CH2O)3FeMo6O18.5(OH)2.5]3– with ten water molecules in the crystal lattice. In the polyanion, molybdenum ions are bonded to two terminal oxygen atoms and two μ2- (Mo-Mo) and μ3- (Mo-Fe-Mo) bridging oxygen atoms. Iron in the center of the polyanion 1 is also surrounded by three deprotonated oxygens of trimethanolamine ligand and three μ3- (Mo-Fe-Mo) bridging oxygen atoms. This new polyoxomolybdate was used as an efficient catalyst in the azide–alkyne cycloaddition reaction to produce various 1,2,3-triazoles with high yields. Additional investigations reveal this catalytic system can be refreshed and utilized for up to six successive applications.


Introduction
The first synthesized polyoxometalate (POM) was reported by chemists in the nineteenth century [1], but the further description of POMs and their systematic study began in the early twentieth century [2].Studies on polyoxometalates showed that there is a great variety in terms of shape, size, and structure, from small to large, in their clusters.Polyoxometalates generally contain transition metals with d 0 layers, such as molybdenum, vanadium, and tungsten, which are produced using bottom-up physical techniques.Their applications are mostly in materials science, biology, catalysis, imaging, drug delivery, etc. [3][4][5][6][7].Also, many studies have been conducted in the field of the electrochemical activity of polyoxometalates as self-assembled materials [8][9][10][11][12] and organocatalyst functionalization of different CH bonds and developed an NH catalyzed esterification reaction [13].
Due to the high catalytic activity of some polyoxometalates, they are used in the production of 1,2,3-triazoles, having three nitrogens in the five-membered ring.In addition to biological activities, 1,2,3-triazoles are used as precursors in the production of drugs such as Mobritinib and Tazobactam [11].Mubritinib is a protein kinase inhibitor with the brand name TAK-165, first proposed by Takeda for cancer treatment; the drug Tazobactam is a highly modified penicillin and sulfone that is added to certain antibiotics to make them resistant to antimicrobial agents [14][15][16][17][18][19][20].
Here, a new polyoxometalate containing iron and molybdenum was synthesized and characterized.This prepared compound was used as a catalyst in azide-alkyne cycloaddition reactions.

Materials and methods
Sodium molybdate, Fe(NO 3 ) 2 Á9H 2 O, and ammonium acetate were purchased from Merck, and anhydrous acetic acid was obtained from Iranian Vision Pars Delta Company.Energy dispersive spectrometry (EDS) spectrum was obtained using an EDAX Pegasus XM4 spectrometer with an SDD Apollo 4D detector mounted on an FEI Nova NanoSEM 230 microscope.FTIR spectrum was obtained as KBr pellets on a Perkin-Elmer 100 FTIR spectrometer from 450 to 4000 cm À1 .

Synthesis of [(
Complex 1 was prepared by addition of sodium molybdate (5.85 mmol) to a solution of glacial acetic acid (15 mL), methanol (2 mL), and water (25 mL) and the resulting solution was stirred for 5 min.Then, iron(III) nitrate nonahydrate (2.06 mmol) was added followed by addition of ammonium acetate (12.97 mmol).After the solution was stirred for 2 h, the mixture was filtered and the filtrate was kept at room temperature to yield crystals.

Single-crystal X-ray data collection
X-ray intensity data for the were collected using graphite monochromatic MoKa radiation on a four-circle j geometry KUMA KM-4 diffractometer with a twodimensional area CCD detector at 295 K and 100 K. Between the room temperature (RT) and low temperature (LT) no structural phase transitions were observed, therefore full data measurement for structural analysis was performed for these crystals at 100 K. Data collections were made using the CrysAlis CCD program [21].Integration, scaling of the reflections, correction for Lorenz and polarization effects, and absorption corrections were performed using the CrysAlis Red program [21].The structure was solved by direct methods using SHELXT-2014/7 [22] and refined using SHELXL-2018/3 [23].The positions of hydrogen atoms were introduced in their geometrical positions and treated as rigid.
The final difference Fourier maps showed no peaks of chemical significance.Details of the data collection parameters, crystallographic data, and final agreement parameters are collected in Table 1.Visualizations of the structures were made with the Diamond 3.0 program [24].

Typical catalytic process for the azide-alkyne cycloaddition reaction
In a general process for synthesis of 1,2,3-triazole, a mixture including terminal alkyne (0.5 mmol), organic halide (0.55 mmol), NaN 3 (0.55 mmol), and 1 (0.00064 mmol) was reacted in an open-air atmosphere at 80 C for the desired reaction period.After termination of the reaction, the reaction vessel was left to cool to ambient temperature and corresponding 1,2,3-triazole was extracted by addition of water (5 mL) and ethyl acetate (15 mL in two steps).

Characterization of 1
The structure of FeMo 6 O 18.5 (OH) 2.5 ] 3-with ten water molecules in the crystal lattice.In the polyanion, molybdenum ions are bonded to two terminal oxygen atoms and two l 2 -(Mo-Mo) and l 3 -(Mo-Fe-Mo) bridging oxygen atoms.Iron in the center of the polyanion 1 is surrounded by three deprotonated oxygens of a trimethanolamine ligand and also three l 3 -(Mo-Fe-Mo) bridging oxygen atoms.In trimethanolamine all three hydroxyls (OH) are deprotonated but one N atom of the two is protonated.Polyoxometalate 1 crystallizes in the centrosymmetric space group I2/m of the monoclinic system and its structure is shown in Figure 1.The crystal data and structure refinement parameters of 1 are summarized in Table 1.Also, according to the FTIR spectrum of 1 (Supplementary Figure S1), peaks observed at 991 cm À1 and 821 cm À1 are, respectively, related to In the polyanion (C 3 H 9 NFeMo 6 O 24 ) 4-, the distances of each Fe-O bond are from 1.939(3) to 2.2032(2) Å and the average length of Mo-O t bonds is 1.7135 Å (range from 1.702(2) to 1.723(3) Å).The bond lengths of bridged oxygens with molybdenum vary from 1.917(2) to 1.954(2) Å with an average length of 1.9346 Å.The angles of O-Fe-O bonds around the central Fe ion, involving its surrounding oxygen atoms, are from 86.52(9) to 173.30 (12) Å which demonstrates there is a distorted octahedral geometry of the iron coordination sphere.Selected bond lengths and angles are given in Table 2.
Quite recently, another heteronuclear polyoxomolybdate complex with the composition of (  polyoxomolybdate with the Co(II) the Mo 6 O 24 skeleton also consists of 18 O 2-and two (not one as for Fe heteronuclear polyoxomolybdate complex) triple deprotonated trimethanolamine anions.Although in both heteronuclear polyoxomolybdate complexes the Mo-O bonds are not significantly different, the lengths of the Fe-O or Co-O bonds show differences.Three Fe-O distances in the present complex are 1.939(3), 1.977(3), and 2.032(2) Å, whereas in the Co heteronuclear polyoxomolybdate complex the Co-O are longer by 0.1 Å [26].Moreover, the main difference between these hetero polyoxoheterometalates is in its symmetry; the present Fe heteronuclear polyoxomolybdate [C 3 H 9 NFeMo 6 O 24 ] 3-anionic complex has a plane of symmetry m, whereas the Co heteronuclear polyoxomolybdate [C 6 H 12 N 2 CoMo 6 O 24 ] 4-anionic complex has an inversion center which lies on the Co central ion.In addition, in the present structure the central Fe in the [C 3 H 9 NFeMo 6 O 24 ] 4-anionic complex is coordinated by the O atoms of deprotonated trimethaolamine ligand and by three oxides, while in the cobalt polyoxomolybdate the central cobalt heteroatom in the [C 6 H 12 N 2 CoMo 6 O 24 ] 4-anionic complex is coordinated only by the oxygen atoms of two trimethaolamine ligands [26].

Catalytic effects
1,2,3-Triazole production reactions cannot be done without a catalyst.Polyoxometalates show good performance for such reactions.For this purpose, as the model reaction, benzyl chloride, phenylacetylene, and NaN 3 were used as reactants for the formation of 1,2,3-triazoles.All reactions were carried out in the presence of air without additives.By changing parameters affecting reaction rate, such as temperature, solvent, nature of the catalyst, and the amount of catalysts, the catalytic activity of 1 was optimized (Table 3).
Examining the reaction without a catalyst showed that in the absence of a catalyst, no reaction occurs, but with a catalyst loading of 0.00064 mmol, the reaction is carried out at 80 C in water and under air for 2 h, and the yield of the isolated product is >99%.To obtain the best reaction solvent, acetone, acetylacetone, water, methanol, and ethanol were used (entries 2 and 10-13).Due to the high solubility of sodium azide in water, the highest catalytic activity of 1 was observed in water and low to moderate yields obtained for other solvents.Comparing the efficiency of 1 with metal salts used for its synthesis, such as Fe(NO 3 ) 2 Á9H 2 O and Na 2 MoO 4 , shows the high efficiency of 1 compared to them (entries 14 and 15).Studying the model reaction at different temperatures and reaction times showed that by increasing the temperature to 100 C, product is obtained with 86% yield after 5 min; at 25 C, 1,2,3-triazole was isolated with a yield of 80% in 6 h, which shows the efficiency of 1 at room temperature.
To further test the scope of 1, the cycloaddition reaction was performed on a wide range of substituted phenylacetylenes and a mixture of benzyl halides under optimized conditions (Table 4).The position of the electron-donating or electron-withdrawing groups on the aryl ring in the reaction substrates affects the product yields.Reactions in the presence of electron-donating groups such as methyl show higher efficiency than electron-withdrawing groups such as nitro on benzyl halides.Also, the ortho or para position affects the reactivity of benzyl halides (entries 1-5) with benzyl halides containing ortho substituents having lower efficiency due to steric hindrance.As can be seen from Table 4, by replacing benzyl chloride with benzyl bromide, the products of the cycloaddition reaction were obtained with higher yields (entries 6-8).Based on these observations and the nature of the products, the cycloaddition reaction mechanism was proposed, as shown in Figure 3 [27].
To showcase 1 as catalyst, the results of the azide-alkyne cycloaddition reaction were analyzed under optimal conditions in the present study and compared to previous research, as provided in Table 5.Although our findings are consistent with other data presented in Table 5, our facile and expeditious synthesis approach (i.e. the facial one-pot synthesis protocol) obviates the need for substrate material to provide heterogeneous character (unlike entries 1-4).In addition, our catalytic reaction occurs in an environmentally friendly medium (in contrast to entries 3 and 4), employs low catalyst loading, involves shorter reaction times (in comparison to entries 1-4), and does not necessitate the use of additives (such as the base and ligand in entries 3 and 4).These are the major strengths of our proposed catalytic system, which clearly highlights the immense potential of our prepared catalyst in the production of 1,2,3-triazoles.
To assess the leaching of catalyst into solution of the cycloaddition reaction, the reaction was halted at the midpoint of the reaction duration, which was 1 h.Subsequently, complete separation of the catalyst from the solution was achieved.Following this, the reaction was allowed to continue for an additional hour; however, no product was observed upon extraction and therefore these findings provide confirmation that the synthesized POM is heterogeneous.
To check the reusability and recyclability of the catalyst, the catalyst was easily separated by simply filtering the reaction solution.Then, the catalyst was reused for the next batch of reactions with the addition of fresh substrates.Studies have shown that the catalyst can be used up to six times without significantly reducing the yield of the product (Figure 4).The decrease in activity observed during the process can be attributed to a variety of physical and chemical factors, including the blocking of catalytically active sites and depletion of catalysts during the washing and separation stages.To confirm the changes of the catalyst's structure during the reaction, IR and X-ray diffraction spectra were taken from the catalyst after the reaction.The data obtained indicates that the catalyst's stability diminishes after being utilized repeatedly (Supplementary Figures S2 and S3).

Conclusion
We ] 3-with ten water molecules in its crystal lattice.This new polyoxomolybdate was used as an efficient catalyst in the azide-alkyne cycloaddition reaction to produce various 1,2,3-triazoles with high yields.
The data that has been acquired through utilization of IR and X-ray diffraction spectra indicate that the stability of the catalyst decreases after six subsequent uses.
Mo ¼ O t and Mo-O b -Mo vibrations in 1.The peak at 429 cm À1 is assigned to characteristic vibration bands of Mo-O.The band at 648 cm À1 is due to Fe-O stretch.The peak at 1654 cm À1 corresponds to the asym (O-C-O) [25].The absorption peaks at 2954-3000 cm À1 are due to the asymmetric and symmetric H-C-H stretching vibrations.The elemental analysis confirmed the presence of Mo, Fe, O, and N elements in the synthesized polyoxometalate.The polyoxometalate 1 forms columns along [100] with water molecules and [(C 6 H 12 N 4 -CH 3 ) 2 Na 2 (H 2 O) 8 ] 4þ and (C 6 H 12 N 4 -CH 3 ) þ cations distributed in voids between them (Figure 2).
C 7 H 15 N 4 ) 2 [Co(H 2 O) 6 ][C 6 H 12 N 2 CoMo 6 O 24 ]Á4H 2 O has been characterized by our group [26].Although, these anions of polyoxoheterometalates, [C 3 H 9 NFeMo 6 O 24 ] 3-and [C 6 H 12 N 2 CoMo 6 O 24 ] 4-, have a similar arrangement of six MoO 6 octahedra with shared oxygen atoms surrounding the central Fe in the present structure or Co in the previous complex, however, the backbone of polyoxomolybate Mo 6 O 24 is slightly different in both complexes.In the present complex, [(C 6 H 12 N 4 -CH 3 ) 2 Na 2 (H 2 O) 8 ](C 6 H 12 N 4 -CH 3 ) 2 [(H 0.5 )N(CH 2 O) 3 FeMo 6 O 18.5 (OH) 2.5 ] 2 Á10H 2 O (in the crystal 1), polyoxomolybate Mo 6 O 24 skeleton consists of 18 O 2-, three hydroxyl (OH) groups (one proton of OH is disordered and belongs to two Mo 6 O 24 skeletons), and three O atoms from deprotonated trimethanolamine ligand but protonated on the N atom that is also disordered and belongs also to two Mo 6 O 24 skeletons, whereas in the heteronuclear

Figure 1 .
Figure 1.View of 1 with labeling of the atoms.Anisotropic displacement parameters are shown at the 50% probability level, H atoms with arbitrary radii.The CH 3 group of hexamethylenetetramine in the [(C 6 H 12 N 4 -CH 3 ) 2 Na 2 (H 2 O) 8 ] 4þ occupies two equivalent positions with occupancy of 0.5 (marked in light blue and green), whereas in the (C 6 H 12 N 4 -CH 3 ) þ cation occupies statistically three positions with occupancy 0.33 (marked in yellow, green, and light blue).

Table 1 .
Crystal data and structure refinement parameters for 1. formula C 34 H 94 Fe 2 Mo 12 N 8 Na 2 O 56 Á10 H 2 O

Table 3 .
The results for different conditions on the azide-alkyne cycloaddition in the presence of 1.

Table 4 .
Cycloaddition of various substrates in the presence of 1. Isolated yield, TON ¼ mol product/mol catalyst. a