A new polyoxomolybdate with a formula of [(C6H12N4-CH3)2Na2(H2O)8](C6H12N4-CH3)2 [(H0.5)N(CH2O)3FeMoO18.5(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 FT-IR and EDX analysis. It was confirmed that each unit cell of 1 contains one cation [(C6H12N4-CH3)2Na2(H2O)8]4+ and two hexamethylenetetramine cations (C6H12N4-CH3)+, two polyanions [(H0.5)N(CH2O)3FeMoO18.5(OH)2.5]3- with ten water molecules in the crystal lattice. In polyanion, molybdenum metals 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.
Research Article
Synthesis and characterization a new polyoxomolybdate C34H114Fe2Mo12N18Na2O66 and study of its catalytic activity in the production of 1,2,3-triazoles
https://doi.org/10.21203/rs.3.rs-2623528/v1
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Polyoxomolybdate
Iron
Catalytic reactivity
1
2
3-Triazole
The first synthesized polyoxometalate was reported by chemists in the 19th century [1], but the further description of POMs and their systematic study began in the early 20th 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 d0 layers, such as molybdenum, vanadium, and tungsten, which are produced using bottom-up physical techniques. Their applications are mostly in materials science, biology, catalyst, imaging, drug delivery, etc. [5–8]. Also, many studies have been conducted in the field of the electrochemical activity of polyoxometalates as self-assembled materials [9–12].
Due to the high catalytic activity of some polyoxometalates, they are used in the production of 1,2,3-triazoles. 1,2,3-triazoles having three nitrogens in the five-membered ring, in addition to biological activity, are used as precursors in the production of drugs such as Mobritinib and Tazobactam [13]. Mubritinib is a protein kinase inhibitor with the brand name TAK-165, first proposed by Takeda for cancer treatment, the other drug Tazobactam is a highly modified penicillin and sulfone that is added to certain antibiotics to make them resistant to antimicrobial agents make them resistant [14–19].
Here, a new polyoxometalate containing iron and molybdenum mixed metals was synthesized and characterized. This prepared compound was used as a catalyst in the azide-alkyne cycloaddition reactions.
Materials and Methods
Sodium molybdate, Fe(NO3)2·9H2O, and ammonium acetate were purchased from Merck, and anhydrous acetic acid was prepared 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. FT-IR spectrum was obtained as KBr pellets on a Perkin-Elmer 100 FTIR spectrometer over 450–4000 cm-1.
Synthesis of polyoxomolybdate [(C6H12N4-CH3)2Na2(H2O)8](C6H12N4-CH3)2 [(H0.5)N(CH2O)3FeMoO18.5(OH)2.5]2·10H2O (1)
The 1 was prepared by the 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 kept for stirring for 5 min. Then, iron(III) nitrate nonahydrate (2.06 mmol) was added followed by the addition of ammonium acetate (12.97 mmol). After the solution was stirred for 2 h, the mixture was filtered and the color filtrate was kept at room temperature to yield crystals.
Single crystal X-ray data collection
X-ray intensity data for the [(C6H12N4-CH3)2Na2(H2O)8](C6H12N4-CH3)2 [(H0.5)N(CH2O)3FeMoO18.5(OH)2.5]2·10H2O crystal (1) were collected using graphite monochromatic MoKa radiation on a four-circle k geometry KUMA KM-4 diffractometer with a two-dimensional area CCD detector at RT (295 K) and LT (100 K) temperatures. Between the RT and 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 [20]. Integration, scaling of the reflections, correction for Lorenz and polarisation effects and absorption corrections were performed using the CrysAlis Red program [20]. The structure was solved by the direct methods using SHELXT-2014/7 [21] and refined using SHELXL-2018/3 program [22]. 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 [23].
Typical catalytic process for the azide-alkyne cycloaddition reaction
In a general progress for the synthesis of 1,2,3-triazole, a mixture including terminal alkyne (0.5 mmol), organic halide (0.55 mmol), NaN3 (0.55 mmol) and complex 1 (0.9 mol%) 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 down to ambient temperature and corresponding 1,2,3-triazole was extracted by the addition of water (5 mL) and ethyl acetate (15 mL in two-step).
Characterization of 1
The structure of the synthesized polyoxometalate 1 contains one cation [(C6H12N4-CH3)2Na2(H2O)8]4+ and two hexamethylenetetramine cations (C6H12N4-CH3)+, two polyanions [(H0.5)N(CH2O)3FeMoO18.5(OH)2.5]3- with ten water molecules in the crystal lattice. In polyanion, molybdenum metals 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 surrounded by three deprotonated oxygens of a trimethanolamine ligand and also three μ3- (Mo-Fe-Mo) bridging oxygen atoms. In trimethanolamine all three hydroxyls (OH) groups are deprotonated but the N atom in one 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 Fig. 1. The crystal data and structural refinements of 1 are summarized in Table 1. Also, according to the FT-IR spectrum of 1 (Fig. S1), the peaks observed in the region 991 cm-1 and 821 cm-1 are respectively related to Mo=Ot and Mo–Ob–Mo vibrations in 1. The peak at 429 cm-1 assigned to the characteristic vibration bands of Mo-O. The band 648 cm-1 is due to Fe-O stretching vibration. The peak at 1654 is corresponded to the νasym(O-C-O) [24]. The absorption peaks at 2954-3000 cm-1 are due to the asymmetric and symmetric H-C-H stretching vibration.
Table 1. Crystal data and structure refinement parameters for 1.
|
Empirical formula |
C34H94Fe2Mo12N8Na2O56·10 H2O |
|
Formula weight (g·mol–1) |
3140.39 |
|
Crystal system, space group |
Monoclinic, I2/m |
|
a, b, c (Å) |
14.2349 (4), 13.2036 (3), 25.0904 (7) |
|
β (º) |
100.200(3) |
|
V(Å3) |
4641.3 (2) |
|
Z |
2 |
|
Dcalc (g cm-3)(Dx) |
2.247 |
|
µ (mm–1) |
1.99 |
|
F(000) |
3100 |
|
Crystal size (mm) |
0.20 × 0.17 × 0.12 |
|
Radiation type, wavelength, λ (Å) |
Mo Kα radiation, λ = 0.71073 Å |
|
Temperature (K) |
100 |
|
θ range(o) |
2.9–29.1° |
|
(sin θ/λ)max (Å-1) |
0.692 |
|
Absorption correction |
Multi-scan |
|
Tmin/Tmax |
0.895, 1.000 |
|
Reflections collected / unique / observed |
33190, 6164, 4960 |
|
Reflections/ parameters/ restraint |
6164/ 368/ 1 |
|
Rint |
0.037 |
|
L.S. parameters |
Refinement on F2 |
|
R[F2>2σ(F2)] |
0.0335 |
|
wR(F2 all reflections)a |
0.0778 |
|
Goodness-of-fit, S |
1.003 |
|
∆ρmax, ∆ρmin (e Å–3) |
+0.867, -0.675 |
|
(Δ/σ)max |
0.001 |
a) w = 1/[σ2(Fo2) + (0.0288P)2 + 22.9971P] where P = (Fo2 + 2Fc2)/3.
The polyoxometalate 1 forms columns along [100] with water molecules and [(C6H12N4-CH3)2Na2(H2O)8]4+ and (C6H12N4-CH3)+ cations distributed in voids between them (Fig. 2).
In the polyanion (C3H9NFeMo6O24)4-the distances of each of Fe-O bonds are from 1.939 (3) to 2.2032 (2) Å and the average length of Mo-Ot 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 bond around the central Fe atom, involving its surrounding oxygen atoms, are from 86.52 (9) to 173.30 (12) which demonstrates there is a distorted octahedral geometry of the iron coordination sphere. Selected bond lengths and angles are given in Table 2.
Table 2. Selected bond lengths [Å] and angles [°] for 1
|
Bond lengths (Å) |
|
Bond angles (°) |
|
|
Mo1—O1 |
2.251 (2) |
O2—Mo1—O1 |
72.63 (9) |
|
Mo1—O2 |
1.9467 (15) |
O1—Mo1—O13 |
73.29 (9) |
|
Mo1—O5 |
1.917 (2) |
O2—Mo1—O13 |
83.00 (10) |
|
Mo1—O13 |
2.309 (2) |
O5—Mo1—O13 |
71.65 (8) |
|
Fe—O1 |
1.939 (3) |
Fe—O1—Mo1 |
102.38 (10) |
|
Fe—O12 |
1.977 (2) |
Fe—O13—Mo1 |
97.58 (8) |
|
Fe—O13 |
2.032 (2) |
Fe—O13—Mo2 |
101.18 (8) |
Catalytic effects
1,2,3- Triazoles production reactions cannot be done without a catalyst. Polyoxometalates show good performance for this group of reactions; For this purpose, as the model reaction, benzyl chloride, phenylacetylene, and NaN3 were used as reactants for the formation of 1,2,3-triazoles. All reactions were carried out in the presence of air without any additives. By changing the effective parameters in the 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 between the reactants 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 hours, and the yield of the isolated product is >99%. To obtain the best reaction solvent, various solvents such as acetone, acetylacetone, water, methanol and ethanol have been used (entries 2, 10-13). It was expected that 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 were obtained for other solvents. Comparing the efficiency of polyoxometalate 1 with metal salts used for its synthesis, such as Fe(NO3)2·9H2O and Na2MoO4, shows the high efficiency of catalyst 1 compared to them (entries 14-15). Studying the model reaction at different temperatures and reaction times showed that by increasing the temperature up to 100°C, the product is obtained with 86 % yield after 5 minutes. Of course, it should be mentioned that at 25°C, 1,2,3-triazole was isolated with a yield of 80% in 6 hours, which shows the excellent efficiency of catalyst 1 at room temperature.
Table 3. The results for different conditions on the azide–alkyne cycloaddition in the presence of the 1
To further test the scope of 1, the cycloaddition reaction was successfully performed on a wide range of substituted phenylacetylenes and a mixture of benzyl halides under optimized conditions (Table 4).
Table 4. Cycloaddition of various substrates in the presence of the 1
|
Entry(%) |
Benzyl halide |
Alkyne |
%Yield (TON) |
|
1 2 3 4 5 6 7 8 |
2-nitro benzyl chloride 4-nitro benzyl chloride 2-methyl-benzyl chloride benzyl chloride benzyl chloride benzyl bromide benzyl chloride benzyl bromide |
phenylacetylene phenylacetylene phenylacetylene 3-butyn-2-ol 2-methyl-3-butyn-2-ol phenylacetylene propargyl alcohol propargyl alcohol |
33 (256) 51 (398) 46 (360) 96 (750) 39(305) 90(703) 90 (703) 56 (438) |
aIsolated yield, TON= mol product/ mol catalyst.
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), benzyl halides containing ortho substituents have lower efficiency due to steric hindrance. As can be seen from Table 3, 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 Fig. 3. [25].
In order to check the leaching of the catalyst in the cycloaddition reaction solution, the reaction was stopped in half of the reaction time of 1 hour and the catalyst was completely separated from the solution. The reaction was continued for another 1 h, but no product was observed after extraction (Fig. 4). These results confirmed that the synthesized POMs are heterogeneous.
In order 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 three times without significantly reducing the yield of the product (Fig. 5).
In conclusion, we have synthesized and characterized new polyoxomolybdate [(C6H12N4-CH3)2Na2(H2O)8](C6H12N4-CH3)2[(H0.5)N(CH2O)3FeMoO18.5(OH)2.5]2•10H2O (1). Single crystal X-ray diffraction analysis confirmed that each unit cell of 1 contains one cation [(C6H12N4-CH3)2Na2(H2O)8]4+ and two hexamethylenetetramine cations (C6H12N4-CH3)+, two polyanions [(H0.5)N(CH2O)3FeMoO18.5(OH)2.5]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.
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Availability of data and material
All data generated or analysed during this study are included in this published article [and its supplementary information files].
Competing interests
The authors declare that they have no competing interests.
Funding
Not applicable
Authors' contributions
Asmaa Yousofvand: writing the main manuscript text; Mojtaba Amini: Supervision; Mojtaba Hosseinifard: Writing – Review & editing; Jan Janczak prepared figures 1 and 2 and Table 1. All authors reviewed the manuscript.
Acknowledgments
The authors are grateful to the University of Tabriz for financial support of this work.
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No competing interests reported.