Acidic cesium salt of Preyssler nanoparticles: a new, green and recyclable nanocatalyst for the synthesis of 6-aryl-1H-pyrazolo[3,4-d]pyrimidin-4[5H]-ones

Introduction

Polyoxometalates (POMs) have attracted much attention as building blocks for functional composite materials due to their interesting nanosized structures [ ¹ ].They are superlative models for the construction of hybrid systems, hence they are considered as the potential candidates to be converted into the nanometer-sized materials. In recent years, considerable effort has been devoted to the design and controlled fabrication of nanostructured POMs for being used in green reactions. These waves of interest have resulted in the development of numerous strategies for the synthesis of nanostructured materials in a wide range of sizes. Therefore, the subject of nano POMs and their applications continues to attract dramatic attentions and the number of publications and patents continues to grow day to day, and new researchers are attracted to the field. Thus, plenty of room exists for extending the exploration of these materials and further expansion of new POMs is still in much demands. However, in spite of extensive investigations on using of Keggin-type heteropolyacids in nanotechnology [ ² , ³ ], the role of nano Preyssler has been largely overlooked. Recently, we have reported the application of Preyssler catalyst, H ₁₄ [NaP ₅ W ₃₀ O ₁₁₀ ], in various fields of nanotechnology [ ⁴ – ⁹ ]. In our attempts to use Preyssler as catalyst in organic reactions, we reported that Preyssler-type heteropolyacid, H ₁₄ [NaP ₅ W ₃₀ O ₁₁₀ ], shows good catalytic activity in the synthesis of heterocyclic compounds [ ¹⁰ – ¹⁴ ]. Encouraged by these experiences, it was of great interest using Preyssler’s anion in nanoform in the synthesis of heterocyclic compounds.

Herein, in continuation of our interest in catalytic applications of H ₁₄ [NaP ₅ W ₃₀ O ₁₁₀ ] in the synthesis of heterocyclic compounds [ ¹⁰ – ¹⁴ ] and extending the applications of this catalyst in nanotechnology, and because of importance of Pyrazolo[3,4-d]pyrimidines in medicine as anti-tumor, and anti-leukemia activities [ ¹⁵ – ¹⁹ ], we wish to report the synthesis of some derivatives of 1H-pyrazolo[3,4-d]pyrimidin-4(5H)-ones, by acidic cesium salt of Preyssler nanoparticles, Cs ₁₂ H ₂ [NaP ₅ W ₃₀ O ₁₁₀ ], as a green and eco-friendly solid nanocatalyst. In general, acid-catalyzed condensation of 5-amino-1-phenyl-1H-pyrazolo-4-carboxamide 1 with aromatic aldehydes and carboxylic acids is one of the simplest and most applicable approaches for synthesis of Pyrazolo[3,4-d]pyrimidines. Veeranagaiah and co-workers [ ²⁰ ] have also used acetic acid or hydrochloric acid and poly phosphoric acid or poly phosphate ester as an acid catalyst. These catalysts are toxic and dangerous and the applications of them cause environmental problems. The mentioned reactions were carried out in refluxing acetic acid during 2–4 h in the presence of hydrochloric acid and 5 h refluxing in acetic acid in the absence of catalyst in 47–60 % yields [ ²⁰ ].

Our findings indicate that our green used nanocatalyst is effective for synthesis of 1H-pyrazolo[3,4-d]pyrimidin-4(5H)-ones in excellent yields.

Experimental

Results and discussions

Recently, we described synthesis of some new derivatives of 1H-pyrazolo[3,4-d]pyrimidin-4(5H)-ones by heteropolyacids such as H ₃ [PW ₁₂ O ₄₀ ] and H ₁₄ [NaP ₅ W ₃₀ O ₁₁₀ ] as acidic catalysts instead of mineral acids like HCl or polyphosphoric acid as a more eco-friendly and suitable method [ ²¹ , ²² ]. Therefore, it is of great interest to know, what occurs if we check Preyssler nanocatalyst in these reactions. Also, another goal in this research was an answer to this question: Do surface area in the Preyssler structure, which is responsible for surface type reactions has an important role on yields?

In the preparation of nanoparticles of Cs ₁₂ H ₂ [NaP ₅ W ₃₀ O ₁₁₀ ], to balance anions charge, the stoichiometric amounts of aqueous solutions of cations were used (1:14). However, due to the charge and size differences between Cs ⁺ and H ⁺ ions, different amounts of cations appeared in the Preyssler and the rest of charges were compensated for by protons to form acid salts. The number of Cs ions obtained was 12, by ICP measurement and titration method. Figure  ¹ shows the FT-IR spectrum of the Cs ₁₂ H ₂ [NaP ₅ W ₃₀ O ₁₁₀ ]. In the nano Cs-Preyssler, the Preyssler’s anion displays vibrations at 1161.96, 1087.62, and 1023.33 cm ⁻¹ for P–O stretching, 956.60 and 907.76 cm ⁻¹ for W–O–W stretching, and 772.78 cm ⁻¹ for W=O stretching.

Fig. 1

From Fig.  ¹ , it can be seen that the nano Cs-Preyssler shows all the IR vibration peaks assigned to a Preyssler structure, and the locations of featured peaks are in well agreement with its structure [ ²³ ]. TEM image of nano Cs-Preyssler is shown in Fig.  ² . The TEM image shows the spherical structures which have a diameter in the range of 5–25 nm.

Fig. 2

When the mixture of 5-amino-1-phenyl-1H-pyrazolo-4-carboxamide 1 and aromatic aldehyde 2 in acetic acid were refluxed in the presence of catalytic amounts of mentioned nanoheteropolyacid for 1–3 h, 6-aryl-1H-pyrazolo[3,4-d]pyrimidin-4[5H]-ones 3 were obtained (Scheme  ¹ ).

Scheme 1

The results are shown in Tables  ¹ and ² . No other by products were recognized.

In spite of obtaining good to moderate yields of products in our earlier works [ ²¹ , ²² ], we obtained higher yields using nano Preyssler catalyst (Table  ¹ ). It is suggested that, as the particle size decreases, the surface area increases, and thus activity increases. The larger surface area of cesium nano Preyssler in comparison with pure Preyssler acid was confirmed by BET analysis. The BET surface area of Cs ₁₂ H ₂ [NaP ₅ W ₃₀ O ₁₁₀ ] and H ₁₄ [NaP ₅ W ₃₀ O ₁₁₀ ] obtained was 2.61 and 0.78 m ² /g, respectively. Surface-type and bulk-type catalysis are the two types of catalysis for heterogeneous acid catalysis by heteropolyacids [ ²⁴ , ²⁵ ]. In surface type catalysis, the reactions occur on the surface of bulk or supported heteropoly compounds and the catalytic activity usually depends on the surface acidity of heteropolyacid. We believe that the larger surface area in nano Cs ₁₂ H ₂ [NaP ₅ W ₃₀ O ₁₁₀ ] is responsible for absorbing a greater number of reactant molecules.

The selectivity toward 6-aryl-1H-pyrazolo[3,4-d]pyrimidin-4[5H]-ones remained 100 % in all cases.

Conclusion

We reported here a catalytic method for synthesis of the 6-substituted-pyrazolo[3,4-d]pyrimidin-4(5H)-ones using acidic cesium salt of Preyssler nanoparticles which is found as a more effective catalyst than the other heteropolyacids in our earlier work as well as classical acids.

This nanocatalyst is not soluble in acetic acid and is eligible to an easy separation and recovery by filtering for their further reuse without any appreciable decrease of the catalyst activity.