Catalytic performance of a Keplerate-type, giant-ball nanoporous isopolyoxomolybdate as a highly efficient recyclable catalyst for the synthesis of biscoumarins

1 Introduction

The development of heterogeneous catalysts and how they affect specific transformations in chemical synthesis has become a major area of research. The potential advantages of these materials over homogeneous systems, in terms of their simplified recovery and reusability, could potentially allow for the development of environmentally benign chemical procedures in both academic and industrial settings. Catalysts of this type have the potential to make the processes in which they are applied cleaner, safer, higher-yielding, and relatively inexpensive [1–4]. Polyoxometalates (POMs) are a large family of metal-oxygen clusters of early transition metals. These compounds have stimulated many current research activities in a broad range of fields such as catalysis, magnetism, materials science and biomedicine [5–8]. Such a stimulation is due to diverse and highly modifiable sizes, shapes, charge densities, acidities and reversible redox potentials of POMs [9, 10]. In recent years, the synthesis of nanotubular materials consisting of POMs such as POMs-including titania nanotubes [11] and POMs-organic hybrid nanotubes [12] has been a subject of increasing interest. These new types of nanotubes show not only the functional properties of POMs but also the advantages of tubular systems in the application, for example, of catalytic and photochemical properties [11–13].

Giant nanosized porous Keplerate-type POMs, reported for the first time by Müller and co-workers [14], show a large variety of applications in fundamental and applied science, such as in modeling passive cation transport through membranes, encapsulation, nanoseparation chemistry, and magnetic and optics properties [15, 16]. In spite of these valuable properties, to the authors’ knowledge, there are only three references in the literature on the use of giant nanosized porous POMs as a catalyst in organic transformations [17–19].

Coumarins are a large group of heterocycles with diverse and interesting biological activities. These compounds are reported to possess significant anticoagulant, insecticidal, antihelminthic, hypnotic, antifungal, and HIV protease inhibition activities [20–24]. Biscoumarins, the bridge substituted dimers of 4-hydroxycoumarin, have enormous potential as anticoagulants [25, 26]. A number of biscoumarins have also been found to be urease inhibitors [27].

These compounds are generally synthesized via the reaction of 4-hydroxycoumarin with aldehydes in the presence of various Brønsted and Lewis acidic catalysts such as Brønsted-acidic ionic liquids based on ammonium [28], imidazolium [29], and benzimidazolium [30] cations, [P₄VPy-BuSO₃H]HSO₄, a supported acidic ionic liquid [31], TiO₂-SO₃H [32], tetrabutylammonium hexatungstate [TBA]₂[W₆O₁₉] [33], RuCl₃·nH₂O [34], I₂ [35], and sulfonated rice husk ash (RHA-SO₃H) [36]. Catalyst-free synthesis of these compounds with long reaction times has also been reported [37].

During the course of our recent studies directed towards the development of practical and environmentally friendly procedures for the synthesis of organic compounds using reusable catalysts [38–46], we investigated the application of (NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀ (H₂O)₇₂], a Keplerate-type giant-ball nanoporous isopolyoxomolybdate denoted as {Mo₁₃₂}, as a catalyst for a series of organic transformations. This new reusable catalyst performed well and showed a high level of catalytic activity in the synthesis of 1,2,4,5-tetrasubstituted imidazoles, 1,8-dioxo-octahydroxanthenes and 1,8-dioxodecahydroacridines [18, 19]. This fact prompted us to investigate the catalytic activity of this material in the synthesis of biscoumarins (Scheme 1). For the first time, this ball-shaped POM has been characterized using the transmission electron microscopy (TEM) image by Polarz et al. [47]. The TEM picture clearly shows a periodic structure with an average size of approximately 3 nm in diameter. This experimentally obtained diameter fits nicely with the theoretical value for the inner diameter of this ball-shaped POM that was calculated to be 2.9 nm [14, 48].

Scheme 1:

{Mo₁₃₂} catalyzed synthesis of biscoumarins ({Mo₁₃₂} ≡ (NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]).

2 Results and discussion

The catalyst {Mo₁₃₂} was characterized by FT-IR and UV/Vis spectroscopy as reported in our previous work [19]. At the beginning of this study, 4-chlorobenzaldehyde (2e) was employed as the model aldehyde and reacted with 4-hydroxycoumarin (1). In order to get the effective reaction conditions, the reaction was optimized in terms of various parameters such as catalyst amount, effect of solvent and influence of temperature (Table 1). Low yields of the product 3e were obtained in the absence of the catalyst in refluxing EtOH or H₂O (entries 1 and 2) or in the presence of the catalyst under solvent-free conditions at high temperatures (entries 3 and 4), indicating that the catalyst and solvent are necessary for the reaction. As can be seen from Table 1, among the tested solvents such as H₂O, EtOH, MeOH, CHCl₃, CH₃CN, and also solvent-free conditions and various amounts of the catalyst, the reaction was more facile and proceeded to give the highest yield, using 0.07 g of {Mo₁₃₂} in EtOH at reflux temperature (entry 12). All subsequent reactions were carried out in these optimized conditions.

Encouraged by the remarkable results obtained with the above reaction conditions, and in order to show the generality and scope of this new protocol, we extended the reaction of 4-hydroxycoumarin (1) with a range of other aromatic or aliphatic aldehydes 2a–l under the optimized reaction conditions, and the results are summarized in Table 2. As shown, all reactions proceed very cleanly to give the corresponding biscoumarins 3a–l in high yields over short reaction times. Purity checks with melting points, TLC and the ¹H NMR spectroscopic data reveal that only one product is formed in all cases and no undesirable side products are observed. The structures of all known products 3a–l were deduced from their ¹H NMR and FT-IR spectral data and a comparison of their melting points with those of authentic samples. For example, as shown in Fig. 1, the ¹H NMR spectrum of 3f in CDCl₃ showed a sharp ¹H NMR signal at δ = 6.09 ppm for the methine proton along with two sharp singlets in the down-field region of the spectrum at δ = 11.35 and 11.57 ppm for hydroxyl groups as well as the signals in the aromatic region due to 12 aromatic protons, indicating the formation of the compound 3f.

Fig. 1:

The ¹H NMR spectrum of compound 3f in CDCl₃.

To test the recyclability of {Mo₁₃₂}, after completion of the model reaction, the catalyst was recovered according to the procedure described in the Experimental section. The separated catalyst was dried at 60 °C under vacuum for 1 h before being reused in a similar reaction. The catalyst could be used at least five times without changing the yield of the product significantly (97 % for 1^st use; 97 % for 2^nd use; 96 % for 3^rd use; 95 % for 4^th use; 93 % for 5^th use). Furthermore, the FT-IR spectra of the recovered catalyst (Fig. 2b–e) were almost identical to that of the fresh catalyst (Fig. 2a), which indicated that the structure of the catalyst was unchanged by the reaction.

Fig. 2:

FT-IR spectra of a fresh catalyst {Mo₁₃₂} (run 1, a) and recovered catalysts (runs 2–5, b–e) for the synthesis of compound 3e in the model reaction.

Although we did not investigate the reaction mechanism, a plausible mechanism for this reaction may proceed as depicted in Scheme 2. On the basis of our previous reports [18, 19], it is reasonable to assume that several accessible Mo sites and NH₄ groups in {Mo₁₃₂} could act as Lewis acid and Brönsted acid centers, respectively, and therefore promote the necessary reactions. The catalyst would play a significant role in increasing the electrophilic character of the electrophiles in the reaction. According to this mechanism, the {Mo₁₃₂} catalyst would facilitate the formation of intermediates I, II and III. Under these conditions, however, attempts to isolate the proposed intermediates failed even after careful monitoring of the reactions.

Scheme 2:

Plausible mechanism for the {Mo₁₃₂}-catalyzed formation of biscoumarins 3a–l.

3 Conclusion

In conclusion, in this paper, we showed that {Mo₁₃₂}, a Keplerate-type giant-ball nanoporous isopolyoxomolybdate, as a highly effective heterogeneous catalyst effectively catalyzes the reaction of 4-hydroxycoumarin and aromatic or aliphatic aldehydes in refluxing ethanol. This method provided the biscoumarins products in high yields over short reaction time, following a facile work-up process. The catalyst is inexpensive and easily obtained, stable and storable, easily recycled and reused for several cycles with consistent activity.

4 Experimental section